Important Questions and Answers — Class 11 Biology Chapter 16 Notes

How to Use This Section

This chapter is two grids and a list of numbers. Almost nothing in it is hard to follow; almost everything in it is easy to get half right. The marks go in the same four places - a segment of the tubule given the wrong job, a limb of Henle's loop given the wrong permeability, a hormone given the wrong gland, and a real number from the chapter written under the wrong owner.

Three habits will carry you through it.

First, learn every number with its unit and its per-what attached. Do not memorise "125"; memorise a GFR of approximately 125 mL of filtrate per MINUTE. Do not memorise "180"; memorise 180 LITRES of filtrate per DAY. Do not memorise "200"; memorise approximately 200 mL of carbon dioxide removed by the LUNGS per MINUTE. Every wrong option in this chapter is a genuine figure from the chapter wearing somebody else's name.

Second, say the swapped pairs out loud, both halves in one sentence. The DESCENDING limb is permeable to water and almost impermeable to electrolytes; the ASCENDING limb is impermeable to water but transports electrolytes. CORTICAL nephrons have a short loop and little or no vasa recta; JUXTA MEDULLARY nephrons have a long loop running deep into the medulla. Renin comes from the JG cells; aldosterone comes from the adrenal cortex. ADH conserves water; ANF lowers blood pressure. Each of those four pairs is a guaranteed distractor, and the only defence is to have learnt both halves together.

Third, treat the tubule as a walk, not as a list. Bowman's capsule, PCT, descending limb, ascending limb, DCT, collecting duct, renal pelvis. Once the order is fixed in your head, every "what does this segment do" question becomes a question about a place you can already find, and the segment grid stops being memory work.

The items below run in three tiers.

Tier 1 - short recall. The definitions, the counts, the measurements, the addresses and the orders. Answer these aloud until none of them needs thinking about.
Tier 2 - applied reasoning. A patient, a drug 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 - the filtrate formed per day worked out from the GFR per minute, the volume reabsorbed per day worked out from 180 litres and 99 per cent, and the share of the heart's output that reaches the kidneys - 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.

One of the chapter-end exercises is answered here and nowhere else in the chapter - the match-the-column item. Every other set exercise is answered in full inside one of the eleven 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. Rank ammonia, urea and uric acid by toxicity, and say what each one costs the animal in water.

Answer. Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by animals, and the two properties that separate them run in opposite directions.

Waste	Toxicity	Water needed to excrete it
Ammonia	the MOST toxic	a large amount
Urea	less toxic - intermediate	moderate
Uric acid	the LEAST toxic	a minimum loss of water

The rule in one line: the more toxic the waste, the more water it takes to get rid of it safely. A very toxic waste has to leave in a very dilute solution, and a dilute solution is mostly water. That single sentence tells you which animal can afford which waste, and you never have to memorise the animal lists as lists.

Question 2

Q. Name the three excretory patterns and put every animal group the chapter names under the right one.

Answer.	Pattern	What is excreted
Ammonotelism	ammonia	many bony fishes, aquatic amphibians, aquatic insects
Ureotelism	urea	mammals, many terrestrial amphibians, marine fishes
Uricotelism	uric acid, as a pellet or paste	reptiles, birds, land snails, insects

Three details go with the table.

Ammonia, being readily soluble, is generally excreted by DIFFUSION across body surfaces or through gill surfaces as ammonium ions, and the kidneys do not play any significant role in its removal.
In ureotelic animals ammonia is converted into urea in the LIVER, released into the blood, and filtered and excreted out by the kidneys.
Marine fishes are ureotelic, not ammonotelic, even though they live in water. Sea water is saltier than their body fluids, so water is scarce for them too, and they cannot spend it on flushing out ammonia.

Question 3

Q. Give the excretory structure of a flatworm, an earthworm, a cockroach, a prawn and a human, and name the job the chapter gives each structure.

Answer. In most of the invertebrates these structures are simple tubular forms, whereas vertebrates have complex tubular organs called kidneys.

Animal	Excretory structure	What the chapter credits it with
Flatworm, for example Planaria	protonephridia, also called flame cells	primarily ionic and fluid volume regulation, that is OSMOREGULATION
Earthworm and other annelids	nephridia	removal of nitrogenous wastes AND fluid and ionic balance
Cockroach and most insects	Malpighian tubules	removal of nitrogenous wastes AND osmoregulation
Prawn and other crustaceans	antennal glands, also called green glands	the excretory function
Human and other vertebrates	kidneys	complex tubular organs - the rest of the chapter

Protonephridia are also found in rotifers, some annelids and the cephalochordate Amphioxus, and Amphioxus is the chordate on the flame-cell list.

Question 4

Q. Only one of the invertebrate structures is described as primarily osmoregulatory. Which one, and what are the others credited with?

Answer. Protonephridia, the flame cells, are the ones described as PRIMARILY concerned with ionic and fluid volume regulation, that is osmoregulation. Waste removal is not their headline job.

Nephridia - remove nitrogenous wastes AND maintain a fluid and ionic balance. Both jobs are named.
Malpighian tubules - remove nitrogenous wastes AND carry out osmoregulation. Both jobs again.
Antennal glands or green glands - the excretory function is the only job named for them.

So the count is: one structure is primarily osmoregulatory, two do both jobs, and one is named for excretion alone. That distinction is the most examined line in the whole survey, because an option list will offer you all four and change only the word "primarily".

Question 5

Q. List the parts of the human excretory system, and say which of them are paired and which are single.

Answer. In humans the excretory system consists of a pair of KIDNEYS, one pair of URETERS, a URINARY BLADDER and a URETHRA.

Part	How many	What it is for
Kidneys	a pair	the organs that make the urine
Ureters	one pair - one leaving each kidney	carry urine down to the bladder
Urinary bladder	single	stores urine until the signal comes
Urethra	single	the tube through which urine finally leaves the body

Count it as two, two, one, one. An option list that offers "a pair of urethras" or "two urinary bladders" is testing exactly that count.

Question 6

Q. Write a data card for the human kidney - its colour, its shape, where it lies, and how big it is.

Answer.	Feature	The fact
Colour and shape	reddish brown, bean shaped structures
Vertebral level	between the levels of the LAST THORACIC and the THIRD LUMBAR vertebra
Position in the body cavity	close to the DORSAL INNER WALL of the abdominal cavity
Length	10-12 cm
Width	5-7 cm
Thickness	2-3 cm
Average weight	120-170 g
Nephrons	nearly one million in each kidney

Give every measurement as the range it is - do not round it and do not average it. The three dimensions fall in a tidy order, 10-12, then 5-7, then 2-3, so if you remember that the kidney is longest top to bottom and thinnest front to back you can rebuild the whole row from one figure. The word DORSAL is examined on its own - the kidneys lie against the back wall of the abdominal cavity, not the front.

Question 7

Q. Work in from the outside of the kidney - name the wrapping, the notch, the space behind the notch and the projections of that space, and say what passes through the notch.

Answer. The inward order is capsule, then hilum, then renal pelvis, then calyces.

Structure	The one fact it is asked on
Capsule	the tough OUTER LAYER of the kidney
Hilum	a NOTCH towards the centre of the inner concave surface; the URETER, the BLOOD VESSELS and the NERVES all enter through it
Renal pelvis	a broad FUNNEL SHAPED SPACE lying inner to the hilum
Calyces (singular calyx)	the projections of the renal pelvis

Hilum and renal pelvis are routinely swapped, so hold on to what kind of thing each one is. The hilum is a NOTCH - a doorway in the wall. The renal pelvis is a SPACE - the broad funnel lying just inside that doorway. Urine collected in the calyces drains into the renal pelvis and leaves through the hilum in the ureter, so the traffic runs outwards along the same line you learnt inwards.

Question 8

Q. Name the two zones seen inside a kidney, and the two interlocking structures that run between them, saying which zone each of the two belongs to.

Answer. Inside the kidney there are two zones - an outer CORTEX and an inner MEDULLA.

Structure	What it is	Which tissue it belongs to
Cortex	the outer zone	-
Medulla	the inner zone	-
Medullary pyramids	a few conical masses into which the medulla is divided, projecting into the calyces	MEDULLA pushing inwards
Columns of Bertini	the renal columns - the cortex extending in between the medullary pyramids	CORTEX pushing downwards

They interlock like the fingers of two hands, which is why a diagram question can point at either one and expect a different answer. Ask yourself which tissue you are standing in: a pyramid is medulla, a column of Bertini is cortex, even though the column lies deep inside the kidney.

Question 9

Q. Write a data card for the nephron - how many there are, its two parts, the vessels at each end of the glomerulus, and the other name for the glomerulus together with its capsule.

Answer.	Feature	The fact
Number	each kidney has nearly ONE MILLION complex tubular structures called NEPHRONS, which are the functional units
The two parts	the GLOMERULUS and the RENAL TUBULE
What the glomerulus is	a tuft of capillaries formed by the AFFERENT ARTERIOLE, a fine branch of the renal artery
Blood leaves by	an EFFERENT ARTERIOLE
Where the tubule begins	BOWMAN'S CAPSULE, a double walled cup-like structure which encloses the glomerulus
Glomerulus plus Bowman's capsule	the MALPIGHIAN BODY, also called the RENAL CORPUSCLE

Two things about the glomerulus are worth saying explicitly. Both vessels at its ends are ARTERIOLES, which is unusual - a capillary bed normally lies between an arteriole and a venule - and it is precisely that arrangement which lets the tuft be held under pressure. And AFFERENT brings blood in, EFFERENT takes blood away; the first letters run in alphabetical order in the direction of flow.

Question 10

Q. A drop of filtrate is formed in Bowman's capsule. Name, in order, every part it passes through until it reaches the renal pelvis.

Answer. Bowman's capsule -> PCT -> descending limb of Henle's loop -> ascending limb of Henle's loop -> DCT -> collecting duct -> renal pelvis.

Step	Segment	Its shape
1	Bowman's capsule	a double walled cup enclosing the glomerulus
2	Proximal convoluted tubule (PCT)	a highly coiled network
3	Descending limb of Henle's loop	the arm going down into the medulla
4	Ascending limb of Henle's loop	the arm coming back up; the loop as a whole is hairpin shaped
5	Distal convoluted tubule (DCT)	another highly coiled region
6	Collecting duct	a straight tube that the DCTs of many nephrons open into
7	Renal pelvis	reached by many collecting ducts converging through the medullary pyramids in the calyces

Two of the seven are coiled and one is straight - the PCT and the DCT coil, the collecting duct is straight, and Henle's loop is the hairpin between them. The collecting ducts open into the renal pelvis through the medullary pyramids in the calyces, not straight into the ureter.

Question 11

Q. Say which parts of the nephron lie in the cortex and which part dips into the medulla, and then give the two kinds of nephron with the difference in their loop and their vasa recta.

Answer. The Malpighian corpuscle, the PCT and the DCT are situated in the CORTICAL region, whereas the loop of Henle dips into the MEDULLA. Three parts in the cortex, one in the medulla - and the loop is the only part of the nephron proper that leaves the outer zone, which is why the medulla can be given a job the cortex cannot do.

	CORTICAL nephrons	JUXTA MEDULLARY nephrons
How common	the MAJORITY of nephrons	some of the nephrons
Loop of Henle	too short, extending only very little into the medulla	very long, running deep into the medulla
Vasa recta	absent or highly reduced	present, running parallel to the long loop

The trap is the word "majority". It is the cortical nephrons that are in the majority, even though the juxta medullary ones get all the attention later in the chapter, because they are the ones that make a concentrated urine possible.

Question 12

Q. Where do the peritubular capillaries and the vasa recta come from, and what shape is each?

Answer. The EFFERENT arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the PERITUBULAR CAPILLARIES. A minute vessel of this network runs parallel to Henle's loop forming a U SHAPED VASA RECTA. Vasa recta is absent or highly reduced in cortical nephrons.

The blood route in order: renal artery -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries and vasa recta.

Keep two facts about the vasa recta straight, because both are set as options.

It comes from the EFFERENT arteriole, by way of the peritubular network - not from the afferent one.
It is a BLOOD VESSEL, not a part of the tubule. It only looks like part of the loop because it is shaped like one and lies right beside it. Filtrate runs in the loop; blood runs in the vasa recta.

Question 13

Q. Name the three layers through which blood is filtered at the glomerulus, say which lies in the middle, and explain why the process is called ultra filtration.

Answer. The glomerular capillary blood pressure causes filtration of blood through 3 LAYERS.

The ENDOTHELIUM of the glomerular blood vessels - the wall of the capillary itself.
A BASEMENT MEMBRANE - and the chapter places it between the other two.
The EPITHELIUM of Bowman's capsule - the wall of the capsule.

The epithelial cells of Bowman's capsule are called PODOCYTES, and they are arranged in an intricate manner so as to leave some minute spaces called FILTRATION SLITS or SLIT PORES. Podocytes are the cells; filtration slits are the gaps left between them.

Blood is filtered so finely through these membranes that almost all the constituents of the plasma EXCEPT THE PROTEINS pass into the lumen of Bowman's capsule. Therefore it is considered a process of ULTRA FILTRATION.

The name is earned by one fact only - the proteins stay behind. Glucose, amino acids, urea, water and ions all pass through and are dealt with further down the tubule, so an option calling the filtrate glucose-free or urea-free is wrong.

Question 14

Q. This chapter has three filtration figures that look alike. Give all three and say exactly what each one measures.

Answer.	Figure	What it measures
1100-1200 mL per minute	the volume of BLOOD filtered by the kidneys per minute - roughly one-fifth of the blood pumped out by each ventricle of the heart in a minute
125 mL per minute	the volume of FILTRATE formed per minute - the glomerular filtration rate, or GFR, in a healthy individual
180 litres per day	the same GFR expressed per day

The first is a volume going IN, the second is a volume coming OUT, and the third is the second one in different units. Option lists put all three side by side on purpose.

And the last two are one fact, not two. 125 mL per minute multiplied by 1440 minutes in a day is 180000 mL, which is 180 litres. Carry both, because either can be asked.

Question 15

Q. Which substances are reabsorbed actively and which passively, which substances are secreted, and what is the one-line rule that keeps reabsorption and secretion apart?

Answer. The rule first: REABSORPTION takes things OUT of the filtrate and back into the blood; SECRETION puts things INTO the filtrate. Get the direction right and half the wrong options fall away before you even read the substance.

Substance	Reabsorbed how
Glucose	ACTIVELY
Amino acids	ACTIVELY
$\mathrm{Na^+}$	ACTIVELY
Nitrogenous wastes	by PASSIVE transport
Water, in the initial segments of the nephron	PASSIVELY

Read the pattern instead of the list. The things the body wants back - glucose, amino acids, sodium ions - are pulled back actively, at a cost of energy. The things it is trying to get rid of drift back passively.

In tubular secretion the tubular cells secrete $\mathrm{H^+}$ , $\mathrm{K^+}$ and ammonia INTO the filtrate, and this helps in the maintenance of the ionic and acid base balance of body fluids. "Urea is actively reabsorbed" and "glucose is secreted" are both standard wrong options.

Question 16

Q. Give the permeability of each limb of Henle's loop, and then give the osmolarity gradient the loop helps build and how concentrated the urine can finally become.

Answer.	Limb	Water	Electrolytes	What happens to the filtrate
DESCENDING limb	permeable	almost IMPERMEABLE	the filtrate gets CONCENTRATED as it moves down
ASCENDING limb	IMPERMEABLE	transports them, actively or passively	the concentrated filtrate gets DILUTED as it passes upward, due to the passage of electrolytes into the medullary fluid

Say it in one line: the descending limb loses water, the ascending limb loses salt.

The salt that leaves the ascending limb is exactly what keeps the medullary interstitial fluid at a high osmolarity, and the result is a gradient:

300 mOsmol per litre in the CORTEX, rising to
about 1200 mOsmol per litre in the INNER MEDULLA,
mainly caused by sodium chloride and urea.

With that gradient outside it, water leaves the collecting duct easily, and human kidneys can produce urine nearly FOUR TIMES concentrated than the initial filtrate formed. Note also that reabsorption is MINIMUM in the ascending limb and yet the region matters more than any other - it is building the gradient, not taking back the filtrate.

Question 17

Q. Write a data card for normal human urine, and then distinguish micturition from the micturition reflex.

Answer.	Property	The value as the chapter gives it
Volume per day	an adult human excretes on an average 1 to 1.5 litres of urine per day
Colour	light yellow
Physical nature	a watery fluid
Reaction	slightly ACIDIC, at pH 6.0
Odour	a characteristic odour
Urea excreted	on an average 25-30 g of urea is excreted out per day

Do not let the two daily figures collide: 1 to 1.5 LITRES is the volume of urine, 25-30 GRAMS is the mass of urea in it.

And the two names are not interchangeable.

MICTURITION is the process of release of urine - the event itself.
The MICTURITION REFLEX is the neural mechanism causing it - the machinery behind the event.

Urine is stored in the urinary bladder till a VOLUNTARY signal is given by the central nervous system, which is why the kidney can work without a break while the body releases urine only now and then.

Question 18

Q. Other than the kidneys, which organs help in excretion? Give what each one eliminates and by what route.

Answer. Other than the kidneys, the LUNGS, LIVER and SKIN also help in the elimination of excretory wastes, and small amounts of nitrogenous wastes can be eliminated through SALIVA too.

Organ or gland	What it eliminates	By what route
Lungs	large amounts of carbon dioxide - approximately 200 mL per minute - and also significant quantities of water every day	expired air
Liver, the LARGEST GLAND in our body	bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs	BILE; most of these ultimately pass out along with the digestive wastes
Sweat glands of the skin	sodium chloride, small amounts of urea, lactic acid and so on	SWEAT, a watery fluid
Sebaceous glands of the skin	sterols, hydrocarbons and waxes	SEBUM, which provides a protective oily covering for the skin
Salivary glands	small amounts of nitrogenous wastes	SALIVA

One careful word sits in the sweat row. The PRIMARY function of sweat is to facilitate a COOLING EFFECT on the body surface; it also helps in the removal of some of these wastes, but sweating is not primarily an excretory process. Sweat is watery and cools; sebum is oily and protects.

Question 19

Q. Name the four disorders of the excretory system the chapter gives, with the one phrase that identifies each.

Answer.	Disorder	What it is
Uremia	malfunctioning of the kidneys leads to the accumulation of UREA IN BLOOD; highly harmful, and may lead to kidney failure	urea in the BLOOD
Renal calculi	stone, or insoluble mass of crystallised salts such as OXALATES, formed within the kidney	stone, oxalates
Glomerulonephritis	inflammation of the GLOMERULI of the kidney	inflammation
Acute renal failure	kidney failure, of which KIDNEY TRANSPLANTATION is the ultimate method of correction	transplantation

The endings do the sorting for you. -emia is in the blood, so uremia is urea in the BLOOD. -uria is in the urine, so glycosuria and ketonuria are glucose and ketone bodies in the URINE. -itis is inflammation. Calculus is simply the Latin for a small stone.

Urea in the urine is normal - 25-30 g of it leaves every day. It is urea piling up in the BLOOD that is the disorder.

Tier 2 - Applied Reasoning

Question 20

Q. A patient's glomerular filtration rate is measured at 100 mL per minute. Work out the volume of filtrate formed per day, and say what share of the normal value that is.

Answer. Step 1 - turn the rate per minute into a rate per day. There are 60 minutes in an hour and 24 hours in a day, so 1440 minutes in a day.

100 mL per minute multiplied by 1440 minutes = 144000 mL of filtrate per day.

Step 2 - convert to litres. 1000 mL make 1 litre, so 144000 mL = 144 litres per day.

Step 3 - compare with the normal value. GFR in a healthy individual is approximately 125 mL per minute, that is 180 litres per day.

144 divided by 180 = 0.8, which is 80 per cent.

So this patient is filtering 144 litres per day, which is 80 per cent of the normal 180 litres per day. Do the conversion in that order every time - rate per minute, then multiply by 1440, then divide by 1000 - and you can move between the chapter's two GFR figures in either direction.

Question 21

Q. 180 litres of filtrate are formed per day and nearly 99 per cent of it is reabsorbed. Work out how much comes back and how much leaves. Then work out what would leave if reabsorption fell to 98 per cent.

Answer. Step 1 - the volume reabsorbed. 99 per cent of 180 litres = 0.99 multiplied by 180 = 178.2 litres reabsorbed per day.

Step 2 - the volume left. 180 minus 178.2 = 1.8 litres per day. That sits just beside the chapter's own figure, an average of 1 to 1.5 litres of urine per day - and the small difference is only because 99 per cent is itself a rounded figure. Work it the other way and the rounding disappears: 180 litres in, 1.5 litres out, so 178.5 litres come back, and 178.5 divided by 180 is 0.9917, which is why the chapter says nearly 99 per cent.

Step 3 - now drop the reabsorption to 98 per cent. 98 per cent of 180 = 176.4 litres reabsorbed, so 180 minus 176.4 = 3.6 litres of urine per day.

A fall of one percentage point in reabsorption roughly DOUBLES the urine output. That is the real lesson in the arithmetic: the kidney is a recovery organ far more than a disposal organ, and because it throws away barely one part in a hundred of what it filters, a tiny change in the reabsorbed fraction makes an enormous change in what leaves.

Question 22

Q. The kidneys filter 1100-1200 mL of blood per minute, and the cardiac output is about 5 litres per minute. Work out what share of the heart's output reaches the kidneys, and check it against the chapter's own statement.

Answer. Step 1 - put both figures in the same unit. Cardiac output is about 5 litres per minute, which is 5000 mL per minute.

Step 2 - divide.

At the lower end: 1100 divided by 5000 = 0.22, that is 22 per cent.
At the upper end: 1200 divided by 5000 = 0.24, that is 24 per cent.

Step 3 - compare with what the chapter says. The chapter states that the 1100-1200 mL per minute constitutes roughly ONE-FIFTH of the blood pumped out by each ventricle of the heart in a minute, and one-fifth is 20 per cent. Our sum gives between about 22 and 24 per cent, so the two agree to within the roughness of the word "roughly".

The point to carry away is the size of the share, not the second decimal place. A pair of organs weighing 120-170 g each takes about a fifth of everything the heart pumps - which is why the kidney can filter the whole blood volume many times over in a day.

Question 23

Q. Match Column I with Column II. Column I: (a) Ammonotelism, (b) Bowman's capsule, (c) Micturition, (d) Uricotelism, (e) ADH. Column II: (i) Birds, (ii) Water reabsorption, (iii) Bony fish, (iv) Urinary bladder, (v) Renal tubule. This is one of the chapter-end exercises.

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

Column I	Column II	Why they go together
(a) Ammonotelism	(iii) Bony fish	Many bony fishes are ammonotelic - they excrete ammonia, which being readily soluble diffuses out across the gill surfaces as ammonium ions, and the kidneys play no significant role. An animal surrounded by water can afford the large amount of water that ammonia costs.
(b) Bowman's capsule	(v) Renal tubule	The renal tubule BEGINS with Bowman's capsule, a double walled cup-like structure which encloses the glomerulus. The capsule is the first segment of the tubule, so the tubule is the structure it belongs to.
(c) Micturition	(iv) Urinary bladder	Micturition is the process of release of urine, and urine is stored in the urinary bladder until the reflex empties it. The stretching of the bladder starts the signal, and the contraction of its smooth muscles with the simultaneous relaxation of the urethral sphincter releases the urine.
(d) Uricotelism	(i) Birds	Birds excrete nitrogenous wastes as uric acid, in the form of a pellet or paste, with a minimum loss of water. Reptiles, land snails and insects are uricotelic too, but birds are the group offered here.
(e) ADH	(ii) Water reabsorption	ADH, the antidiuretic hormone, also called vasopressin, facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis. Conserving water is its defining action.

Two of the five pairs can be settled by elimination alone, which is worth practising. Bowman's capsule is the only structure in Column I, and the renal tubule is the only structure in Column II, so that pair fixes itself. Micturition is the only event, and the urinary bladder is the only organ it could belong to. That leaves the two excretory patterns and ADH, and ammonotelism must take the aquatic animal while uricotelism takes the bird, because ammonia costs a large amount of water and uric acid costs the least.

Question 24

Q. Two nephrons are taken from the same kidney. One has a long loop running deep into the medulla with a vessel running beside it; the other has a short loop and almost no such vessel. Name each nephron and say what each can and cannot do.

Answer. The first is a JUXTA MEDULLARY nephron and the second is a CORTICAL nephron.

	The one with the long loop	The one with the short loop
Name	juxta medullary nephron	cortical nephron
Loop of Henle	very long, running deep into the medulla	too short, extending only very little into the medulla
Vasa recta	present, running parallel to the loop	absent or highly reduced
How common	some of the nephrons	the MAJORITY of nephrons

What each can do follows from the loop. The counter current mechanism needs both a long Henle's loop and a vasa recta lying beside it, because the gradient is built by the two hairpins running past each other in opposite directions. Only the juxta medullary nephron has both, so it is the one that builds and holds the medullary osmolarity gradient that lets the kidney make a concentrated urine.

The cortical nephron filters, reabsorbs and secretes perfectly well - it simply has almost nothing dipping into the medulla and so contributes little to the gradient. The majority of nephrons are of the kind that does not concentrate the urine, and the minority does the concentrating for all of them.

Question 25

Q. A patient's urine is found to contain glucose and amino acids, although the filtration barrier is intact and no plasma protein appears in the urine. Which step of urine formation has failed, where does it normally happen, and by what mechanism?

Answer. Filtration is working. The step that has failed is REABSORPTION, and specifically reabsorption in the PROXIMAL CONVOLUTED TUBULE.

Work through it in order.

Step 1 - filtration is normal. Almost all the constituents of the plasma except the proteins pass into the lumen of Bowman's capsule, so glucose and amino acids are SUPPOSED to be in the filtrate. Their presence in the filtrate is not the fault. And no protein has appeared in the urine, which tells us the three layers of the barrier are still holding proteins back, so the glomerulus is intact.

Step 2 - what should have happened next. The PCT reabsorbs nearly all of the essential nutrients, and glucose and amino acids are reabsorbed ACTIVELY, at a cost of energy.

Step 3 - the conclusion. Since these substances entered the filtrate normally and were then not taken back, the active reabsorption in the PCT has failed.

Contrast this with the other case, so the two are never confused. Protein in the urine points at the FILTRATION barrier, because proteins should never have crossed it. Glucose and amino acids in the urine point at REABSORPTION, because they were meant to cross and then come back.

Question 26

Q. A drug stops the ascending limb of Henle's loop from transporting electrolytes out into the medullary interstitium. Predict what happens to the medullary gradient and to the urine.

Answer. The gradient collapses, and the urine becomes large in volume and dilute.

Step 1 - what the ascending limb was doing. The ascending limb is impermeable to water but transports electrolytes actively or passively, and the electrolytes that leave it are exactly what keeps the medullary interstitial fluid at a high osmolarity. Reabsorption is minimum in the ascending limb, yet the region plays a significant role in the maintenance of high osmolarity of the medullary interstitial fluid.

Step 2 - the gradient. With no salt leaving, the rise from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla cannot be built, since the gradient is mainly caused by sodium chloride and urea and the sodium chloride circuit has been broken at its source.

Step 3 - the collecting duct. The purpose of the interstitial gradient is the easy passage of water out of the collecting tubule, thereby concentrating the urine. With a flat interstitium there is nothing to pull water out, so water stays in the duct.

Step 4 - the urine. A large volume of dilute urine leaves - the kidney can no longer produce urine nearly four times concentrated than the initial filtrate. That is exactly how a class of diuretic drugs works, and it shows that the ascending limb matters not for what it reabsorbs but for what it puts into the medulla.

Question 27

Q. A person drinks three litres of water in a short time. Trace what the body does about it, naming the receptor, the hormone and the effect on the urine.

Answer. Step 1 - the change is detected. Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. Here the body fluid volume has risen and the ionic concentration has fallen.

Step 2 - the hormone is switched off. An increase in body fluid volume switches off the osmoreceptors and suppresses ADH release. ADH, the antidiuretic hormone or vasopressin, is released by the hypothalamus from the NEUROHYPOPHYSIS, and here the release is turned down rather than turned up.

Step 3 - the tubule. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis. With less ADH, less water is reabsorbed at the distal parts and the collecting duct.

Step 4 - the urine. A large volume of dilute urine is passed, and the extra water leaves the body. That is diuresis, and it is exactly what ADH normally prevents.

Note that this is a FEEDBACK, not a one-way push. Excessive loss of fluid turns the osmoreceptors ON and raises ADH; an increase in body fluid volume turns them OFF and suppresses it. Both halves of the loop are examinable, and the half that gets forgotten is the switching off.

Question 28

Q. A person loses a large volume of blood and the blood pressure and GFR fall. Trace the chain of events that corrects it, and name the mechanism that later holds that correction in check.

Answer. The correcting chain is the RENIN-ANGIOTENSIN MECHANISM, and the check on it is the ANF mechanism.

Step 1. A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release RENIN.

Step 2. Renin converts ANGIOTENSINOGEN in blood to ANGIOTENSIN I and further to ANGIOTENSIN II.

Step 3. Angiotensin II, being a powerful VASOCONSTRICTOR, increases the glomerular blood pressure and thereby the GFR.

Step 4. Angiotensin II also activates the ADRENAL CORTEX to release ALDOSTERONE.

Step 5. Aldosterone causes reabsorption of $\mathrm{Na^+}$ and water from the distal parts of the tubule, which also leads to an increase in blood pressure and GFR.

Now the check. An increase in blood flow to the ATRIA of the heart can cause the release of ATRIAL NATRIURETIC FACTOR (ANF). ANF causes VASODILATION and thereby DECREASES the blood pressure. The ANF mechanism therefore acts as a check on the renin-angiotensin mechanism.

Read the two together: renin-angiotensin pushes the blood pressure up, ANF pulls it back down, and between them the pressure is held steady. Keep the sources apart - renin from the JG cells, aldosterone from the adrenal cortex, ANF from the atria of the heart, ADH from the neurohypophysis.

Question 29

Q. A question names a "Malpighian" structure without saying which one, and another offers you both "protonephridia" and "nephridia". Sort out both pairs.

Answer. Two pairs of look-alike names, and each pair belongs to two different animals or two different places.

Pair one - Malpighian tubules against the Malpighian body.

Name	What it is	Where
Malpighian tubules	the excretory structures of most insects, including cockroaches; they remove nitrogenous wastes and carry out osmoregulation	an insect
Malpighian body, also called the renal corpuscle	the GLOMERULUS together with BOWMAN'S CAPSULE	the human nephron, in the cortex

They are named after the same anatomist and that is the only thing they have in common. If the stem says "tubules" it is an insect; if it says "body" or "corpuscle" it is a nephron.

Pair two - protonephridia against nephridia.

Name	What it is	Which animals	Its job
Protonephridia, the flame cells	the simpler, blind-ending structures	Platyhelminthes such as Planaria, rotifers, some annelids and the cephalochordate Amphioxus	primarily ionic and fluid volume regulation, that is osmoregulation
Nephridia	the tubular excretory structures	earthworms and other annelids	remove nitrogenous wastes AND maintain fluid and ionic balance

The prefix is the whole difference, and an option list will always offer you both.

Question 30

Q. Give the owner of each of these figures: 125 mL per minute, 180 litres per day, 200 mL per minute, 1 to 1.5 litres per day, 25-30 g per day, 120-170 g, 1200 mOsmol per litre, 70-80 per cent.

Answer. Every wrong option in this chapter is a genuine figure wearing somebody else's name, so learn each number with its owner attached.

Figure	Its owner
125 mL per minute	the GFR - the volume of FILTRATE formed by the kidneys per minute
180 litres per day	the same GFR expressed per day
200 mL per minute	the carbon dioxide removed by the LUNGS
1 to 1.5 litres per day	the volume of URINE an adult human excretes on an average
25-30 g per day	the mass of UREA excreted on an average
120-170 g	the average WEIGHT of one kidney
1200 mOsmol per litre	the osmolarity of the INNER MEDULLA, against 300 mOsmol per litre in the cortex
70-80 per cent	the share of ELECTROLYTES AND WATER reabsorbed in the PCT

Three more belong on the same card. 1100-1200 mL per minute is the BLOOD filtered, roughly one-fifth of the blood pumped out by each ventricle in a minute. Nearly 99 per cent of the filtrate is reabsorbed. pH 6.0 is urine, slightly acidic.

Before you pick an option, read its unit and its per-what. A gram figure cannot be a volume, and a per-minute figure cannot be a per-day figure. That single check disposes of most of the distractors in this chapter.

Question 31

Q. Put these five events of micturition in order, and name the one swap that is offered as a wrong option: relaxation of the urethral sphincter; signals from the stretch receptors to the CNS; release of urine; filling and stretching of the urinary bladder; contraction of the smooth muscles of the bladder.

Answer. The correct order is:

Filling and STRETCHING of the urinary bladder as urine collects in it - the trigger is mechanical, not chemical.
The STRETCH RECEPTORS on the walls of the bladder send signals to the CNS.
The CNS passes on motor messages, causing the CONTRACTION of the smooth muscles of the bladder and
the SIMULTANEOUS RELAXATION of the urethral sphincter.
Release of urine.

Steps 3 and 4 happen together, and the word SIMULTANEOUS is doing real work - a bladder that squeezes against a shut sphincter releases nothing.

The swap that is offered as a distractor reverses those two muscle events - it gives you relaxation of the bladder with contraction of the sphincter. Fix it with one sentence: the BLADDER CONTRACTS and the SPHINCTER RELAXES.

One more detail is worth having ready. Urine is stored in the bladder till a VOLUNTARY signal is given by the central nervous system, so the emptying is a reflex that we can hold back - which is why the chapter calls the signal voluntary and the mechanism a reflex in the same breath.

Question 32

Q. The liver appears twice in this chapter in two quite different roles. Give both, and say where the wastes of each role finally leave the body.

Answer. Role one - the liver makes urea.

In ureotelic animals, ammonia produced by metabolism is converted into UREA in the LIVER. The urea is released into the blood, which is filtered and excreted out by the kidneys. So in this role the liver is a chemical converter: it turns the most toxic waste into a less toxic one that the body can carry in the blood and get rid of with only a moderate loss of water. These wastes leave in the URINE - 25-30 g of urea per day.

Role two - the liver secretes bile.

The liver is the LARGEST GLAND in our body, and it eliminates bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs through BILE. Most of these substances ultimately pass out along with the digestive wastes - that is, in the faeces, not in the urine.

Role	What the liver handles	Where it goes next	Where it leaves the body
Urea formation	ammonia converted into urea	into the BLOOD, then to the kidneys	the URINE
Bile secretion	bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs	into the BILE, then to the gut	the FAECES, with the digestive wastes

The two roles use two different exits, and that is the part a question tests. Do not send the bile pigments out in the urine, and do not send the urea out in the faeces.

Tier 3 - Longer Written Answers

Question 33

Q. Describe the structure of a nephron in full, from the vessel that brings blood to it to the duct that carries the urine away, and include the blood supply.

Answer. Each kidney has nearly ONE MILLION complex tubular structures called NEPHRONS, which are the functional units. Each nephron has two parts - the GLOMERULUS and the RENAL TUBULE.

The glomerulus. The glomerulus is a tuft of capillaries formed by the AFFERENT ARTERIOLE, a fine branch of the renal artery. Blood from the glomerulus is carried away by an EFFERENT ARTERIOLE. A capillary bed lying between two arterioles is unusual, and it is what allows the tuft to be held under pressure, which is the pressure that drives filtration.

The renal tubule, segment by segment.

The renal tubule begins with a double walled cup-like structure called BOWMAN'S CAPSULE, which encloses the glomerulus. The glomerulus along with Bowman's capsule is called the MALPIGHIAN BODY or RENAL CORPUSCLE.
The tubule continues as a highly coiled network, the PROXIMAL CONVOLUTED TUBULE (PCT).
A hairpin shaped HENLE'S LOOP is the next part, with a DESCENDING and an ASCENDING limb.
The ascending limb continues as another highly coiled region, the DISTAL CONVOLUTED TUBULE (DCT).
The DCTs of many nephrons open into a straight tube called the COLLECTING DUCT, many of which converge and open into the renal pelvis through the medullary pyramids in the calyces.

Where the parts lie. The Malpighian corpuscle, the PCT and the DCT are situated in the CORTICAL region, whereas the loop of Henle dips into the MEDULLA.

The two kinds of nephron.

	CORTICAL	JUXTA MEDULLARY
How common	the majority	some of the nephrons
Loop of Henle	too short, extending only very little into the medulla	very long, running deep into the medulla
Vasa recta	absent or highly reduced	present

The blood supply. The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the PERITUBULAR CAPILLARIES. A minute vessel of this network runs parallel to Henle's loop forming a U shaped VASA RECTA. Vasa recta is absent or highly reduced in cortical nephrons.

The full blood route: renal artery -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries and vasa recta.

Keep the two routes apart when you write this out. The tubule carries FILTRATE; the vessels carry BLOOD. They run side by side the whole way, and every exchange in the chapter is a movement between one and the other.

Question 34

Q. Give a full account of urine formation, from blood entering the kidney to urine reaching the renal pelvis.

Answer. Urine formation involves THREE main processes - GLOMERULAR FILTRATION, REABSORPTION and SECRETION - that take place in different parts of the nephron.

1. Glomerular filtration.

The first step in urine formation is the filtration of blood, which is carried out by the glomerulus. On an average, 1100-1200 mL of blood is filtered by the kidneys per minute, which constitutes roughly one-fifth of the blood pumped out by each ventricle of the heart in a minute.

The glomerular capillary blood pressure causes filtration of blood through 3 layers - the endothelium of the glomerular blood vessels, the epithelium of Bowman's capsule, and a basement membrane between these two layers. The epithelial cells of Bowman's capsule, called PODOCYTES, are arranged in an intricate manner so as to leave some minute spaces called FILTRATION SLITS or SLIT PORES.

Blood is filtered so finely that almost all the constituents of the plasma EXCEPT THE PROTEINS pass into the lumen of Bowman's capsule, and the process is therefore called ULTRA FILTRATION.

The amount of the filtrate formed by the kidneys per minute is called the GLOMERULAR FILTRATION RATE (GFR), and GFR in a healthy individual is approximately 125 mL per minute, that is 180 litres per day.

2. Reabsorption.

A comparison of the volume of filtrate formed per day, 180 litres, with the urine released, 1.5 litres, shows that nearly 99 PER CENT of the filtrate has to be reabsorbed by the renal tubules. The tubular epithelial cells in different segments perform this either by active or passive mechanisms. Glucose, amino acids and $\mathrm{Na^+}$ are reabsorbed ACTIVELY; the nitrogenous wastes are absorbed by PASSIVE transport; water in the initial segments is reabsorbed PASSIVELY.

3. Tubular secretion.

During urine formation the tubular cells SECRETE substances like $\mathrm{H^+}$ , $\mathrm{K^+}$ and ammonia INTO the filtrate. Tubular secretion is an important step in urine formation, as it helps in the maintenance of the ionic and acid base balance of body fluids.

Why secretion is needed at all. Filtration is unselective - it takes everything except the proteins. Reabsorption then takes back what the body wants, and secretion is the kidney's chance to add in exactly what still needs to go. Only after all three have acted is the fluid in the tubule really urine.

4. Concentrating the urine and carrying it away.

Water then leaves the collecting duct into the concentrated medullary interstitium, so that human kidneys can produce urine nearly four times concentrated than the initial filtrate formed. The collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces, and from the pelvis the urine passes into the ureter.

Question 35

Q. Explain how the human kidney manages to produce a urine far more concentrated than the filtrate it started with. Name the structures involved and trace both of the circuits that build the gradient.

Answer. Mammals have the ability to produce a concentrated urine. The HENLE'S LOOP and the VASA RECTA play a significant role in this. Both are hairpins, and in a hairpin the fluid coming down runs right beside the fluid going up.

Step 1 - two counter currents.

The flow of filtrate in the two limbs of Henle's loop is in OPPOSITE DIRECTIONS and thus forms a COUNTER CURRENT. The flow of blood through the two limbs of the vasa recta is also in a counter current pattern.

Step 2 - the gradient that results.

The proximity between Henle's loop and the vasa recta, as well as the counter current in them, helps in maintaining an increasing osmolarity towards the inner medullary interstitium - from 300 mOsmol per litre in the CORTEX to about 1200 mOsmol per litre in the INNER MEDULLA. This gradient is mainly caused by SODIUM CHLORIDE and UREA.

Step 3 - the two circuits, kept separate.

Substance	Where it leaves or enters	What returns it to the interstitium
Sodium chloride	transported by the ASCENDING LIMB of Henle's loop, and exchanged with the DESCENDING LIMB of the vasa recta	the ASCENDING PORTION of the vasa recta
Urea	small amounts enter the THIN SEGMENT OF THE ASCENDING LIMB of Henle's loop	the COLLECTING TUBULE

Neither substance simply leaks into the medulla and stays there - each goes round a small circuit instead of being washed away.

Step 4 - why running two streams in opposite directions works.

At any one level the ascending limb only has to make the interstitium around it slightly saltier than the fluid inside it - a small and easy step. The fluid moving down the descending limb, and the blood moving down the vasa recta, then carry that slightly higher concentration a little deeper, where the next small step is added on top of it. Step after small step, stacked down the length of the hairpin, the total becomes enormous. And a straight vessel running through the medulla would simply wash the salt and urea away; the vasa recta, being a hairpin, carries them down one limb and brings them back up the other, so the medulla keeps what it has built.

Step 5 - what it is all for.

The transport of substances facilitated by the special arrangement of Henle's loop and the vasa recta is called the COUNTER CURRENT MECHANISM. The presence of such an interstitial gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate, that is the urine. Human kidneys can produce urine nearly FOUR TIMES concentrated than the initial filtrate formed.

The gradient is a means, not an end. A question asking what it is for is answered with water leaving the collecting duct, not with salt reabsorption.

Question 36

Q. Write a segment-by-segment account of what each part of the renal tubule does.

Answer. This grid is the single most examined object in the chapter, so write it in order down the tubule.

Segment	What it does
Proximal Convoluted Tubule (PCT)	lined by simple cuboidal BRUSH BORDER epithelium, which increases the surface area for reabsorption; nearly all of the essential nutrients, and 70-80 PER CENT of electrolytes and water, are reabsorbed here; maintains pH and ionic balance by selective secretion of $\mathrm{H^+}$ and ammonia into the filtrate and by absorption of $\mathrm{HCO_3^-}$ from it
Henle's Loop	reabsorption is MINIMUM in its ascending limb; the region plays a significant role in the maintenance of high osmolarity of the medullary interstitial fluid
Distal Convoluted Tubule (DCT)	CONDITIONAL reabsorption of $\mathrm{Na^+}$ and water; also capable of reabsorption of $\mathrm{HCO_3^-}$ and selective secretion of $\mathrm{H^+}$ , $\mathrm{K^+}$ and ammonia, to maintain the pH and sodium-potassium balance in blood
Collecting Duct	extends from the CORTEX of the kidney to the INNER PARTS OF THE MEDULLA; large amounts of water could be reabsorbed here to produce a concentrated urine; allows the passage of small amounts of UREA into the medullary interstitium to keep up the osmolarity; maintains pH and ionic balance by selective secretion of $\mathrm{H^+}$ and $\mathrm{K^+}$

The two limbs of Henle's loop need a row each of their own, because swapping them is the commonest error in the chapter.

Limb	Water	Electrolytes	The filtrate
DESCENDING	permeable	almost impermeable	gets CONCENTRATED as it moves down
ASCENDING	impermeable	transported actively or passively	gets DILUTED as it passes upward, due to the passage of electrolytes into the medullary fluid

Two words in the grid are examinable on their own.

BRUSH BORDER - the PCT lining, and the reason given is that it increases the surface area for reabsorption.
CONDITIONAL - the DCT's reabsorption of sodium and water is not fixed; it depends on the body's needs at the time and is placed under hormonal control. The PCT reabsorbs regardless; the DCT reabsorbs on condition.

And one job belongs to one segment alone. Only the collecting duct is credited with passing urea into the medullary interstitium. Do not hand that job to Henle's loop.

Question 37

Q. Give a full account of the hormonal regulation of kidney function, naming all three routes, their triggers, their sources and their effects.

Answer. The functioning of the kidneys is efficiently monitored and regulated by HORMONAL FEEDBACK MECHANISMS involving the HYPOTHALAMUS, the JGA and, to a certain extent, the HEART. Name those three first - the list itself is asked.

Route 1 - ADH, the water route.

OSMORECEPTORS in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid can activate these receptors, which stimulate the hypothalamus to release ANTIDIURETIC HORMONE (ADH), or VASOPRESSIN, from the NEUROHYPOPHYSIS. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing DIURESIS. An increase in body fluid volume can switch off the osmoreceptors and suppress ADH release, to complete the feedback.

ADH has a second action as well. Its CONSTRICTORY effects on blood vessels cause an increase in blood pressure, and an increase in blood pressure can increase the glomerular blood flow and thereby the GFR.

Route 2 - the renin-angiotensin mechanism.

The JGA plays a complex regulatory role, and its sequence must be written in order.

A fall in glomerular blood flow, glomerular blood pressure or GFR can activate the JG cells to release RENIN.
Renin converts ANGIOTENSINOGEN in blood to ANGIOTENSIN I and further to ANGIOTENSIN II.
Angiotensin II, being a powerful VASOCONSTRICTOR, increases the glomerular blood pressure and thereby the GFR.
Angiotensin II also activates the ADRENAL CORTEX to release ALDOSTERONE.
Aldosterone causes reabsorption of $\mathrm{Na^+}$ and water from the distal parts of the tubule, which also leads to an increase in blood pressure and GFR.

This complex mechanism is generally known as the RENIN-ANGIOTENSIN MECHANISM.

Route 3 - ANF.

An increase in blood flow to the ATRIA of the heart can cause the release of ATRIAL NATRIURETIC FACTOR (ANF). ANF can cause VASODILATION - dilation of blood vessels - and thereby DECREASE the blood pressure. The ANF mechanism therefore acts as a CHECK on the renin-angiotensin mechanism.

The three side by side.

Route	Trigger	Source	Net effect
ADH, or vasopressin	osmoreceptors activated by changes in blood volume, body fluid volume and ionic concentration	the NEUROHYPOPHYSIS, on stimulation of the hypothalamus	conserves water; also raises blood pressure and the GFR
Renin-angiotensin	a fall in glomerular blood flow, glomerular blood pressure or GFR	RENIN from the JG cells; ALDOSTERONE from the ADRENAL CORTEX	RAISES blood pressure and GFR
ANF	an increase in blood flow to the ATRIA of the heart	the ATRIA of the heart	DECREASES blood pressure - a check on the renin-angiotensin mechanism

Four sources get swapped in every option list: renin from the JG cells, aldosterone from the adrenal cortex, ADH from the neurohypophysis, ANF from the atria of the heart. And keep the directions straight - renin-angiotensin raises blood pressure while ANF lowers it, and ADH conserves water rather than eliminating it.

Question 38

Q. A patient's blood urea is rising steadily. Name the condition, describe in full how the urea can be removed, and say what the ultimate correction would be.

Answer. The condition is UREMIA. Malfunctioning of the kidneys can lead to the accumulation of UREA IN BLOOD, a condition called uremia, which is highly harmful and may lead to kidney failure. Note the ending - -emia is in the blood. Urea in the urine is perfectly normal; 25-30 g of it leaves every day.

In such patients urea can be removed by a process called HAEMODIALYSIS, and the machine works in a fixed order.

The blood drained from a convenient ARTERY is pumped into a dialysing unit called an ARTIFICIAL KIDNEY, after adding an ANTICOAGULANT LIKE HEPARIN, so that it does not clot inside the machine.
The unit contains a coiled CELLOPHANE TUBE surrounded by a fluid, the DIALYSING FLUID, having the same composition as that of plasma EXCEPT THE NITROGENOUS WASTES.
The porous cellophane membrane of the tube allows the passage of molecules based on CONCENTRATION GRADIENT.
As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood.
The cleared blood is pumped back to the body through a VEIN after adding ANTI-HEPARIN to it, which restores normal clotting.

Going in	Coming out
from a convenient ARTERY	back through a VEIN
HEPARIN added - an anticoagulant	ANTI-HEPARIN added - restores clotting

The design of the dialysing fluid is the clever part, and it is worth saying why. It matches plasma in everything except the nitrogenous wastes. Because the wastes are missing on the outside, there is a steep concentration gradient for them and they move out. Because everything else matches, glucose, salts and the other useful constituents have no gradient to move down and are not stripped from the blood. If the fluid were plain water, the patient would lose those too.

The ultimate correction is a transplant. KIDNEY TRANSPLANTATION is the ultimate method in the correction of acute renal failures. A functioning kidney is used in transplantation from a donor, preferably a CLOSE RELATIVE, to minimise its chances of REJECTION by the immune system of the host. The relative is preferred because a close relative's tissues are genetically more similar, not for convenience. Dialysis keeps a uremic patient alive; a transplant is the actual correction.

Question 39

Q. Osmoregulation runs through the whole of this chapter. Show what the word means for a flatworm, for a marine fish and for a human being on a hot day.

Answer. Osmoregulation is the regulation of the water and ionic content of the body fluids - keeping the concentration of the body's fluids steady, whatever the animal takes in or loses. Excretion and osmoregulation are done by the same organs at the same time, which is why the chapter keeps naming them together.

The flatworm. Protonephridia, the flame cells of Platyhelminthes such as Planaria, rotifers, some annelids and the cephalochordate Amphioxus, are PRIMARILY concerned with ionic and fluid volume regulation, that is osmoregulation. A freshwater flatworm is surrounded by water far more dilute than its own body fluids, so water is constantly entering it and has to be pushed out again. For this animal osmoregulation is the main job and waste removal is the side effect - the only structure in the chapter of which that is said.

The marine fish. Marine fishes are UREOTELIC, not ammonotelic, although they live in water. Sea water is saltier than their body fluids, so water is scarce for them too, and they cannot afford the large amount of water that excreting ammonia costs. Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water, and the marine fish faces the same shortage for a different reason. Being in water is not the same as having water to spare.

The human being on a hot day. Here three of the chapter's mechanisms act at once.

Sweat is lost, and its PRIMARY function is to facilitate a COOLING EFFECT on the body surface, though it also removes sodium chloride, small amounts of urea and lactic acid.
The loss of fluid activates the OSMORECEPTORS, which stimulate the hypothalamus to release ADH from the NEUROHYPOPHYSIS. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis.
The counter current mechanism does the rest. The gradient from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla allows an easy passage of water out of the collecting tubule, so the kidney returns water to the body and passes a small volume of concentrated urine - nearly four times concentrated than the initial filtrate.

One idea, three answers. The animal that has water to spare lets its waste go cheaply; the animal that has none rebuilds its whole excretory chemistry and its whole kidney around saving it.

Where Every Chapter-End Exercise Is Answered

This chapter has twelve exercises at the end. Counted properly they come to twenty-one questions, because exercise 3, the true-or-false item, has five parts a to e, exercise 11, the name-the-following item, has three parts a to c, and exercise 12, the fill-in-the-gaps item, has four parts a to d. Every other exercise is a single part.

Twenty of those twenty-one parts are answered in full inside the eleven teaching sections of this chapter. The remaining one - exercise 7, the match-the-column item - is answered in this section and nowhere else.

The parts of the three multi-part exercises are deliberately split across different sections, because each part belongs with the topic it tests. Exercise 3(b) sits with the hormones and 3(d) with the tubules; exercise 12(c) sits with dialysis and 12(d) with the composition of urine. So do not go looking for all five parts of exercise 3 in one place.

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	Define GFR	Section 5 - Glomerular Filtration and the GFR	Question 8
2	Explain the autoregulatory mechanism of GFR	Section 5 - Glomerular Filtration and the GFR	Question 9
3 (a)	True or false - micturition is carried out by a reflex	Section 9 - Micturition and the Composition of Urine	Question 7
3 (b)	True or false - ADH helps in water elimination	Section 8 - Regulation of Kidney Function	Question 4
3 (c)	True or false - protein-free fluid is filtered into Bowman's capsule	Section 5 - Glomerular Filtration and the GFR	Question 10
3 (d)	True or false - Henle's loop concentrates the urine	Section 6 - Reabsorption, Secretion and the Function of the Tubules	Question 9
3 (e)	True or false - glucose is actively reabsorbed in the PCT	Section 6 - Reabsorption, Secretion and the Function of the Tubules	Question 10
4	A brief account of the counter current mechanism	Section 7 - The Counter Current Mechanism	Question 11
5	The role of liver, lungs and skin in excretion	Section 10 - The Role of Other Organs in Excretion	Question 14
6	Explain micturition	Section 9 - Micturition and the Composition of Urine	Question 6
7	Match Column I with Column II	Section 12 - Important Questions and Answers	Question 23
8	What osmoregulation means	Section 1 - Nitrogenous Wastes and the Three Excretory Patterns	Question 10
9	Why terrestrial animals are not ammonotelic	Section 1 - Nitrogenous Wastes and the Three Excretory Patterns	Question 11
10	The significance of the juxta glomerular apparatus	Section 8 - Regulation of Kidney Function	Question 7
11 (a)	Name a chordate having flame cells	Section 2 - Excretory Structures Across the Animal Kingdom	Question 4
11 (b)	Name the cortical portions between the medullary pyramids	Section 3 - The Human Excretory System - The Kidney's Gross Structure	Question 10
11 (c)	Name a capillary loop running parallel to Henle's loop	Section 4 - The Nephron - The Functional Unit	Question 12
12 (a)	Fill in - ascending and descending limb permeability to water	Section 6 - Reabsorption, Secretion and the Function of the Tubules	Question 11
12 (b)	Fill in - the hormone facilitating water reabsorption at the distal parts	Section 8 - Regulation of Kidney Function	Question 11
12 (c)	Fill in - dialysis fluid contains all the constituents of plasma except	Section 11 - Disorders of the Excretory System	Question 9
12 (d)	Fill in - grams of urea excreted per day	Section 9 - Micturition and the Composition of Urine	Question 10

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

Eight of the twenty-one parts - exercises 1, 2, 3(c), 3(d), 3(e), 4, 10 and 12(a) - come out of the working of the nephron, that is, filtration, the GFR and its autoregulation, the tubule segments and the counter current mechanism. Learn the three processes of urine formation, the segment grid and the two limbs of Henle's loop, and well over a third of the exercise set is answered before you start.

Four - exercises 3(b), 10, 12(b) and, from the other side, 2 - turn on the hormones. ADH conserves water, aldosterone brings back sodium and water at the distal parts, renin comes from the JG cells and ANF checks the renin-angiotensin mechanism. Two of those four are one-word answers, so they are the cheapest marks in the set.

Five are pure naming - exercises 11(a), 11(b), 11(c), 12(c) and 12(d). Amphioxus, the columns of Bertini, the vasa recta, the nitrogenous wastes, and 25-30 g. Each is worth exactly one mark and takes exactly one line, and there is no partial credit for nearly remembering a name.

Three - exercises 8, 9 and 5 - sit at the two ends of the chapter, at osmoregulation and the excretory patterns on one side and the liver, lungs and skin on the other. Those are the parts students skip because they come before and after the kidney, and they carry three of the twenty-one parts between them.

The remaining two - exercises 6 and 3(a) - are both micturition, asked once as an account and once as a true-or-false. One answer serves for both.

One last thing worth knowing about the exercise set: it never asks about the disorders except through the dialysis gap in 12(c), and it never asks for the gross anatomy of the kidney except through 11(b). Do not read that as permission to skip them. The measurements of the kidney, the columns of Bertini, uremia, renal calculi and glomerulonephritis carry a 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.