Summary and Exam Tips

The Chapter in One Page

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. It counts as a connective tissue because it has a matrix, because it is mesodermal in origin and because it connects and serves every part of the body. Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood, and the formed elements make up nearly 45 per cent. Plasma is 90-92 per cent water and 6-8 per cent proteins. The major plasma proteins are fibrinogen, needed for the coagulation or clotting of blood; globulins, primarily involved in the defense mechanisms of the body; and albumins, which help in osmotic balance. Plasma also carries minerals such as sodium ions, calcium ions, magnesium ions, bicarbonate ions and chloride ions, and glucose, amino acids and lipids in transit. Plasma also contains the clotting factors, in an inactive form. Plasma without the clotting factors is called serum.

The formed elements are the erythrocytes, the leucocytes and the platelets. Erythrocytes or red blood cells are the most abundant of all the cells in blood. A healthy adult man has 5 million to 5.5 million RBCs per cubic millimetre of blood. They are formed in the red bone marrow in the adults, are devoid of a nucleus in most mammals and are biconcave in shape. They contain haemoglobin, a red coloured iron containing complex protein, and a healthy individual has 12-16 grams of haemoglobin in every 100 mL of blood. These molecules play a significant role in the transport of respiratory gases. RBCs have an average life span of 120 days, after which they are destroyed in the spleen, the graveyard of RBCs. Leucocytes or white blood cells are colourless due to the lack of haemoglobin, are nucleated, are relatively fewer at 6000-8000 per cubic millimetre of blood, and are generally short lived. They are of two main categories - granulocytes, that is neutrophils, eosinophils and basophils, and agranulocytes, that is lymphocytes and monocytes. Neutrophils are the most abundant cells, 60-65 per cent of the total WBCs, and along with the monocytes, 6-8 per cent, they are phagocytic and destroy foreign organisms entering the body. Basophils are the least, 0.5-1 per cent, and secrete histamine, serotonin and heparin, and are involved in inflammatory reactions. Eosinophils, 2-3 per cent, resist infections and are also associated with allergic reactions. Lymphocytes, 20-25 per cent, are of two major types, B and T forms, and both are responsible for immune responses of the body. Platelets, also called thrombocytes, are cell fragments produced by megakaryocytes, special cells in the bone marrow. Blood normally contains 1,50,000 to 3,50,000 platelets per cubic millimetre, and they can release a variety of substances most of which are involved in the coagulation or clotting of blood; a reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.

Two groupings of blood are widely used all over the world - the ABO and the Rh. ABO grouping is based on the presence or absence of two surface antigens, A and B, on the RBCs. An antigen is a chemical that can induce an immune response; an antibody is a protein produced in response to it. The antigens sit on the RBCs and the antibodies are carried in the plasma, and the one line that generates the whole table is that the antibody a person carries is directed against the antigen they do not have. Group O individuals are called universal donors and group AB individuals universal recipients. The blood of a donor has to be carefully matched with the blood of a recipient before it is transfused, to avoid severe problems of clumping, that is destruction of RBC. A second antigen, similar to one present in Rhesus monkeys, is observed on the surface of the RBCs of nearly 80 per cent of humans - they are Rh positive, and those who lack it are Rh negative. An Rh negative person exposed to Rh positive blood forms specific antibodies against the Rh antigens, so the Rh group must also be matched before a transfusion. A special case of Rh incompatibility is erythroblastosis foetalis, in which an Rh negative mother carries an Rh positive foetus. In the first pregnancy the Rh antigens of the foetus do not get exposed to the mother's blood, as the two are well separated by the placenta. 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, and the mother starts preparing antibodies against the Rh antigen. In her subsequent pregnancies the Rh antibodies from the mother can leak into the blood of the foetus and destroy the foetal RBCs, which could be fatal to the foetus or cause severe anaemia and jaundice to the baby. This condition is avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Blood exhibits coagulation or clotting in response to an injury or trauma, and this is a mechanism to prevent excessive loss of blood from the body. A clot or coagulum is formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped, and at the site of a cut it appears as a dark reddish brown scum. Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. Thrombins themselves are formed from another inactive substance present in the plasma called prothrombin. An enzyme complex, thrombokinase, is required for the conversion of prothrombin into thrombin. Thrombokinase is formed by a series of linked enzymic reactions, a cascade process, involving a number of factors present in the plasma in an inactive state. An injury or a trauma stimulates the platelets in the blood 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 coagulation. Calcium ions, $\mathrm{Ca^{2+}}$, play a very important role in clotting.

As blood passes through the capillaries in tissues, some water along with many small water soluble substances moves out into the spaces between the cells of the tissues, 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, and it has the same mineral distribution as that in plasma. Exchange of nutrients, gases and other substances 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 the lymph. Lymph is a colourless fluid containing specialised lymphocytes which are responsible for the immune responses of the body, and it is also an important carrier for nutrients, hormones and so on. Fats are absorbed through lymph in the lacteals present in the intestinal villi.

The circulatory patterns of animals are of two types, open or closed. Open circulatory system is present in arthropods and molluscs, in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Closed circulatory system is present in annelids and chordates, in which the blood pumped by the heart is always circulated through a closed network of blood vessels, and this pattern of blood circulation is more advantageous as the flow of fluid can be more precisely regulated. All vertebrates possess a muscular chambered heart. Fishes have a 2-chambered heart with an atrium and a ventricle. Amphibians and the reptiles, except crocodiles, have a 3-chambered heart with two atria and a single ventricle. Crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart - single circulation. In amphibians and reptiles, the left atrium receives oxygenated blood from the gills, lungs or skin and the right atrium gets the deoxygenated blood from other body parts, but they get mixed up in the single ventricle which pumps out mixed blood - incomplete double circulation. In birds and mammals the ventricles pump out blood without any mixing up, so two separate circulatory pathways are present - double circulation.

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. The heart, the mesodermally derived organ, is situated in the thoracic cavity in between the two lungs, slightly tilted to the left, and has the size of a clenched fist. It is protected by a double walled membranous bag, the pericardium, enclosing the pericardial fluid. It has four chambers - two relatively small upper chambers, the atria, and two larger lower chambers, the ventricles. A thin, muscular wall called the inter-atrial septum separates the right and the left atria, whereas a thick-walled inter-ventricular septum separates the left and the right ventricles. The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum, and each of these septa is provided with an opening through which the two chambers of the same side are connected. The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle. The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves. 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 the aorta, and prevent any backward flow. The entire heart is made of cardiac muscles, and the walls of the ventricles are much thicker than that of the atria.

A specialised cardiac musculature called the nodal tissue is also distributed in the heart. A patch of this tissue is present in the right upper corner of the right atrium, called the sino-atrial node (SAN). Another mass of this tissue is seen in the lower left corner of the right atrium, close to the atrio-ventricular septum, called the atrio-ventricular node (AVN). A bundle of nodal fibres, the atrio-ventricular bundle (AV bundle), continues from the AVN, which 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, called the Purkinje fibres, and these fibres along with the right and left bundles are known as the bundle of His. The nodal musculature has the ability to generate action potentials without any external stimuli, that is it is autoexcitable. The number of action potentials that could be generated in a minute varies at different parts of the nodal system. 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, and is therefore called the pacemaker. Our heart normally beats 70-75 times in a minute, with an average of 72 beats per minute.

The sequential event in the heart which is cyclically repeated is called the cardiac cycle, and it consists of the systole and diastole of both the atria and the ventricles. Begin from a state when all four chambers are in a relaxed state, that is joint diastole. The tricuspid and bicuspid valves are open and the semilunar valves are closed, and blood from the pulmonary veins and the vena cava flows into the left and the right atrium respectively, and then on into the ventricles through the open atrio-ventricular valves. The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction, the atrial systole, and this increases the flow of blood into the ventricles by about 30 per cent. 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, causing the ventricular systole, during which the atria undergo relaxation, the atrial diastole. The 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, and as the pressure rises 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. The ventricles now relax, the ventricular diastole, and the ventricular pressure falls, causing the closure of the semilunar valves which prevents the backflow of blood into the ventricles. 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, blood once again moves freely to the ventricles, and the heart is now ready for the next cycle. The heart beats 72 times per minute, so the same number of cardiac cycles are performed per minute, and the duration of one cardiac cycle is therefore 0.8 seconds. During one cardiac cycle each ventricle pumps out approximately 70 mL of blood, which is called the stroke volume. The volume of blood pumped out by each ventricle per minute is called the cardiac output, and it is the product of the stroke volume and the heart rate, averaging 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, and the cardiac output of an athlete will be much higher than that of an ordinary man. During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope - the first heart sound, lub, is associated with the closure of the tricuspid and bicuspid valves, and the second heart sound, dub, is associated with the closure of the semilunar valves. These sounds are of clinical diagnostic significance.

An electrocardiograph is used to obtain an electrocardiogram (ECG), which is a graphical representation of the electrical activity of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine with three electrical leads, one to each wrist and one to the left ankle, that continuously monitor the heart activity, and for a detailed evaluation of the heart's function, multiple leads are attached to the chest region. Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart. The P-wave represents the electrical excitation, or depolarisation, of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction, and the contraction starts shortly after Q and marks the beginning of the systole. The T-wave represents the return of the ventricles from the excited to the normal state, that is repolarisation, and the end of the T-wave marks the end of the systole. By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. 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, and hence it is of a great clinical significance.

The blood pumped by the heart is distributed through a network of arteries, veins and capillaries. Each artery and vein has three layers - the tunica intima, the inner lining of squamous endothelium; the tunica media, a middle layer of smooth muscle and elastic fibres, which is comparatively thin in the veins; and the tunica externa, an external layer of fibrous connective tissue with collagen fibres. The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta. 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, and this pathway constitutes the pulmonary circulation. 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, and this is the systemic circulation. The systemic circulation provides nutrients, oxygen and other essential substances to the tissues and takes carbon dioxide and other harmful substances away for elimination. A unique vascular connection exists between the digestive tract and the liver, called the hepatic portal system, and the hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation. A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.

The normal activities of the heart are regulated intrinsically, that is they are auto regulated by specialised muscles, the nodal tissue, and hence the heart is called myogenic. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system (ANS). Neural signals through the sympathetic nerves can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output. Parasympathetic neural signals 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.

Finally, the disorders. High blood pressure, that is hypertension, is the term for blood pressure that is higher than normal, which is 120/80 - 120 mm Hg is the systolic or pumping pressure and 80 mm Hg is the diastolic or resting pressure. If repeated checks of blood pressure of an individual is 140/90 or higher, it shows hypertension, and high blood pressure leads to heart diseases and also affects vital organs like the brain and the kidney. Coronary Artery Disease, often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle, and it is caused by deposits of calcium, fat, cholesterol and fibrous tissues, which makes the lumen of the arteries narrower. Angina, also called angina pectoris, is a symptom of acute chest pain that appears when not enough oxygen is reaching the heart muscle; it can occur in men and women of any age but is more common among the middle-aged and elderly, and it occurs due to conditions that affect the blood flow. Heart failure is the state of the heart when it is not pumping blood effectively enough to meet the needs of the body, and it is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms. Heart failure is not the same as cardiac arrest, when the heart stops beating, or a heart attack, when the heart muscle is suddenly damaged by an inadequate blood supply.

The Number Sheet

Every figure in the chapter, in one place. Learn them exactly as they are printed - a range stays a range, and the words nearly, about and approximately stay in front of the figures that carry them.

Blood and plasma

The three counts, and they are the three most swapped numbers in the chapter

The two red-cell figures. Haemoglobin: 12-16 grams in every 100 mL of blood in a healthy individual. Average life span of an RBC: 120 days, after which it is destroyed in the spleen.

The five white cells, in descending order of abundance - learn the order and the figures together

Blood groups. Nearly 80 per cent of humans carry the Rh antigen on their RBCs and are Rh positive; the rest are Rh negative.

The heart, the cycle and the pressure

The vertebrate hearts, as a number line

Three arithmetic checks worth carrying into the exam hall. One, the cycle duration is 60 divided by the heart rate - at 72 beats per minute that is 0.8 seconds, and at 60 beats per minute it is 1.0 second. Two, the cardiac output is the stroke volume multiplied by the heart rate - 70 multiplied by 72 gives about 5040 mL, which is the 5 litres the chapter quotes. Three, a differential count is a percentage of the total - with a total WBC count of 8000 per cubic millimetre, neutrophils at 60-65 per cent are 4800 to 5200, and basophils at 0.5-1 per cent are only 40 to 80.

The Address Sheet

One line per structure - what it is and exactly where it sits. Most of what this chapter asks beyond the numbers is an address, and the paper tests it by moving a structure to the other side.

• Inter-atrial septum - a thin, muscular wall that separates the right and the left atria. Thin.
• Inter-ventricular septum - a thick-walled septum that separates the left and the right ventricles. Thick.
• Atrio-ventricular septum - a thick fibrous tissue that separates the atrium and the ventricle of the same side. Fibrous, and it does not conduct. Each of the three septa has an opening through which the two chambers of that side are connected.
• 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 the left ventricles into the pulmonary artery and the aorta respectively. All the valves together allow the flow of blood in one direction only and prevent any backward flow.
• Pericardium - the double walled membranous bag that protects the heart, enclosing the pericardial fluid.
• Sino-atrial node (SAN) - a patch of nodal tissue in the RIGHT UPPER CORNER of the RIGHT ATRIUM; the pacemaker, generating 70-75 action potentials per minute.
• Atrio-ventricular node (AVN) - a mass of nodal tissue in the LOWER LEFT CORNER of the RIGHT ATRIUM, close to the atrio-ventricular septum. The words lower left name a corner of the right atrium, not the left atrium.
• Atrio-ventricular bundle (AV bundle) - 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.
• Purkinje fibres - minute fibres spread throughout the ventricular musculature of the respective sides, arising from the right and the left bundles; these fibres along with the right and left bundles are known as the bundle of His.
• Tunica intima - the innermost layer of an artery or a vein; an inner lining of squamous endothelium.
• Tunica media - the middle layer; smooth muscle and elastic fibres, and comparatively thin in the veins.
• Tunica externa - the outermost layer; fibrous connective tissue with collagen fibres.
• Pulmonary artery - carries the deoxygenated blood pumped by the RIGHT ventricle to the lungs. The one artery that carries deoxygenated blood.
• Pulmonary veins - carry the oxygenated blood from the lungs into the LEFT ATRIUM. The one veins that carry oxygenated blood.
• Hepatic portal vein - the vessel of the hepatic portal system, the unique vascular connection between the digestive tract and the liver; it carries blood from the intestine to the liver before it is delivered to the systemic circulation.
• Coronary system - a special system of blood vessels present exclusively for the circulation of blood to and from the cardiac musculature.
• Medulla oblongata - the site of the special neural centre that can moderate the cardiac function through the autonomic nervous system.
• P-wave - the first wave of the ECG; the electrical excitation, or depolarisation, of the atria, which leads to the contraction of both the atria.
• QRS complex - the depolarisation of the ventricles, which initiates the ventricular contraction; the contraction starts shortly after Q and marks the BEGINNING of the systole.
• T-wave - the return of the ventricles from the excited to the normal state, that is REPOLARISATION; the end of the T-wave marks the END of the systole.

Four pairs to rehearse aloud, because each one is examined as a swap. Tricuspid on the right, bicuspid on the left. SAN in the right upper corner, AVN in the lower left corner, both of them in the right atrium. Lub is the tricuspid and bicuspid valves closing, dub is the semilunar valves closing. QRS is depolarisation of the ventricles, T is repolarisation of the ventricles.

The ABO Grid

Reproduce this table exactly. Almost every ABO question in every shape is one row of it read in one direction or the other.

One line generates the whole table: the antibody a person carries is against the antigen they do NOT have. Say it once and you never have to memorise the second column. Group A lacks the B antigen, so it carries anti-B. Group B lacks the A antigen, so it carries anti-A. Group AB lacks neither, so it carries no antibody at all. Group O lacks both, so it carries both anti-A and anti-B.

The donor column follows from the same line, read the other way. A person can receive only cells that carry no antigen against which they hold an antibody. Group O has no antigens on its RBCs, so its blood can be given to everyone - group O individuals are called universal donors. Group AB has no antibodies in its plasma, so it can receive from everyone - group AB individuals are called universal recipients. O gives to all and receives only from O; AB receives from all and gives only to AB. The blood of a donor has to be carefully matched with the blood of a recipient before it is transfused, to avoid severe problems of clumping, that is destruction of RBC.

And the Rh line sits on top of the ABO grid, not inside it. Nearly 80 per cent of humans carry the Rh antigen and are Rh positive. An Rh negative person exposed to Rh positive blood will form specific antibodies against the Rh antigens, so the Rh group must be matched as well - a group O recipient who is Rh negative needs a group O, Rh negative donor, not simply a group O donor.

The Mistakes That Cost Marks in This Chapter

1. Putting the tricuspid valve on the left. This is the single commonest error in the chapter, and it is worth -1 in a paper marked +4 and -1. The tricuspid valve, formed of three muscular flaps or cusps, guards the opening between the RIGHT atrium and the RIGHT ventricle. The bicuspid or mitral valve guards the opening between the LEFT atrium and the LEFT ventricle. Fix the pair once, with whatever device works for you, and say both halves together every time - tricuspid right, bicuspid left.
2. Placing the AVN in the left atrium because its address reads "lower left corner". The atrio-ventricular node lies in the lower left corner of the RIGHT ATRIUM, close to the atrio-ventricular septum. Lower left names a corner of a chamber, not a chamber. Both nodes are in the right atrium - the SAN in its right upper corner and the AVN in its lower left corner - and an option that puts either of them in the left atrium is wrong however confident it looks.
3. Pairing lub with the semilunar valves. The FIRST heart sound, lub, is associated with the closure of the tricuspid and bicuspid valves, which happens as ventricular systole begins. The SECOND heart sound, dub, is associated with the closure of the semilunar valves, which happens as ventricular diastole begins. First sound, atrio-ventricular valves. Second sound, semilunar valves.
4. Calling the T-wave a depolarisation. It is not. The T-wave represents the return of the ventricles from the excited to the normal state, that is REPOLARISATION, and the end of the T-wave marks the end of the systole. The depolarisation of the ventricles is the QRS complex, and the depolarisation of the atria is the P-wave. There is also no separate wave for the repolarisation of the atria - it is hidden within the QRS complex - so an option offering one is always wrong.
5. Reversing an inactive form and its active product in the clotting cascade. The direction is fixed and it runs only one way. The inactive plasma protein fibrinogen is converted into fibrin by the enzyme thrombin. The inactive plasma substance prothrombin is converted into thrombin by the enzyme complex thrombokinase. Fibrinogen becomes fibrin; prothrombin becomes thrombin - never the reverse, and thrombokinase is the enzyme complex, not one of the inactive proteins.
6. Forgetting that crocodiles are four-chambered. The rule reads amphibians and reptiles, EXCEPT crocodiles, have a 3-chambered heart, and the exception is the whole reason the sentence is examined. Crocodiles sit with the birds and the mammals at four chambers - two atria and two ventricles. A reptile that is not a crocodile, such as a lizard or a snake, has three chambers and incomplete double circulation.
7. Saying cardiac output is the volume pumped by both ventricles together. It is not. Cardiac output is the volume of blood pumped out by EACH ventricle per minute, and it is the stroke volume multiplied by the heart rate, averaging 5000 mL or 5 litres in a healthy individual. The stroke volume itself, approximately 70 mL, is also per ventricle per cardiac cycle. Doubling either figure to account for two ventricles is the trap.
8. Treating heart failure, cardiac arrest and a heart attack as the same thing. They are three different conditions and the chapter separates them in as many words. Heart failure is the state of the heart when it is not pumping blood effectively enough to meet the needs of the body, and is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms. Cardiac arrest is when the heart stops beating. A heart attack is when the heart muscle is suddenly damaged by an inadequate blood supply.

Writing the Chapter-End Exercises Well

Class 11 has no board paper, but the chapter-end exercises and your school tests are still written answers, marked by a person reading for particular words. Everything below is about getting those words onto the page in the order a marker looks for them.

For a "write the differences" exercise, draw an actual two-column table, and name the basis of each row. This chapter sets that instruction more often than any other - blood and lymph, an open and a closed circulatory system, the tricuspid and the bicuspid valve, systemic and pulmonary circulation, the P-wave and the T-wave, an artery and a vein. A paragraph that describes both things one after the other rarely scores full marks, because the marker is looking for matched pairs of statements, not for two descriptions. Draw three columns if it helps - the basis of comparison on the left, and the two items in the next two columns - and make every row a genuine pair. For blood and lymph, the rows are colour (blood is red because of the haemoglobin in the RBCs, lymph is colourless), cells present (blood carries RBCs, all five kinds of WBC and platelets; lymph carries only specialised lymphocytes), proteins (blood plasma is 6-8 per cent proteins, lymph has far less because the larger proteins are left behind in the vessels), the route of flow (blood circulates in a closed circuit of vessels, lymph flows one way from the tissue spaces back to the major veins) and the function (blood transports respiratory gases, nutrients and wastes; lymph carries nutrients, hormones and absorbed fats and holds the lymphocytes responsible for immune responses). Five named rows will always beat five unnamed sentences.

For an address question, name the structure and its exact location in the same sentence. A vague location scores nothing, and this chapter is almost entirely made of addresses. Write "the sino-atrial node, a patch of nodal tissue in the right upper corner of the right atrium", not "a node in the atrium". Write "the atrio-ventricular node, in the lower left corner of the right atrium, close to the atrio-ventricular septum". Write "the tricuspid valve, guarding the opening between the right atrium and the right ventricle, formed of three muscular flaps or cusps". Write "the semilunar valves, at the openings of the right and the left ventricles into the pulmonary artery and the aorta respectively". The pattern is always the same - the name, the exact place, and the one defining feature, in one sentence - and each of those three parts is usually worth a separate mark. When the exercise asks you to trace a path rather than to place one structure, write the path as a numbered chain and name every station on it: for the conduction of the impulse, SAN, then the atrial musculature, then the AVN, then the AV bundle, then the right and left bundles, then the Purkinje fibres, then the ventricular musculature, and add the line that the atrio-ventricular septum is fibrous and does not conduct, which is why the AVN and the AV bundle are the only electrical connection between the atria and the ventricles.

For a definition question, quote the defining sentence rather than paraphrasing it. These definitions are short, they are exact, and a paraphrase almost always drops the one word the marker is counting. A cardiac cycle is the sequential event in the heart which is cyclically repeated, and it consists of the systole and diastole of both the atria and the ventricles. The stroke volume is the volume of blood, approximately 70 mL, pumped out by each ventricle during one cardiac cycle. The cardiac output is the volume of blood pumped out by each ventricle per minute, and it is the product of the stroke volume and the heart rate. Double circulation means that the blood passes through the heart twice in one complete round of the body, in two separate circulatory pathways - the pulmonary circulation and the systemic circulation. An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. Serum is plasma without the clotting factors. Notice how much work single words are doing in those sentences - each, per minute, both, twice, without - and how a loose rewording loses exactly those words.

Three habits that pay across the whole exercise set. When an exercise asks for a value, give the figure with its unit and its qualifier, exactly as the chapter prints it - nearly 55 per cent, 5 million to 5.5 million per cubic millimetre, 12-16 grams per 100 mL, 0.8 seconds, approximately 70 mL, 5000 mL or 5 litres per minute, 120/80 - and never round a range down to a single number. When an exercise names a process, write it in the order the events happen, one event per line, because a marker awards the sequence as much as the content - the cardiac cycle from joint diastole through atrial systole, ventricular systole, closure of the atrio-ventricular valves, opening of the semilunar valves, ventricular diastole and closure of the semilunar valves back to joint diastole. And when an exercise asks you to draw, describe what you draw in the labels - for the standard ECG, a flat baseline, a small upward P, then the QRS complex with its small downward Q, its tall narrow R and its downward S, and then the rounded upward T - with P labelled atrial depolarisation, QRS ventricular depolarisation and the beginning of systole, and T ventricular repolarisation and the end of systole. An unlabelled diagram in this chapter is worth almost nothing; the labels are the answer.

The Night Before - What to Revise, in Order

Read in this order and stop when the list runs out. Nothing new goes in tonight.

1. The number sheet, top to bottom. Twenty minutes, and the most valuable twenty in the chapter. The three counts first - 5 million to 5.5 million RBCs, 6000-8000 WBCs and 1,50,000 to 3,50,000 platelets, all per cubic millimetre - because those three are the ones the paper offers you in the same option list. Then the percentages - 55 and 45, 90-92 and 6-8, the five white cells in descending order, and the 80 per cent who are Rh positive. Then the heart's figures - 70-75 action potentials, 72 beats, 0.8 seconds, about 30 per cent, 70 mL, 5 litres, 120/80 and 140/90. Write them out from memory and check that every range is still a range.

2. The address sheet, said aloud as a recitation. Fifteen minutes. Go down the list and give where it is and what it does for each - the three septa, the three valve sets, the pericardium, the SAN, the AVN, the AV bundle, the Purkinje fibres, the three vessel layers, the pulmonary artery and veins, the hepatic portal vein, the coronary system, the medulla oblongata and the three ECG waves. Spend the last five minutes on the four pairs that get swapped.

3. The ABO grid, rebuilt from the one line rather than recalled. Ten minutes. The antibody a person carries is against the antigen they lack. Build all four rows from that sentence, then add the donor column, then add the Rh line on top - nearly 80 per cent are Rh positive, and the Rh group must be matched as well as the ABO group.

4. The eight mistakes above. Ten minutes. These are the marks you are most likely to lose while knowing the material perfectly well, which makes them the cheapest ones to save.

5. The cardiac cycle as a chain, and the ECG on top of it. Ten minutes. Say the cycle from joint diastole back to joint diastole, naming which valves are open and which are closed at every step, and then lay the trace over it - P before atrial contraction, QRS at the beginning of the systole, the end of T at the end of the systole.

6. The clotting cascade and the two comparisons. Five minutes. Thrombokinase, then prothrombin to thrombin, then fibrinogen to fibrin, with calcium ions playing a very important role throughout. Then run blood against lymph and open against closed circulation once each, as tables in your head.

If you have ten minutes and no more, read the number sheet and the four swapped pairs. This chapter rewards exact figures and correct sides over everything else, and a student who can place every valve and every node and reproduce the counts will out-score one who has read the whole chapter through again.