Solved Examples — Class 11 Biology Chapter 15 Notes

How to Use This Section

This chapter is a chapter of lists, addresses and numbers. Almost nothing in it is hard to understand; almost everything in it is easy to get slightly wrong. The marks are lost in the same four places every year - a percentage written for the wrong cell, a valve put on the wrong side, a node put in the wrong corner, and a wave of the ECG matched to the wrong event.

Three habits will carry you through it.

First, learn every number with its owner attached. Do not memorise "60-65 per cent"; memorise neutrophils 60-65 per cent, the most abundant WBC. Do not memorise "70 mL"; memorise stroke volume, approximately 70 mL pumped by each ventricle in each cardiac cycle. Every wrong option in this chapter is a real figure from the chapter wearing someone else's name.

Second, say the paired addresses out loud, both halves together. Tricuspid valve on the RIGHT, bicuspid or mitral valve on the LEFT. SAN in the right upper corner of the right atrium, AVN in the lower left corner of the right atrium. Lub with the closure of the tricuspid and bicuspid valves, dub with the closure of the semilunar valves. P-wave for atrial depolarisation, T-wave for ventricular repolarisation. Each of those four pairs is a two-mark question and a guaranteed distractor, and the only defence is to have learnt both halves as one sentence.

Third, treat the ABO table as a piece of logic, not a piece of memory. Your plasma carries the antibody against the antigen you do not have. That single line generates the whole of Table 15.1, tells you at once why O is the universal donor and why AB is the universal recipient, and saves you from ever having to recall four rows under pressure.

The items below run in three tiers.

Tier 1 - short recall. The definitions, the counts, the percentages, the addresses and the orders. Answer these aloud until none of them needs thinking about.
Tier 2 - applied reasoning. A patient, a trace or a set of figures is put in front of you and you have to work out what follows. Three of these are numerical items - cardiac output from stroke volume and heart rate, the duration of one cardiac cycle from the heart rate, and a full differential white cell count worked out from a total count - because those are the three sums this chapter actually sets.
Tier 3 - longer written answers. Full accounts written the way a written paper wants them, with the marking-scheme words in place.

Two of the chapter-end exercises are answered here and nowhere else in the chapter - the match-the-column item and the comparison of blood with lymph. Every other set exercise is answered in full inside one of the twelve teaching sections, and the last block of this section is a table telling you exactly which section and which question number holds each one. Work the exercises on paper first, then use that table to check yourself against a complete answer.

Tier 1 - Short Recall

Question 1

Q. Define blood, and give the two fractions it separates into with the share of each.

Answer. Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. It is a connective tissue because, like every connective tissue, it is cells sitting in a matrix - the difference is only that here the matrix is a liquid instead of a solid.

Spin a sample down and it splits in two:

Fraction	What it is	Share of the blood
Plasma	the straw coloured, viscous fluid matrix	nearly 55 per cent
Formed elements	erythrocytes, leucocytes and platelets	nearly 45 per cent

Learn the pair together - 55 and 45 - because the commonest error is to swap them and make the cells the bigger half.

Question 2

Q. Give the composition of plasma in full - water, proteins, minerals and everything else it carries - and say what serum is.

Answer. Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood.

Water: 90-92 per cent of plasma.
Proteins: 6-8 per cent of plasma. The three major ones are fibrinogen, globulins and albumins.
Fibrinogens are needed for clotting or coagulation of blood.
Globulins are primarily involved in the defence mechanisms of the body.
Albumins help in osmotic balance.
Minerals in small amounts: $\mathrm{Na^+}$ , $\mathrm{Ca^{2+}}$ , $\mathrm{Mg^{2+}}$ , $\mathrm{HCO_3^-}$ , $\mathrm{Cl^-}$ and others.
Glucose, amino acids and lipids, which are present because they are always in transit in the body.
The factors for coagulation, carried in an inactive form.

Serum is plasma without the clotting factors. That is the whole definition, and it is asked in exactly those words.

Three proteins, three jobs, and they are never interchangeable: fibrinogen clots, globulin defends, albumin balances.

Question 3

Q. Write a data card for the red blood cell - its count, its haemoglobin, its shape and nucleus, where it is made, how long it lives and where it dies.

Answer. Erythrocytes or red blood cells are the most abundant of all the cells in blood.

Feature	The fact
Number	5 million to 5.5 million per cubic millimetre of blood in a healthy adult man, on an average
Haemoglobin	12-16 grams in every 100 mL of blood in a healthy individual
What haemoglobin is	a red coloured, iron containing complex protein - hence the colour and the name of the cell
Its job	transport of respiratory gases
Nucleus	absent in most mammals
Shape	biconcave
Formed in	the red bone marrow in adults
Life span	an average of 120 days
Destroyed in	the spleen, the graveyard of RBCs

Two of those numbers look alike and are constantly swapped: 5 million to 5.5 million is a count per cubic millimetre; 12-16 grams is a weight per 100 mL. One is cells, the other is protein.

Question 4

Q. Name the five white blood cells in descending order of abundance with the percentage of each, and give the total white cell count.

Answer. The total leucocyte count averages 6000-8000 per cubic millimetre of blood, which is far fewer than the red cells.

Rank	Cell	Share of the total WBCs	Category
1 - most abundant	Neutrophils	60-65 per cent	granulocyte
2	Lymphocytes	20-25 per cent	agranulocyte
3	Monocytes	6-8 per cent	agranulocyte
4	Eosinophils	2-3 per cent	granulocyte
5 - least abundant	Basophils	0.5-1 per cent	granulocyte

The two ends of that ladder are the whole question: neutrophils most abundant, basophils least abundant.

The two categories split three against two. Granulocytes are neutrophils, eosinophils and basophils. Agranulocytes are lymphocytes and monocytes.

Leucocytes are colourless because they lack haemoglobin, they are nucleated, and they are generally short lived - three differences from the red cell in one line.

Question 5

Q. Give one job for each of the five white blood cells.

Answer.	Cell	What it does
Neutrophils	phagocytic - they destroy foreign organisms entering the body
Monocytes	phagocytic, the same job as the neutrophil
Basophils	secrete histamine, serotonin and heparin, and are involved in inflammatory reactions
Eosinophils	resist infections and are associated with allergic reactions
Lymphocytes	of two major types, B and T, and both are responsible for the immune responses of the body

The two phagocytes are neutrophils and monocytes - one from each category. Being asked "which cells are phagocytic" and answering only "neutrophils" loses half the mark.

Question 6

Q. What are platelets, where do they come from, how many are there, and what goes wrong if the number falls?

Answer. Platelets, also called thrombocytes, are cell fragments produced from megakaryocytes, which are special cells in the bone marrow. They are fragments, not whole cells, and that word is worth a mark on its own.

Number: 1,50,000 to 3,50,000 platelets per cubic millimetre of blood.
Function: platelets can release a variety of substances, most of which are involved in the coagulation or clotting of blood.
A reduction in their number leads to clotting disorders, which lead to excessive loss of blood from the body.

Note the odd position of the platelet in the chapter - it is counted among the formed elements, but it is not a cell.

Question 7

Q. Reproduce the table of the four blood groups with their antigens, their antibodies and the groups they may receive from.

Answer. ABO grouping is based on the presence or absence of two surface antigens on the RBCs, namely A and B, and on the two natural antibodies in the plasma.

Blood group	Antigens on RBCs	Antibodies in plasma	Donor's group
A	A	anti-B	A, O
B	B	anti-A	B, O
AB	A, B	nil	AB, A, B, O
O	nil	anti-A, anti-B	O

The whole table comes out of one line: your plasma carries the antibody against the antigen you do not have. Group A lacks antigen B, so it carries anti-B. Group AB lacks nothing, so it carries no antibody at all. Group O has neither antigen, so it carries both antibodies.

An antigen is a chemical that can induce an immune response. An antibody is a protein produced in response to an antigen. Those two definitions are asked as often as the table itself.

Question 8

Q. Explain, in one line each, why group O is called the universal donor and group AB the universal recipient.

Answer. - Group O blood can be donated to persons with any other blood group, and hence O group individuals are called universal donors. The reason is that the O red cell carries no antigen at all, so there is nothing on it for the recipient's antibodies to attack.

Persons with AB group can accept blood from persons with AB as well as the other groups of blood, and are therefore called universal recipients. The reason is that AB plasma carries no antibody at all, so there is nothing in it to attack an incoming red cell.

Say the rule cleanly: donation is decided by the DONOR'S ANTIGEN, reception is decided by the RECIPIENT'S ANTIBODY. That is why the same person cannot be both.

Question 9

Q. What is the Rh antigen, how many people carry it, and what happens when an Rh negative person is exposed to Rh positive blood?

Answer. The Rh antigen is another antigen on the surface of the RBCs, similar to one present in Rhesus monkeys - hence the name Rh.

It is present in the majority, nearly 80 per cent, of humans. Such individuals are called Rh positive; those in whom the antigen is absent are called Rh negative.
An Rh negative person, if exposed to Rh positive blood, will form specific antibodies against the Rh antigens.
Therefore the Rh group should also be matched before transfusions, and not the ABO group alone.

Nearly 80 per cent is the figure that gets asked, and the distractor is always a percentage borrowed from the white cell list.

Question 10

Q. Give the sequence of erythroblastosis foetalis in short, and say how it is avoided.

Answer. It is a special case of Rh incompatibility between the Rh negative blood of a pregnant mother and the Rh positive blood of the foetus.

In the first pregnancy the Rh antigens of the foetus do not get exposed to the mother's blood, as the two bloods are well separated by the placenta.
During the delivery of the first child there is a possibility of exposure of the maternal blood to small amounts of Rh positive blood from the foetus.
The mother then starts preparing antibodies against the Rh antigen in her blood.
In her subsequent pregnancies these Rh antibodies can leak from the mother into the blood of the foetus and destroy the foetal RBCs.
This could be fatal to the foetus, or could cause severe anaemia and jaundice to the baby. The condition is called erythroblastosis foetalis.

It is avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

The examinable point is the timing: the danger is created at the first DELIVERY and it strikes in a LATER pregnancy.

Question 11

Q. Write the clotting cascade in the order in which the events happen, showing the inactive form and the active product at each step.

Answer. Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body.

Written forwards, in the order the events actually occur:

Step	Inactive form present in plasma	Enzyme or complex that acts	Active product
1	a number of factors in an inactive state	a series of linked enzymic reactions - the cascade process	thrombokinase, an enzyme complex
2	prothrombin	thrombokinase	thrombin
3	fibrinogen	thrombin	fibrins

The end product is the clot or coagulam - a dark reddish brown scum formed mainly of a network of threads called fibrins, in which the dead and damaged formed elements of blood are trapped.

Papers usually print the cascade backwards, the way the chapter narrates it - fibrin from fibrinogen, thrombin from prothrombin, thrombokinase from the cascade. Turn it round before you answer, because "write the order of events" means forwards.

Question 12

Q. What starts coagulation, and what part do calcium ions play in it?

Answer. Two things can start it, and both are worth naming.

An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation.
Certain factors released by the tissues at the site of injury can also initiate coagulation.

Calcium ions, $\mathrm{Ca^{2+}}$ , play a very important role in clotting. They are needed for the cascade to run at all, which is why removing calcium from a collected sample stops it from clotting.

Note why the factors are stored inactive. They are present in the plasma in an inactive state so that blood does not clot while it is flowing normally inside the vessels - the cascade is a loaded mechanism waiting for a trigger.

Question 13

Q. What is tissue fluid, what is lymph, and what does lymph do?

Answer. As the blood passes through the capillaries in the tissues, some water along with many small water soluble substances moves out into the spaces between the cells, leaving the larger proteins and most of the formed elements in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid.

It has the same mineral distribution as that in plasma.
Exchange of nutrients, gases and so on between the blood and the cells always occurs through this fluid.
An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called lymph.

So tissue fluid and lymph are the same fluid at two addresses - in the tissue spaces it is tissue fluid, inside the lymphatic vessels it is lymph.

What lymph is and does:

Lymph is a colourless fluid, because it has no haemoglobin containing RBCs.
It contains specialised lymphocytes, which are responsible for the immune responses of the body.
Lymph is also an important carrier for nutrients, hormones and so on.
Fats are absorbed through lymph in the lacteals present in the intestinal villi.

Learn that last line as a four word chain - fats, lymph, lacteals, villi. It is the single most asked sentence on lymph.

Question 14

Q. Distinguish the open circulatory system from the closed one, and say which animals have each.

Answer. The circulatory patterns are of two types, open or closed.

Feature	Open system	Closed system
Route of the blood	blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses	blood pumped by the heart is always circulated through a closed network of blood vessels
Contact with tissues	blood bathes the tissues directly in the sinuses	blood stays inside vessels; exchange is through the tissue fluid
Animals	arthropods and molluscs	annelids and chordates

The closed pattern is considered to be more advantageous as the flow of fluid can be more precisely regulated. That one clause is the whole answer to "why is the closed system better" - precise regulation of flow, not speed and not pressure.

Sinuses are the give-away word: if a question mentions sinuses, the system is open and the animal is an arthropod or a mollusc.

Question 15

Q. Give the number of heart chambers in each group of vertebrates, and the type of circulation each has.

Answer. All vertebrates possess a muscular chambered heart.

Group	Chambers	What they are	Circulation
Fishes	2-chambered	an atrium and a ventricle	single circulation
Amphibians and reptiles, EXCEPT crocodiles	3-chambered	two atria and a single ventricle	incomplete double circulation
Crocodiles, birds and mammals	4-chambered	two atria and two ventricles	double circulation

The crocodile is the whole trap. It is a reptile with a 4-chambered heart, so the rule has to be stated with its exception attached: amphibians and reptiles except crocodiles are 3-chambered.

In the 3-chambered heart the left atrium receives oxygenated blood and the right atrium the deoxygenated blood, but they get mixed up in the single ventricle, which pumps out mixed blood. That mixing is the reason the word incomplete is used.

Question 16

Q. Give the address of every chamber, septum and valve of the human heart in one list, saying which side each valve is on.

Answer. The heart is a mesodermally derived organ, situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist, and is protected by a double walled membranous bag, the pericardium, enclosing the pericardial fluid.

Structure	Where it is and what it does
Atria	the two relatively small upper chambers
Ventricles	the two larger lower chambers
Inter-atrial septum	a thin, muscular wall separating the right and left atria
Inter-ventricular septum	a thick-walled septum separating the left and right ventricles
Atrio-ventricular septum	a thick fibrous tissue separating the atrium and the ventricle of the same side
Tricuspid valve	guards the opening between the RIGHT atrium and the RIGHT ventricle; formed of three muscular flaps or cusps
Bicuspid or mitral valve	guards the opening between the LEFT atrium and the LEFT ventricle
Semilunar valves	guard the openings of the right and left ventricles into the pulmonary artery and the aorta respectively

Each septum is provided with an opening through which the two chambers of the same side are connected, and each of those openings carries a valve.

The valves in the heart allow the flow of blood only in one direction - from the atria to the ventricles, and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.

The entire heart is made of cardiac muscles, and the walls of the ventricles are much thicker than those of the atria.

Say tricuspid-right and bicuspid-left as one breath. It is the most swapped pair in the chapter.

Question 17

Q. Where exactly do the SAN and the AVN lie, and write the conducting pathway of the heart in order.

Answer. A specialised cardiac musculature called the nodal tissue is distributed in the heart.

The sino-atrial node (SAN) is a patch of this tissue present in the RIGHT UPPER CORNER of the RIGHT ATRIUM.
The atrio-ventricular node (AVN) is another mass of this tissue in the LOWER LEFT CORNER of the RIGHT ATRIUM, close to the atrio-ventricular septum.

Both nodes lie in the right atrium. That is the fact the distractors are built on - "lower left corner" tempts you into saying left atrium, and it is wrong.

The conducting pathway, in order:

SAN -> atrial musculature -> AVN -> AV bundle -> bundle of His -> right and left bundle branches -> Purkinje fibres -> ventricular musculature.

The atrio-ventricular bundle is a bundle of nodal fibres continuing from the AVN. It passes through the atrio-ventricular septa to emerge on the top of the inter-ventricular septum and immediately divides into a right and a left bundle.
These branches give rise to minute fibres throughout the ventricular musculature of the respective sides, and these are called Purkinje fibres.

The nodal musculature has the ability to generate action potentials without any external stimuli, that is, it is autoexcitable. The SAN can generate the maximum number of action potentials, 70-75 per minute, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore it is called the pacemaker. Our heart normally beats 70-75 times a minute, average 72 beats per minute.

Question 18

Q. List every number attached to the cardiac cycle and to blood pressure, with the name each one belongs to.

Answer.	Number	What it belongs to
70-75 action potentials per minute	the maximum the SAN can generate, which is why it is the pacemaker
70-75 beats a minute, average 72	the normal human heart rate, and therefore 72 cardiac cycles per minute
0.8 seconds	the duration of one cardiac cycle, deduced from 72 beats a minute
about 30 per cent	the increase in the flow of blood into the ventricles produced by atrial systole
approximately 70 mL	the stroke volume - the blood pumped out by each ventricle during one cardiac cycle
5000 mL or 5 litres per minute	the average cardiac output of a healthy individual
120/80	normal blood pressure; 120 is the systolic or pumping pressure, 80 is the diastolic or resting pressure, both in millimetres of mercury
140/90 or higher on repeated checks	hypertension

Cardiac output is the volume of blood pumped out by each ventricle per minute, and it equals the stroke volume multiplied by the heart rate. The body can alter both the stroke volume and the heart rate, and thereby the cardiac output - which is why the cardiac output of an athlete is much higher than that of an ordinary man.

Question 19

Q. Name the three waves of an ECG with what each represents, and name the two heart sounds with what each is caused by.

Answer. An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle, recorded by an electro-cardiograph. Each peak is identified with a letter from P to T.

Wave	What it represents	What it leads to
P-wave	the electrical excitation, or depolarisation, of the ATRIA	the contraction of both the atria
QRS complex	the DEPOLARISATION of the VENTRICLES	ventricular contraction, which starts shortly after Q and marks the BEGINNING of systole
T-wave	the return of the VENTRICLES from the excited to the normal state - REPOLARISATION	its end marks the END of systole

The two sounds, each produced by a valve closing:

The first heart sound, lub, is associated with the closure of the tricuspid and bicuspid valves.
The second heart sound, dub, is associated with the closure of the semilunar valves.
These sounds are of clinical diagnostic significance.

Two pairings, both routinely swapped: P is atrial DEPOLARISATION while T is ventricular REPOLARISATION, and lub goes with the tricuspid and bicuspid valves while dub goes with the semilunar valves.

Tier 2 - Applied Reasoning

Question 20

Q. Match Column I with Column II. Column I: (a) Eosinophils, (b) RBC, (c) AB Group, (d) Platelets, (e) Systole. Column II: (i) Coagulation, (ii) Universal Recipient, (iii) Resist Infections, (iv) Contraction of Heart, (v) Gas transport. This is one of the chapter-end exercises.

Answer. The matched pairs are (a)-(iii), (b)-(v), (c)-(ii), (d)-(i) and (e)-(iv).

Column I	Column II	Why they go together
(a) Eosinophils	(iii) Resist Infections	Eosinophils, 2-3 per cent of the WBCs, resist infections and are also associated with allergic reactions. They are not the phagocytes - those are the neutrophils and monocytes.
(b) RBC	(v) Gas transport	The RBC carries haemoglobin, a red coloured, iron containing complex protein, and these molecules play a significant role in the transport of respiratory gases.
(c) AB Group	(ii) Universal Recipient	A person of group AB has no antibody in the plasma, so can accept blood from persons with AB as well as the other groups. The universal DONOR is group O, which is the trap in this row.
(d) Platelets	(i) Coagulation	Platelets can release a variety of substances, most of which are involved in the coagulation or clotting of blood, and a fall in their number leads to clotting disorders.
(e) Systole	(iv) Contraction of Heart	Systole is contraction; diastole is relaxation. Atrial systole is the contraction of the atria, ventricular systole the contraction of the ventricles.

Work an item like this by elimination on the two rows you are surest of. RBC to gas transport and platelets to coagulation are the two nobody misses, and once those two are gone, eosinophils can only take "resist infections" and AB can only take "universal recipient", leaving systole with "contraction of heart".

The one deliberate trap is (c). AB is the recipient, O is the donor - if you write "universal donor" against AB, the whole row is lost.

Question 21

Q. An athlete at rest has a stroke volume of 100 mL and a heart rate of 50 beats per minute. In hard exercise his stroke volume rises to 150 mL and his heart rate to 180 beats per minute. Work out his cardiac output in each state, and compare both with an ordinary man whose cardiac output is 5 litres per minute. Then find the stroke volume of a person whose cardiac output is 5 litres per minute at a heart rate of 80.

Answer. Write the formula down first, every time. Cardiac output = stroke volume multiplied by heart rate, and cardiac output is the volume of blood pumped out by each ventricle per minute.

At rest. Cardiac output = 100 mL multiplied by 50 = 5000 mL per minute = 5 litres per minute.

In hard exercise. Cardiac output = 150 mL multiplied by 180 = 27000 mL per minute = 27 litres per minute.

Working backwards for the third person. Stroke volume = cardiac output divided by heart rate = 5000 divided by 80 = 62.5 mL.

Person and state	Stroke volume	Heart rate	Cardiac output
Athlete at rest	100 mL	50 per minute	5000 mL, or 5 litres per minute
Athlete in hard exercise	150 mL	180 per minute	27000 mL, or 27 litres per minute
Ordinary healthy man	approximately 70 mL	72 per minute	about 5000 mL, or 5 litres per minute
Third person	62.5 mL	80 per minute	5000 mL, or 5 litres per minute

Read the table across and two teaching points fall out.

First, the athlete at rest delivers exactly the same 5 litres a minute as an ordinary man, but does it with a much larger stroke volume and a much slower heart. His heart does the same work in fewer, bigger beats.

Second, the reason the cardiac output of an athlete will be much higher than that of an ordinary man is that the body has the ability to alter the stroke volume as well as the heart rate, and thereby the cardiac output. In exercise both factors rise together, and because they multiply, the output rises more than five-fold.

Never quote 5 litres as if it were a fixed constant. It is an average for a healthy individual at rest, and it is the product of two numbers that both move.

Question 22

Q. Work out the duration of one cardiac cycle for a heart rate of 75 beats per minute, for the chapter's average of 72, for a patient at 100 beats per minute and for a resting athlete at 50. Say in each case what has been squeezed or stretched.

Answer. One heart beat is one cardiac cycle, so the duration of one cycle = 60 seconds divided by the heart rate.

Heart rate	Arithmetic	Duration of one cardiac cycle
75 beats per minute	60 divided by 75	0.8 seconds
72 beats per minute (the chapter's average)	60 divided by 72	about 0.83 seconds, which is why the chapter gives the duration as 0.8 seconds
100 beats per minute	60 divided by 100	0.6 seconds
50 beats per minute	60 divided by 50	1.2 seconds

Where the chapter's 0.8 seconds comes from: the heart beats 72 times per minute, that is, 72 cardiac cycles are performed per minute, and from this it is deduced that the duration of a cardiac cycle is 0.8 seconds. The number is not measured separately - it is derived from the heart rate, and that is exactly how a paper wants it justified.

What gets squeezed. The cycle runs joint diastole, atrial systole, ventricular systole, ventricular diastole and back to joint diastole. When the rate rises to 100, the whole cycle shortens to 0.6 seconds, and it is mainly the relaxation phase, the diastole, that is cut. That matters because the ventricles fill during diastole. A very fast heart has less filling time, so the stroke volume can actually fall.

In the resting athlete at 50 the cycle stretches to 1.2 seconds, giving a long diastole and a large, unhurried filling - which is why his stroke volume is high.

Question 23

Q. A patient's total white blood cell count is reported as 8000 per cubic millimetre. Work out the number of each of the five white cells that this total should contain, and say what you would suspect if the eosinophil count came back at 800.

Answer. Take each cell's printed percentage range and apply it to the total of 8000 per cubic millimetre.

Cell	Share of total WBCs	Arithmetic	Expected number per cubic millimetre
Neutrophils	60-65 per cent	60 hundredths of 8000 to 65 hundredths of 8000	4800 to 5200
Lymphocytes	20-25 per cent	20 hundredths of 8000 to 25 hundredths of 8000	1600 to 2000
Monocytes	6-8 per cent	6 hundredths of 8000 to 8 hundredths of 8000	480 to 640
Eosinophils	2-3 per cent	2 hundredths of 8000 to 3 hundredths of 8000	160 to 240
Basophils	0.5-1 per cent	half of one hundredth of 8000 to one hundredth of 8000	40 to 80

Check the answer the way you would check any percentage sum. The five lower limits add to 7080 and the five upper limits add to 8160. The true total, 8000, sits between them, as it must - the five printed ranges are ranges, so only a set of values chosen from inside them will add to exactly 100 per cent. Do not expect the lower limits to add to 8000; if they did, the ranges would not be ranges.

The last part. An eosinophil count of 800 out of 8000 is 10 per cent, against an expected 2-3 per cent - well over three times the top of the normal range. Eosinophils resist infections and are also associated with allergic reactions, so a raised eosinophil count points to an allergic condition or a parasitic infection.

The lesson of the sum is that a differential count is read as a proportion, not as a raw number. A count of 800 means nothing until you know the total it came out of.

Question 24

Q. A patient of blood group B needs an urgent transfusion. A donor of group A steps forward. Say whether the transfusion can be given, what would happen if it were, and list every group this patient can safely receive from.

Answer. The transfusion must not be given.

Work it from the recipient's plasma. A person of group B carries the anti-A antibody in the plasma. A donor of group A carries antigen A on the surface of the RBCs. Put them together and the recipient's anti-A antibody meets the donor's antigen A, giving severe problems of clumping, that is, destruction of the RBCs. This is exactly why the blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion.

A group B patient can receive from group B and from group O only.

Offered donor	Antigen the donor's cells carry	Meets recipient's anti-A?	Safe?
B	B	no	yes
O	none	no	yes
A	A	yes	no
AB	A and B	yes	no

And the Rh group must be matched as well. An Rh negative person exposed to Rh positive blood will form specific antibodies against the Rh antigens, so a transfusion that is correct on ABO can still be wrong on Rh. A complete answer names both groupings.

Question 25

Q. An Rh negative woman has already delivered one Rh positive child, safely. She is now carrying a second Rh positive child, and her doctor is worried. Explain why the first child was safe, why the second is at risk, and what should have been done.

Answer. The first child was safe because the two bloods never met while the pregnancy lasted. The Rh antigens of the foetus do not get exposed to the Rh negative blood of the mother in the first pregnancy, as the two bloods are well separated by the placenta. The mother therefore made no antibody during that pregnancy, and the baby was never attacked.

The exposure happened at the delivery, not during the pregnancy. During the delivery of the first child there is a possibility of exposure of the maternal blood to small amounts of the Rh positive blood from the foetus. In such cases the mother starts preparing antibodies against the Rh antigen in her blood. From that moment she is sensitised, and she stays sensitised.

The second child is at risk because the mother's antibodies now exist and can cross to the foetus. In her subsequent pregnancies the Rh antibodies from the Rh negative mother can leak into the blood of the Rh positive foetus and destroy the foetal RBCs. This could be fatal to the foetus, or could cause severe anaemia and jaundice to the baby. The condition is called erythroblastosis foetalis.

What should have been done. Anti-Rh antibodies should have been administered to the mother immediately after the delivery of the first child. Given in time, they clear the foetal cells from her circulation before her own immune system can respond, so she never becomes sensitised and the later pregnancies are safe.

The single idea worth carrying away: the damage is done at the FIRST DELIVERY and it is paid for in a LATER PREGNANCY. The treatment is therefore given at the moment of the first delivery, not when the second pregnancy is discovered.

Question 26

Q. A blood sample is drawn into a tube that contains a substance which binds up all the calcium ions in it. The sample sits for an hour and does not clot at all. Explain why, and say what this tells you about where the clotting factors normally sit.

Answer. The sample does not clot because $\mathrm{Ca^{2+}}$ has been removed, and calcium ions play a very important role in clotting.

Follow the cascade and you can see exactly what has been stopped. The factors for coagulation are present in the plasma in an inactive form, and they are turned on by a series of linked enzymic reactions - the cascade process - which builds the enzyme complex thrombokinase. That complex is what converts inactive prothrombin into thrombin, and thrombin is what converts inactive fibrinogen into fibrins. Calcium ions are required for the cascade to run. Take the calcium away and the very first stage never completes, so no thrombokinase is made, no thrombin is made, no fibrin threads are made, and there is no network in which the formed elements can be trapped. Without that network there is no clot or coagulam.

What the experiment tells you about the resting state of the blood. Every one of these factors is already present in the flowing plasma, but every one of them is present in an inactive state. That is the design of the system - the whole machinery is loaded and waiting, and only a trigger sets it off. An injury or trauma stimulates the platelets to release certain factors which activate the mechanism of coagulation, and certain factors released by the tissues at the site of injury can also initiate it.

If the factors circulated in their active forms, blood would clot inside intact vessels. It does not, because activation, not manufacture, is the step that is controlled - and calcium is one of the things that step needs.

Question 27

Q. Why is the wall of the left ventricle the thickest part of the heart, while the atria have the thinnest walls? Answer from what each chamber has to push against.

Answer. The rule is simple: the thickness of a chamber's wall matches the distance and the resistance the blood it pumps has to overcome. The entire heart is made of cardiac muscles, and the walls of the ventricles are much thicker than those of the atria.

Chamber	Where it sends blood	Distance and resistance	Wall
Atria	into the ventricle just below, through an open valve	almost none	thinnest
Right ventricle	into the pulmonary artery, to the lungs	a short circuit, to one pair of organs	thick
Left ventricle	into the aorta, to the whole body	the longest circuit, to every tissue	thickest

The atria hardly work at all. Blood from the pulmonary veins and the vena cava flows into the ventricles through the atria while all four chambers are relaxed, so the atria are largely a passage. Atrial systole adds only about 30 per cent to the flow already going in. A chamber that adds thirty per cent to a flow that is already happening needs very little muscle.

The ventricles have to generate real pressure, enough to force the semilunar valves open and drive blood out into the great arteries.

And of the two ventricles, the left has by far the harder job. The left ventricle pumps blood into the aorta for the systemic circulation, which carries it by arteries, arterioles and capillaries to every tissue of the body, while the right ventricle pumps into the pulmonary artery, which only has to reach the lungs. Both eject the same stroke volume of approximately 70 mL, but the left does it against a much longer, higher-resistance circuit - so it carries the thickest wall.

Note that the same logic explains a vessel wall too: the tunica media is comparatively thin in the veins, because veins carry blood back at low pressure.

Question 28

Q. In a patient the atrio-ventricular node is destroyed but the SAN is intact and firing normally. Predict what happens to the atria and to the ventricles, and use the answer to explain why the AVN and the AV bundle matter at all.

Answer. The atria will go on contracting normally; the ventricles will lose their signal.

Trace the pathway and the reason is obvious. The SAN generates an action potential which stimulates both the atria to undergo a simultaneous contraction, the atrial systole. That happens in the atrial muscle itself and needs no AVN, so the atria keep beating at the SAN's own rate of 70-75 per minute.

But the atria and the ventricles are separated by the atrio-ventricular septum, a thick fibrous tissue, and fibrous tissue does not conduct. The only route across it is the AVN and the AV bundle: the action potential is conducted to the ventricular side by the AVN and the AV bundle, from where the bundle of His transmits it through the entire ventricular musculature. Destroy the AVN and that single bridge is gone. The action potential cannot reach the ventricles, so the ventricles no longer contract in step with the atria.

The heart does not simply stop, and that is the interesting part. The nodal musculature has the ability to generate action potentials without any external stimuli - it is autoexcitable - but the number of action potentials that could be generated in a minute varies at different parts of the nodal system, and the SAN can generate the maximum. Left to itself, the ventricular tissue beats at its own slower rate, out of step with the atria, so the pumping becomes disorganised and inefficient.

So the significance of the AVN and the AV bundle is that they are the only electrical connection between the atria and the ventricles, and they hand the impulse over in the right order - atria first, ventricles after - so that the ventricles are filled before they are asked to contract. Without that ordering the heart would be a set of chambers contracting at random.

Question 29

Q. An ECG trace shows a normal flat baseline with regular P-waves, but many of the P-waves are not followed by a QRS complex or a T-wave. Which part of the heart is working, which part is failing, and where is the fault?

Answer. The atria are working. The ventricles are not being reached. The fault is in the conduction between them.

Read the trace wave by wave.

The P-wave represents the electrical excitation, or depolarisation, of the atria, which leads to the contraction of both the atria. Regular P-waves therefore mean the SAN is firing normally and the atrial muscle is responding normally.
The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. A missing QRS means the ventricular muscle was never depolarised.
The T-wave represents the return of the ventricles from the excited to the normal state, that is, repolarisation. If the ventricle never depolarised, there is nothing to repolarise, so the T-wave is missing too. The T-wave goes missing WITH the QRS, never on its own - that pairing is itself worth stating.

So the signal is being generated and is crossing the atria, but on many beats it is not crossing into the ventricles. The route it must take is AVN, then AV bundle, then bundle of His, then the right and left bundle branches, then the Purkinje fibres, and the fault lies somewhere along it.

Why the trace is of any use at all: since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence it is of great clinical significance. A P-wave with no QRS after it is a deviation of exactly that kind.

One more reading is available from the same trace. By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual - and here the QRS count, not the P count, gives the rate at which the ventricles are actually pumping blood, which is the rate that matters to the patient.

Question 30

Q. A frog and a pigeon are the same size and both are active. Explain why the pigeon's tissues receive blood that carries more oxygen, using the structure of each heart.

Answer. The difference is one wall - the inter-ventricular septum - and everything else follows from it.

The frog is an amphibian, and amphibians and reptiles, except crocodiles, have a 3-chambered heart with two atria and a single ventricle. The left atrium receives oxygenated blood from the gills, lungs or skin, and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle, which pumps out mixed blood. This is incomplete double circulation. The frog's tissues therefore receive blood that is part oxygenated and part deoxygenated, and no amount of good breathing can change that, because the mixing happens after the lungs have done their work.

The pigeon is a bird, and crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. In birds and mammals the oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up - two separate circulatory pathways are present, hence these animals have double circulation.

Feature	Frog	Pigeon
Chambers	3 - two atria, one ventricle	4 - two atria, two ventricles
Where the two bloods meet	in the single ventricle	nowhere; they are kept apart
Blood sent to the tissues	mixed blood	fully oxygenated blood
Name of the pattern	incomplete double circulation	double circulation

What the separation buys the bird is a higher and steadier oxygen delivery per unit of blood pumped, which is what a warm, constantly active animal needs.

Keep the word "incomplete" attached to its reason. It is not incomplete because there is one circuit - there are two circuits in the frog as well. It is incomplete because the single ventricle lets the two bloods mix.

Question 31

Q. A man's blood pressure is measured on three separate occasions and reads 145/95 each time. Say what each number means, what the readings show, and why it matters.

Answer. Read the two numbers first. In a measurement such as 120/80, the first figure is the systolic, or pumping, pressure and the second is the diastolic, or resting, pressure, both in millimetres of mercury. So 145 is this man's systolic pressure and 95 is his diastolic pressure.

Compare with the printed standard. Normal blood pressure is 120/80. If repeated checks of the blood pressure of an individual are 140/90 or higher, it shows hypertension. This man is at 145/95 on repeated checks, which is above 140/90 on both figures, so he has hypertension - the term for blood pressure that is higher than normal.

The word "repeated" is doing real work here and is often the point of the question. A single high reading is not a diagnosis; the chapter deliberately says repeated checks, and this man has had three.

Why it matters. High blood pressure leads to heart diseases and also affects vital organs like the brain and the kidney. It is dangerous precisely because it produces no sensation of its own - the damage accumulates in the heart and in those organs while the person feels nothing, and it is found only when someone measures it.

Do not confuse hypertension with the other three disorders in this chapter. Hypertension is a pressure that is too high. Coronary Artery Disease is a narrowing of the vessels supplying the heart muscle. Angina is a chest pain from too little oxygen reaching the heart muscle. Heart failure is a heart that is not pumping effectively enough to meet the needs of the body.

Tier 3 - Longer Written Answers

Question 32

Q. Write the differences between Blood and Lymph. This is one of the chapter-end exercises.

Answer. A note before the answer: the chapter sets this same comparison twice. It appears once as exercise 5, "What is the difference between lymph and blood?", and again as exercise 7(a), "Write the differences between Blood and Lymph". They are the same question, so one answer serves both - write this table for either of them, and do not go looking for a second, different answer.

Both are fluid connective tissues, and lymph is derived from blood, which is why they resemble each other in so much. The differences all come from one event: what the capillary wall lets through. As blood passes through the capillaries in the tissues, some water along with many small water soluble substances moves out into the spaces between the cells, leaving the larger proteins and most of the formed elements in the blood vessels.

Feature	Blood	Lymph
Colour	red, because of the haemoglobin in its RBCs	colourless, because it has no haemoglobin containing RBCs
Cells present	RBCs, all five types of WBC, and platelets	only specialised lymphocytes; no RBCs and no platelets
Proteins	more; the plasma carries the larger proteins - fibrinogen, globulins and albumins	fewer, because the larger proteins are left behind in the blood vessels
What it carries	respiratory gases, nutrients, hormones and wastes	nutrients and hormones, and above all the fats absorbed in the lacteals of the intestinal villi
Direction of flow	circulates in a closed circuit - out from the heart and back to it	flows one way only, from the tissue spaces through the lymphatic vessels back to the major veins
Function	transport of substances to and from the cells, and defence	returns the tissue fluid to the blood, absorbs fats, and carries out the immune responses of the body

Three of those rows are one fact seen three times. The capillary wall holds back the large proteins and the formed elements. That is why lymph has no RBCs and so is colourless, why it has fewer proteins, and why the only cells in it are lymphocytes - and the lymphocytes are there because they move in by themselves, not because they leaked out.

Two lines are the marking-scheme lines, and neither should be left out. Lymph contains specialised lymphocytes which are responsible for the immune responses of the body, and fats are absorbed through lymph in the lacteals present in the intestinal villi.

The classic error is to write that lymph has no cells at all. It has lymphocytes. What it lacks is RBCs and platelets.

Question 33

Q. Describe the composition of blood in full - the matrix, the formed elements, and every figure attached to each.

Answer. Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. Plasma constitutes nearly 55 per cent of the blood and the formed elements nearly 45 per cent.

The plasma. A straw coloured, viscous fluid.

90-92 per cent of plasma is water.
Proteins contribute 6-8 per cent of it. Fibrinogen, globulins and albumins are the major proteins. Fibrinogens are needed for clotting or coagulation of blood; globulins are primarily involved in the defence mechanisms of the body; the albumins help in osmotic balance.
Small amounts of minerals such as $\mathrm{Na^+}$ , $\mathrm{Ca^{2+}}$ , $\mathrm{Mg^{2+}}$ , $\mathrm{HCO_3^-}$ and $\mathrm{Cl^-}$ .
Glucose, amino acids and lipids, present because they are always in transit in the body.
The factors for coagulation, present in an inactive form. Plasma without the clotting factors is called serum.

The formed elements. Erythrocytes, leucocytes and platelets are collectively called formed elements.

Element	Numbers	Structure	Main function
Erythrocytes (RBCs)	5 million to 5.5 million per cubic millimetre; 12-16 grams of haemoglobin per 100 mL of blood; life span 120 days	biconcave, devoid of a nucleus in most mammals, formed in the red bone marrow, destroyed in the spleen	transport of respiratory gases, through haemoglobin, a red coloured, iron containing complex protein
Leucocytes (WBCs)	6000-8000 per cubic millimetre	colourless due to the lack of haemoglobin, nucleated, generally short lived; granulocytes are neutrophils, eosinophils and basophils, agranulocytes are lymphocytes and monocytes	defence - see the five jobs below
Platelets (thrombocytes)	1,50,000 to 3,50,000 per cubic millimetre	cell fragments produced from megakaryocytes, special cells in the bone marrow	release substances most of which are involved in the coagulation or clotting of blood

The five white cells with their shares and their jobs.

Neutrophils, 60-65 per cent, the most abundant - phagocytic, destroying foreign organisms entering the body.
Lymphocytes, 20-25 per cent - of two major types, B and T, both responsible for the immune responses of the body.
Monocytes, 6-8 per cent - phagocytic.
Eosinophils, 2-3 per cent - resist infections and are associated with allergic reactions.
Basophils, 0.5-1 per cent, the least abundant - secrete histamine, serotonin and heparin, and are involved in inflammatory reactions.

A reduction in the number of platelets can lead to clotting disorders which will lead to excessive loss of blood from the body.

Question 34

Q. Give a full account of ABO grouping - what it is based on, the table, the logic behind the table, and why matching matters before a transfusion.

Answer. Blood of human beings differs in certain aspects, though it appears to be similar. Two groupings, the ABO and the Rh, are widely used all over the world.

What ABO grouping is based on. It is based on the presence or absence of two surface antigens on the RBCs, namely A and B. Similarly, the plasma of different individuals contains two natural antibodies.

An antigen is a chemical that can induce an immune response.
An antibody is a protein produced in response to an antigen.

The four groups.

Blood group	Antigens on RBCs	Antibodies in plasma	Donor's group
A	A	anti-B	A, O
B	B	anti-A	B, O
AB	A, B	nil	AB, A, B, O
O	nil	anti-A, anti-B	O

The logic that generates the whole table. Your plasma carries the antibody against the antigen your own red cells do not have. Once that line is fixed, every row can be rebuilt from memory of nothing but the group's name.

The two special groups.

Group O blood can be donated to persons with any other blood group, and hence O group individuals are called universal donors. The O cell carries no antigen, so no recipient's antibody has anything to attack.
Persons with AB group can accept blood from persons with AB as well as the other groups of blood, and are therefore called universal recipients. AB plasma carries no antibody, so no incoming cell is attacked.

Why matching matters. During blood transfusion any blood cannot be used. The blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion, to avoid severe problems of clumping, that is, destruction of the RBCs.

And the ABO group is not the whole test. Another antigen, the Rh antigen, similar to one present in Rhesus monkeys, is observed on the surface of the RBCs of the majority, nearly 80 per cent, of humans; such individuals are called Rh positive and those in whom this antigen is absent are called Rh negative. An Rh negative person, if exposed to Rh positive blood, will form specific antibodies against the Rh antigens, and therefore the Rh group should also be matched before transfusions.

Give the direction of each rule when you write this out. Donation is limited by the DONOR'S ANTIGEN; reception is limited by the RECIPIENT'S ANTIBODY. Answers that state the rule without its direction lose the marks for O and AB.

Question 35

Q. Explain erythroblastosis foetalis in full - the setting, the sequence, the outcome and the prevention.

Answer. Erythroblastosis foetalis is a special case of Rh incompatibility, or mismatching, observed between the Rh negative blood of a pregnant mother and the Rh positive blood of the foetus.

The setting. The Rh antigen is present on the RBCs of nearly 80 per cent of humans, who are Rh positive; those without it are Rh negative. An Rh negative person, if exposed to Rh positive blood, will form specific antibodies against the Rh antigens. The situation arises when an Rh negative mother carries an Rh positive foetus.

The sequence, step by step.

In the first pregnancy the Rh antigens of the foetus do not get exposed to the Rh negative blood of the mother, as the two bloods are well separated by the placenta. The first child is therefore not harmed.
During the delivery of the first child, however, there is a possibility of exposure of the maternal blood to small amounts of the Rh positive blood from the foetus.
In such cases the mother starts preparing antibodies against the Rh antigen in her blood. She is now sensitised, permanently.
In the case of her subsequent pregnancies, the Rh antibodies from the mother, who is Rh negative, can leak into the blood of the foetus, which is Rh positive.
These antibodies destroy the foetal RBCs.

The outcome. This could be fatal to the foetus, or could cause severe anaemia and jaundice to the baby. The anaemia follows directly from the destruction of the red cells, and the jaundice from the breakdown products released as they are destroyed.

The prevention. This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child. Given at that moment, they remove the foetal Rh positive cells from her circulation before her own immune system has had time to respond, so she never makes her own antibodies and her later pregnancies are safe.

Three points decide the marks on this question.

The placenta is what protects the first pregnancy - it keeps the two bloods well separated.
The exposure happens at DELIVERY, not during the pregnancy.
The treatment is timed to the FIRST delivery, not to the second pregnancy. By the time the second pregnancy exists, the antibodies already do too.

Question 36

Q. Describe the coagulation of blood in full - what a clot is, the cascade, what starts it, and the part played by calcium.

Answer. Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body.

What the clot is. A dark reddish brown scum forms at the site of a cut or an injury over a period of time. It is a clot or coagulam, formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped. The fibrin threads are the net; the trapped formed elements are what makes the clot dark and solid.

The cascade, in the order the events actually happen.

A number of factors are present in the plasma in an inactive state. A series of linked enzymic reactions - the cascade process - converts them, and an enzyme complex, thrombokinase, is formed.
Thrombokinase acts on prothrombin, another inactive substance present in the plasma, and converts it to thrombin.
Thrombin, an enzyme, converts the inactive fibrinogens in the plasma into fibrins.
The fibrins form a network in which the dead and damaged formed elements are trapped - the clot.

Inactive form	Converted by	Active product
plasma factors	the cascade process	thrombokinase, an enzyme complex
prothrombin	thrombokinase	thrombin
fibrinogen	thrombin	fibrins

What starts it. An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation. Certain factors released by the tissues at the site of the injury can also initiate coagulation. Both routes must be named - platelet factors and tissue factors.

The part played by calcium. Calcium ions, $\mathrm{Ca^{2+}}$ , play a very important role in clotting. Remove them from a sample of blood and the cascade cannot run, so no thrombokinase, no thrombin, no fibrin and no clot.

Why the factors are kept inactive. They circulate in the plasma in an inactive form so that blood does not clot while it is flowing normally through intact vessels. The whole system is loaded and waiting; the injury is the trigger, not the manufacturer.

Watch the endings when you write this out. Fibrinogen is inactive and soluble; fibrin is the active thread. Prothrombin is inactive; thrombin is the active enzyme. One syllable separates each pair, and a paper will hand you the wrong one on purpose.

Question 37

Q. Describe the structure of the human heart in full - where it lies, its covering, its chambers, its septa, its valves and its muscle.

Answer. The human circulatory system, also called the blood vascular system, consists of a muscular chambered heart, a network of closed branching blood vessels, and blood, the fluid which is circulated. Name all three when the system is asked for.

Where the heart lies. The heart is a mesodermally derived organ, situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist.

Its covering. It is protected by a double walled membranous bag, the pericardium, enclosing the pericardial fluid.

The chambers. The heart has four chambers - two relatively small upper chambers called atria and two larger lower chambers called ventricles.

The three septa.

Septum	What it separates	What it is made of
Inter-atrial septum	the right and the left atria	a thin, muscular wall
Inter-ventricular septum	the left and the right ventricles	thick-walled
Atrio-ventricular septum	the atrium and the ventricle of the same side	a thick fibrous tissue

However, each of these septa is provided with an opening through which the two chambers of the same side are connected, and each of those openings is guarded by a valve.

The valves, with their sides.

Valve	Opening it guards	Detail
Tricuspid valve	between the right atrium and the right ventricle	formed of three muscular flaps or cusps
Bicuspid or mitral valve	between the left atrium and the left ventricle	two cusps, hence bicuspid
Semilunar valves	the openings of the right and the left ventricles into the pulmonary artery and the aorta respectively	one on each side

What the valves are for. The valves in the heart allow the flow of blood only in one direction, that is, from the atria to the ventricles and from the ventricles to the pulmonary artery or the aorta. These valves prevent any backward flow.

The muscle. The entire heart is made of cardiac muscles, and the walls of the ventricles are much thicker than those of the atria, because the ventricles have to drive blood out into the great arteries while the atria only pass it down into the ventricles below them.

Say the two valve names with their sides attached, every time: tricuspid on the RIGHT, bicuspid or mitral on the LEFT. The three-cusp valve on the three-letter side is a workable memory hook, and the pair is asked in almost every paper.

Question 38

Q. Describe one complete cardiac cycle from joint diastole back to joint diastole, naming the valve movements at each stage, and give every number attached to the cycle.

Answer. The cardiac cycle is the sequential event in the heart which is cyclically repeated, and it consists of the systole and the diastole of both the atria and the ventricles.

Stage 1 - joint diastole. All four chambers of the heart are in a relaxed state. The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and the vena cava flows into the left and the right ventricle respectively, through the left and right atria. The semilunar valves are closed at this stage. Filling therefore happens without any chamber contracting at all, which is a point students often miss.

Stage 2 - atrial systole. The SAN generates an action potential which stimulates both the atria to undergo a simultaneous contraction - the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent. The atria are topping up a ventricle that is already nearly full.

Stage 3 - ventricular systole. The action potential is conducted to the ventricular side by the AVN and the AV bundle, from where the bundle of His transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract - ventricular systole - while the atria undergo relaxation, diastole, coinciding with the ventricular systole.

Ventricular systole increases the ventricular pressure, causing the closure of the tricuspid and bicuspid valves due to the attempted backflow of blood into the atria. That closure is the first heart sound, lub.
As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery on the right side and the aorta on the left side are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.

Stage 4 - ventricular diastole. The ventricles now relax and the ventricular pressure falls, causing the closure of the semilunar valves, which prevents the backflow of blood into the ventricles. That closure is the second heart sound, dub.

Stage 5 - back to joint diastole. As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria, exerted by the blood which was being emptied into them by the veins. The blood now once again moves freely into the ventricles, and the ventricles and atria are again in a relaxed, joint diastole state. Soon the SAN generates a new action potential and the events are repeated in the same sequence.

The numbers of the cycle.

The heart beats 72 times per minute, so 72 cardiac cycles are performed per minute.
From this it is deduced that the duration of a cardiac cycle is 0.8 seconds.
Atrial systole increases the flow of blood into the ventricles by about 30 per cent.
During a cardiac cycle each ventricle pumps out approximately 70 mL of blood, which is called the stroke volume.
The stroke volume multiplied by the heart rate gives the cardiac output, so cardiac output is the volume of blood pumped out by each ventricle per minute, and it averages 5000 mL or 5 litres in a healthy individual.
The body has the ability to alter the stroke volume as well as the heart rate, and thereby the cardiac output; the cardiac output of an athlete will be much higher than that of an ordinary man.

Every valve movement in this cycle is caused by a pressure difference, never by the valve itself. Say "the pressure rose, so the valve shut" and the whole cycle writes itself.

Question 39

Q. Set the electrical events of the heart against the mechanical events - line up each ECG wave with what the muscle is doing and with the heart sound that belongs there.

Answer. An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. The mechanical events - contraction, relaxation and the slamming of valves - follow the electrical ones, always a little behind. Laying the two side by side is the surest way to stop confusing them.

ECG feature	The electrical event	The mechanical event that follows	Sound
P-wave	the electrical excitation, or depolarisation, of the ATRIA	the contraction of both the atria - atrial systole, which increases the flow of blood into the ventricles by about 30 per cent	none
QRS complex	the DEPOLARISATION of the VENTRICLES	ventricular contraction, which starts shortly after Q and marks the beginning of the systole; the rising pressure closes the tricuspid and bicuspid valves	lub, the first heart sound - the closure of the tricuspid and bicuspid valves
the stretch after QRS, before T	the ventricles remain depolarised	the semilunar valves are forced open and blood is ejected into the pulmonary artery and the aorta	none
T-wave	the return of the VENTRICLES from the excited to the normal state - REPOLARISATION; its end marks the end of systole	ventricular diastole; the falling pressure closes the semilunar valves	dub, the second heart sound - the closure of the semilunar valves

Three things fall straight out of the table, and each is asked on its own.

First, atrial repolarisation has no wave of its own on a standard ECG. The trace names atrial depolarisation (P), ventricular depolarisation (QRS) and ventricular repolarisation (T) - the fourth member of the set is simply not there to be seen, being buried under the much larger QRS.

Second, both heart sounds are valve closures, and neither is a contraction. Lub is the tricuspid and bicuspid valves closing; dub is the semilunar valves closing. These sounds are of clinical diagnostic significance.

Third, the QRS complex is the timekeeper. By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. The count is taken from the QRS and not from the P, because the QRS is what tells you the ventricles actually pumped.

The whole point of recording it: since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence it is of great clinical significance.

Question 40

Q. Give a full account of the blood vessels and of double circulation - the three layers of a vessel, both circuits traced in order, and the two special vascular routes.

Answer. The blood flows strictly by a fixed route through the blood vessels - the arteries and the veins.

The three layers, from the inside out.

Layer	What it is made of
Tunica intima	an inner lining of squamous endothelium
Tunica media	a middle layer of smooth muscle and elastic fibres
Tunica externa	an external layer of fibrous connective tissue with collagen fibres

The tunica media is comparatively thin in the veins, which fits the job - veins return blood at low pressure and need less muscle in the wall.

Pulmonary circulation, traced in order. The blood pumped by the right ventricle enters the pulmonary artery. The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs, from where the oxygenated blood is carried by the pulmonary veins into the left atrium.

Right ventricle -> pulmonary artery -> lungs -> pulmonary veins -> left atrium.

Systemic circulation, traced in order. The left ventricle pumps blood into the aorta. The oxygenated blood entering the aorta is carried by a network of arteries, arterioles and capillaries to the tissues, from where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium.

Left ventricle -> aorta -> arteries -> arterioles -> capillaries -> tissues -> venules -> veins -> vena cava -> right atrium.

What the systemic circuit does. It provides nutrients, $\mathrm{O_2}$ and other essential substances to the tissues, and takes $\mathrm{CO_2}$ and other harmful substances away for elimination.

Why it is called double circulation, and what it is worth. Two circulatory pathways, the pulmonary and the systemic, are present, so blood passes through the heart twice in one complete round of the body. Its significance is that the oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides, and the ventricles pump it out without any mixing up. The tissues therefore receive fully oxygenated blood, which is what birds and mammals have and what an amphibian, with its single ventricle pumping out mixed blood, does not.

The one vessel that breaks the naming rule. The pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood. An artery is defined by carrying blood AWAY from the heart, not by what is in it.

The two special routes.

The hepatic portal system is a unique vascular connection between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation. Absorbed food is therefore processed by the liver before it reaches the rest of the body.
A special coronary system of blood vessels is present exclusively for the circulation of blood to and from the cardiac musculature. The heart is full of blood, but that blood is inside the chambers and cannot feed the thick muscle of the wall, so the wall needs a supply of its own. That is why a narrowing of these vessels - Coronary Artery Disease - starves the heart muscle itself.

Question 41

Q. How is the activity of the heart regulated? Explain why the heart is called myogenic, and give the neural and hormonal controls that act on it.

Answer. The heart is called myogenic because its beat originates in its own muscle and not in any nerve. The normal activities of the heart are regulated intrinsically, that is, auto regulated by specialised muscles - the nodal tissue - hence the heart is called myogenic. The word itself says it: myo, muscle, and genic, originating from.

The evidence sits in the nodal tissue. A specialised cardiac musculature called the nodal tissue is distributed in the heart, and the nodal musculature has the ability to generate action potentials without any external stimuli, that is, it is autoexcitable. The SAN can generate the maximum number of action potentials, 70-75 per minute, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart; therefore it is called the pacemaker. Nothing outside the heart is needed to make it beat - the signal is made inside it, by muscle.

The nerves modify the beat; they do not create it. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system.

Control	What it does
Sympathetic nerves, part of the ANS	increase the rate of heart beat, the strength of ventricular contraction, and thereby the cardiac output
Parasympathetic nerves, another component of the ANS	decrease the rate of heart beat, the speed of conduction of the action potential, and thereby the cardiac output
Adrenal medullary hormones	can also increase the cardiac output

Note the word "moderate" carefully. The neural centre moderates an activity that is already going on. A heart cut off from all its nerves goes on beating; a heart whose SAN is destroyed does not. That contrast is the cleanest way to justify the word myogenic in an answer.

And notice that all three controls act on the same final quantity - the cardiac output - by changing the heart rate, the strength of contraction and hence the stroke volume, or both. Cardiac output is stroke volume multiplied by heart rate, so every regulator in the chapter works through one of those two numbers.

A useful contrast for a two-mark question: a myogenic heart, as in vertebrates, is started by its own nodal muscle, while a neurogenic heart is started by a nerve ganglion outside the muscle. Ours is myogenic.

Question 42

Q. Set the four disorders of the circulatory system side by side - what each is, what causes it, and what distinguishes it from the others.

Answer.	Disorder	What it is
High blood pressure (hypertension)	blood pressure that is higher than normal, which is 120/80	repeated checks reading 140/90 or higher show hypertension; it leads to heart diseases and also affects vital organs like the brain and the kidney
Coronary Artery Disease (CAD)	often referred to as atherosclerosis; it affects the vessels that supply blood to the heart muscle	caused by deposits of calcium, fat, cholesterol and fibrous tissues, which make the lumen of the arteries narrower
Angina, also called angina pectoris	a symptom of acute chest pain that appears when not enough oxygen is reaching the heart muscle	can occur in men and women of any age, but is more common among the middle-aged and elderly; it occurs due to conditions that affect the blood flow
Heart failure	the state of the heart when it is not pumping blood effectively enough to meet the needs of the body	sometimes called congestive heart failure, because congestion of the lungs is one of the main symptoms

Reading the blood pressure numbers. In a measurement of 120/80, the 120 millimetres of mercury is the systolic, or pumping, pressure and the 80 millimetres of mercury is the diastolic, or resting, pressure.

The three that get confused, and how to keep them apart. Heart failure is not the same as cardiac arrest, when the heart stops beating, or as a heart attack, when the heart muscle is suddenly damaged by an inadequate blood supply.

Heart failure - the heart is beating but pumping badly, over a long period.
Cardiac arrest - the heart has stopped beating altogether.
Heart attack - the muscle itself is suddenly damaged, by a supply that is not enough.

Notice how three of the four link back to one set of vessels. The coronary system exists exclusively for the circulation of blood to and from the cardiac musculature. CAD narrows those vessels, angina is the pain felt when they deliver too little oxygen, and a heart attack is the damage done when the supply through them fails. Hypertension stands slightly apart - it is a pressure problem rather than a supply problem, but it is what leads to heart diseases in the first place, so it sits at the head of the list.

Where Every Chapter-End Exercise Is Answered

This chapter has fourteen exercises at the end. Counted properly they come to seventeen questions, because exercise 7 has four parts, a, b, c and d, while every other exercise is a single part.

Fifteen of those seventeen parts are answered in full inside the twelve teaching sections of this chapter. The remaining two - exercise 3, the match-the-column item, and exercise 7(a), the comparison of blood with lymph - are answered in this section and nowhere else.

One warning about exercise 7(a) before you use the table. The chapter sets the same comparison twice - exercise 5 asks for the difference between lymph and blood, and exercise 7(a) asks for the differences between Blood and Lymph. They are one question wearing two numbers. It is answered once in Section 5 - Lymph (Tissue Fluid) and once here, and either answer serves for both exercises.

Attempt each exercise on paper first, then turn to the section named. The answer there is written out in full, with the wording a marking scheme is looking for.

Exercise	The question, in short	Section	Answered as
1	Name the formed elements and one major function of each	Section 2 - Formed Elements - RBC, WBC and Platelets	Question 1
2	The importance of plasma proteins	Section 1 - Blood and Plasma	Question 6
3	Match Column I with Column II	Section 13 - Solved Examples	Question 20
4	Why we consider blood a connective tissue	Section 1 - Blood and Plasma	Question 2
5	The difference between lymph and blood	Section 5 - Lymph (Tissue Fluid)	Question 10
6	What double circulation means, and its significance	Section 11 - Double Circulation and the Blood Vessels	Question 3
7 (a)	The differences between Blood and Lymph	Section 13 - Solved Examples	Question 32
7 (b)	The differences between the Open and Closed systems of circulation	Section 6 - Circulatory Pathways and the Vertebrate Heart	Question 5
7 (c)	The differences between Systole and Diastole	Section 9 - The Cardiac Cycle	Question 10
7 (d)	The differences between the P-wave and the T-wave	Section 10 - The Electrocardiogram	Question 8
8	The evolutionary change in the pattern of the heart among the vertebrates	Section 6 - Circulatory Pathways and the Vertebrate Heart	Question 14
9	Why we call our heart myogenic	Section 12 - Regulation of Cardiac Activity, and Disorders of the Circulatory System	Question 1
10	Why the sino-atrial node is called the pacemaker	Section 8 - The Nodal Tissue and the Conducting System	Question 11
11	The significance of the atrio-ventricular node and the atrio-ventricular bundle	Section 8 - The Nodal Tissue and the Conducting System	Question 12
12	Define a cardiac cycle and the cardiac output	Section 9 - The Cardiac Cycle	Question 11
13	Explain heart sounds	Section 9 - The Cardiac Cycle	Question 12
14	Draw a standard ECG and explain the different segments in it	Section 10 - The Electrocardiogram	Question 9

Read the table as a revision plan, because it tells you where the marks sit.

Six of the seventeen parts - exercises 9, 10, 11, 12, 13 and 7(c) - come out of the heart's own working, that is, the nodal tissue, the cardiac cycle and the heart sounds. Learn the conducting pathway in order, the five stages of one cycle with the valve movement at each, and the two sounds with the valves that make them, and more than a third of the exercise set is answered before you start.

Four more - exercises 6, 8, 14 and 7(b), (d) between them - are the comparison questions: open against closed circulation, the 2, 3 and 4 chambered hearts, the two circuits of double circulation, and the P-wave against the T-wave. Every one of those is best written as a table, and a table earns the marks faster than a paragraph does.

Three - exercises 1, 2 and 4 - are pure blood composition, and all three are answered inside the first two sections. The percentages are the whole of exercise 1 and exercise 2.

The remaining four are the odd ones out. Exercise 3 is the only matching item in the chapter and exercise 7(a) is a repeat of exercise 5, and both of those are answered here in this section. Exercise 5 itself sits in the lymph section, and exercise 14 asks you to draw, which is answered in the electrocardiogram section with the trace described in enough words that you can put it on paper.

One last thing worth knowing about the exercise set: it does not touch coagulation, the blood groups, or the disorders at all. Do not read that as permission to skip them. Those three topics carry a very large share of the objective questions on this chapter, which is why they have full teaching sections and a good many items in the tiers above.