Summary and Exam Tips

The Chapter in One Page

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like $\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Cl^-}$, phosphate and sulphate, either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially, and the process of removing them is called excretion. Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by animals, and the three differ in toxicity and in the amount of water needed to eliminate them, with the two properties running in opposite directions. Ammonia is the most toxic form and requires a large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water.

Ammonotelism is the process of excreting ammonia. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic. Ammonia, as it is 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. Ureotelism is the excretion of urea. Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for the conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the LIVER of these animals and released into the blood, which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity. Uricotelism is the excretion of uric acid. Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water. Osmoregulation, the regulation of the water and ionic content of the body fluids, is carried out by the excretory organs alongside excretion.

A survey of animal kingdom presents a variety of excretory structures. In most of the invertebrates, these structures are simple tubular forms, whereas vertebrates have complex tubular organs called kidneys. PROTONEPHRIDIA or FLAME CELLS are the excretory structures in Platyhelminthes such as Planaria, rotifers, some annelids and the cephalochordate Amphioxus, and they are primarily concerned with ionic and fluid volume regulation, that is osmoregulation. NEPHRIDIA are the tubular excretory structures of earthworms and other annelids, and they help to remove nitrogenous wastes and maintain a fluid and ionic balance. MALPIGHIAN TUBULES are the excretory structures of most of the insects including cockroaches, and they help in the removal of nitrogenous wastes and osmoregulation. ANTENNAL GLANDS or GREEN GLANDS perform the excretory function in crustaceans like prawns.

In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. Kidneys are reddish brown, bean shaped structures situated between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity. Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness, with an average weight of 120-170 g. Towards the centre of the inner concave surface of the kidney is a notch called HILUM, through which ureter, blood vessels and nerves enter. Inner to the hilum is a broad funnel shaped space called the RENAL PELVIS with projections called CALYCES. The outer layer of the kidney is a tough CAPSULE. Inside the kidney, there are two zones - an outer CORTEX and an inner MEDULLA. The medulla is divided into a few conical masses called MEDULLARY PYRAMIDS projecting into the calyces. The cortex extends in between the medullary pyramids as renal columns called COLUMNS OF BERTINI.

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. Glomerulus is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery, and blood from the glomerulus is carried away by an efferent arteriole. The renal tubule begins with a double walled cup-like structure called BOWMAN'S CAPSULE, which encloses the glomerulus, and the glomerulus along with Bowman's capsule is called the MALPIGHIAN BODY or RENAL CORPUSCLE. The tubule continues further to form a highly coiled network - the PROXIMAL CONVOLUTED TUBULE (PCT). A hairpin shaped HENLE'S LOOP is the next part of the tubule, with a DESCENDING and an ASCENDING limb. The ascending limb continues as another highly coiled tubular region called the DISTAL CONVOLUTED TUBULE (DCT). The DCTs of many nephrons open into a straight tube called COLLECTING DUCT, many of which converge and open into the renal pelvis through medullary pyramids in the calyces. The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney whereas the loop of Henle dips into the medulla. In majority of nephrons, the loop of Henle is too short and extends only very little into the medulla - these are called CORTICAL NEPHRONS. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla - these are called JUXTA MEDULLARY NEPHRONS. 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 the Henle's loop forming a U shaped VASA RECTA, and the vasa recta is absent or highly reduced in cortical nephrons.

Urine formation involves three main processes - GLOMERULAR FILTRATION, REABSORPTION and SECRETION - that take place in different parts of the nephron. The first step in urine formation is the filtration of blood, which is carried out by the glomerulus and is called glomerular filtration. 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 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 through these membranes that almost all the constituents of the plasma except the proteins pass onto the lumen of the Bowman's capsule, and therefore it is considered as a process of ULTRA FILTRATION. The amount of the filtrate formed by the kidneys per minute is called GLOMERULAR FILTRATION RATE (GFR). GFR in a healthy individual is approximately 125 mL per minute, that is 180 litres per day. The kidneys have built-in mechanisms for the regulation of glomerular filtration rate, and one such efficient mechanism is carried out by the JUXTA GLOMERULAR APPARATUS (JGA), a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal.

A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres) suggests that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules - this process is called REABSORPTION. The tubular epithelial cells in different segments of the nephron perform this either by active or passive mechanisms. Substances like glucose, amino acids and $\mathrm{Na^+}$ are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport, and reabsorption of water also occurs passively in the initial segments of the nephron. During urine formation, the tubular cells secrete substances like $\mathrm{H^+}$, $\mathrm{K^+}$ and ammonia into the filtrate - TUBULAR SECRETION is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.

Segment by segment, the tubule does this. The PCT is 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 by this segment; and the PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of $\mathrm{H^+}$, ammonia and $\mathrm{K^+}$ into the filtrate and by absorption of $\mathrm{HCO_3^-}$ from it. In Henle's loop, reabsorption is minimum in its ascending limb, but this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes, and this concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively, and therefore, as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes to the medullary fluid. Conditional reabsorption of $\mathrm{Na^+}$ and water takes place in the DCT, which is also capable of reabsorption of $\mathrm{HCO_3^-}$ and selective secretion of $\mathrm{H^+}$ and $\mathrm{K^+}$ ions and ammonia to maintain the pH and sodium-potassium balance in blood. The collecting duct extends from the cortex of the kidney to the inner parts of the medulla; large amounts of water could be reabsorbed from this region to produce a concentrated urine; this segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity; and it also plays a role in the maintenance of pH and ionic balance of blood by selective secretion of $\mathrm{H^+}$ and $\mathrm{K^+}$ ions.

Mammals have the ability to produce a concentrated urine, and the Henle's loop and vasa recta play a significant role in this. 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 vasa recta is also in a counter current pattern. The proximity between the Henle's loop and vasa recta, as well as the counter current in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium, that is from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla. This gradient is mainly caused by $\mathrm{NaCl}$ and urea. $\mathrm{NaCl}$ is transported by the ascending limb of Henle's loop which is exchanged with the descending limb of vasa recta, and it is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop which is transported back to the interstitium by the collecting tubule. The above described transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called the COUNTER CURRENT MECHANISM. This mechanism helps to maintain a concentration gradient in the medullary interstitium, and the presence of such 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 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. Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release ANTIDIURETIC HORMONE (ADH) or VASOPRESSIN from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback. ADH can also affect the kidney function by its constrictory effects on blood vessels, which causes an increase in blood pressure, and an increase in blood pressure can increase the glomerular blood flow and thereby the GFR. The JGA plays a complex regulatory role - a fall in glomerular blood flow, glomerular blood pressure or GFR can activate the JG cells to release renin which 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 GFR. Angiotensin II also activates the adrenal cortex to release ALDOSTERONE, which causes reabsorption of $\mathrm{Na^+}$ and water from the distal parts of the tubule, and this also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the RENIN-ANGIOTENSIN MECHANISM. An increase in blood flow to the atria of the heart can cause the release of ATRIAL NATRIURETIC FACTOR (ANF), which can cause vasodilation, that is dilation of blood vessels, and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism.

Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the urinary bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called MICTURITION and the neural mechanisms causing it is called the MICTURITION REFLEX. An adult human excretes on an average 1 to 1.5 litres of urine per day. The urine formed is a light yellow coloured watery fluid which is slightly acidic, at pH 6.0, and has a characteristic odour. On an average, 25-30 g of urea is excreted out per day. Various conditions can affect the characteristics of urine, and analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney. For example, presence of glucose (GLYCOSURIA) and ketone bodies (KETONURIA) in urine are indicative of DIABETES MELLITUS.

Other than the kidneys, lungs, liver and skin also help in the elimination of excretory wastes. Our lungs remove large amounts of $\mathrm{CO_2}$ - approximately 200 mL per minute - and also significant quantities of water every day. Liver, the largest gland in our body, secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, and most of these substances ultimately pass out along with digestive wastes. The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions. Sweat produced by the sweat glands is a watery fluid containing $\mathrm{NaCl}$, small amounts of urea, lactic acid and so on. Though the primary function of sweat is to facilitate a cooling effect on the body surface, it also helps in the removal of some of the wastes mentioned above - which is why sweating is not primarily an excretory process. Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum, and this secretion provides a protective oily covering for the skin. Small amounts of nitrogenous wastes could be eliminated through saliva too.

Finally, the disorders. Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called UREMIA, which is highly harmful and may lead to kidney failure. In such patients, urea can be removed by a process called HAEMODIALYSIS. Blood drained from a convenient artery is pumped into a dialysing unit called artificial kidney after adding an anticoagulant like heparin. 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, and 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. KIDNEY TRANSPLANTATION is the ultimate method in the correction of acute renal failures, and 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. RENAL CALCULI is a stone or insoluble mass of crystallised salts such as oxalates formed within the kidney. GLOMERULONEPHRITIS is the inflammation of glomeruli of kidney.

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, roughly and approximately stay in front of the figures that carry them.

The kidney

Filtration

Reabsorption and concentration

Urine, and the other organs

Three arithmetic checks worth carrying into the exam hall. One, the GFR converts by multiplying by 60 and then by 24 - 125 mL per minute is 7500 mL per hour and 180000 mL, that is 180 litres, per day, and that single multiplication ties the chapter's two most quoted figures together. Two, the 99 per cent is a subtraction and then a division - 180 litres of filtrate minus 1.5 litres of urine leaves 178.5 litres reabsorbed, and 178.5 out of 180 is nearly 99 per cent. Three, the one-fifth is a ratio against the cardiac output - 1100-1200 mL of blood filtered per minute against a cardiac output of about 5 litres per minute is roughly one-fifth, which is exactly the phrase the chapter uses.

And the four numbers the paper offers you in the same option list, so keep them apart by their unit. 125 mL per minute is a RATE of filtrate formation. 1100-1200 mL per minute is a VOLUME OF BLOOD filtered. 180 litres per day is the filtrate for a DAY. 1 to 1.5 litres per day is the URINE for a day. Say the quantity before you look at the number, every time.

The Address Sheet

One line per structure - what it is and exactly where it sits. Beyond the numbers, most of what this chapter asks is an address, and the paper tests it by moving a structure one station along the tube or into the wrong zone of the kidney.

• Hilum - a notch towards the centre of the inner concave surface of the kidney, through which the ureter, blood vessels and nerves enter.
• Renal pelvis - a broad funnel shaped space lying INNER to the hilum.
• Calyces - the projections of the renal pelvis (singular, calyx). The medullary pyramids project into them.
• Medullary pyramids - the few conical masses into which the MEDULLA is divided, projecting into the calyces. Medullary.
• Columns of Bertini - the renal columns formed where the CORTEX extends in between the medullary pyramids. Cortical.
• Glomerulus - a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery; blood is carried away from it by an efferent arteriole.
• Bowman's capsule - the double walled cup-like structure with which the renal tubule begins, enclosing the glomerulus.
• Malpighian body or renal corpuscle - the glomerulus together with Bowman's capsule. A part of a vertebrate nephron, and nothing to do with the Malpighian tubules of an insect.
• Podocytes - the epithelial cells of Bowman's capsule, arranged in an intricate manner.
• Filtration slits or slit pores - the minute spaces left between the podocytes.
• Proximal convoluted tubule (PCT) - the highly coiled network immediately following Bowman's capsule; lined by simple cuboidal brush border epithelium; in the CORTEX.
• Descending limb of Henle's loop - the limb going down into the medulla; PERMEABLE TO WATER and almost impermeable to electrolytes; the filtrate is concentrated as it moves down.
• Ascending limb of Henle's loop - the limb coming back up; IMPERMEABLE TO WATER but allows the transport of electrolytes actively or passively; the concentrated filtrate gets diluted as it passes upward; reabsorption is minimum here.
• Distal convoluted tubule (DCT) - the second highly coiled region, continuing from the ascending limb; in the CORTEX; conditional reabsorption of $\mathrm{Na^+}$ and water.
• Collecting duct - the straight tube into which the DCTs of many nephrons open; extends from the CORTEX to the INNER PARTS OF THE MEDULLA; many of them converge and open into the renal pelvis through the medullary pyramids in the calyces.
• Peritubular capillaries - the fine capillary network formed around the renal tubule by the efferent arteriole.
• Vasa recta - a minute vessel of the peritubular network that runs parallel to Henle's loop forming a U shape; absent or highly reduced in CORTICAL nephrons.
• Cortical nephrons - the MAJORITY of nephrons; the loop of Henle is too short and extends only very little into the medulla; little or no vasa recta.
• Juxta medullary nephrons - some of the nephrons; the loop of Henle is very long and runs deep into the medulla; a well developed vasa recta.
• Juxta glomerular apparatus (JGA) - a special sensitive region formed by cellular modifications in the DISTAL CONVOLUTED TUBULE and the AFFERENT ARTERIOLE at the location of their contact; its JG cells release renin.

Two lists to say aloud once, because they settle most of the address questions on their own. In the CORTEX - the Malpighian corpuscle, the PCT and the DCT, and the columns of Bertini. In the MEDULLA - the loop of Henle, the medullary pyramids, and the inner end of the collecting duct. Then walk the tube in order, out loud, from the top - Bowman's capsule, PCT, descending limb, ascending limb, DCT, collecting duct, renal pelvis, calyces, ureter - and walk the blood in order too - renal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries and vasa recta. A structure you can place on both walks cannot be moved by an option.

The Hormone Sheet

Four hormones and one enzyme-to-hormone chain run the whole of the regulation section, and every question on it is one row of this table read in one direction or the other. Learn each row as source, then action, then direction.

Read the last column and the whole sheet collapses into one sentence: four of the five push the blood pressure and the GFR UP, and ANF alone pulls them DOWN. That is why an odd-one-out question on this section is almost always ANF, and why an assertion-reason item on it is almost always the opposition between angiotensin II and ANF.

Three sources that are routinely mis-stated, so fix them now. Renin comes from the JG cells, NOT from the adrenal cortex. Aldosterone comes from the adrenal cortex, NOT from the JGA. ADH comes from the neurohypophysis, released under the stimulation of the hypothalamus, NOT from the adrenal gland. And the fourth source is the one students forget the chapter even mentions - ANF comes from the ATRIA OF THE HEART, which is why the chapter says the regulation involves the hypothalamus, the JGA and, to a certain extent, the heart.

One more pairing worth saying aloud. ADH conserves water - it facilitates water reabsorption and prevents diuresis. It does not cause water to be eliminated. A hormone whose very name is ANTI-diuretic cannot be the one that increases urine output, and an option that says it does is the trap the name was built to set.

The Mistakes That Cost Marks in This Chapter

1. Swapping the two limbs of Henle's loop. This is the single commonest error in the chapter, and it is worth -1 in a paper marked +4 and -1. The DESCENDING limb is PERMEABLE TO WATER and almost impermeable to electrolytes, and the filtrate gets CONCENTRATED as it moves down. The ASCENDING limb is IMPERMEABLE TO WATER but allows the transport of electrolytes, and the concentrated filtrate gets DILUTED as it passes upward. Say water goes down, salt comes up once and the swap cannot happen. Reabsorption is minimum in the ascending limb, and yet that same region maintains the high osmolarity of the medullary interstitial fluid - both halves of that sentence get asked.
2. Putting the vasa recta in cortical nephrons. The vasa recta is absent or highly reduced in CORTICAL nephrons. Cortical nephrons are the majority, and in them the loop of Henle is too short and extends only very little into the medulla. It is the JUXTA MEDULLARY nephrons that have a very long loop running deep into the medulla, and a well developed vasa recta beside it. The two facts travel together - a short loop and little or no vasa recta, or a long loop and a full vasa recta - and an option that mixes the halves is wrong however confident it looks.
3. Saying that ADH causes water to be eliminated. It does the opposite. ADH facilitates water REABSORPTION from the latter parts of the tubule, thereby PREVENTING DIURESIS. Its other name, vasopressin, points at its second action - constrictory effects on blood vessels, which increase blood pressure and thereby the glomerular blood flow and the GFR. The hormone that lowers blood pressure in this chapter is ANF, by vasodilation, and it is the only one that does.
4. Sourcing renin from the adrenal cortex. Renin is released by the JG CELLS, when a fall in glomerular blood flow, glomerular blood pressure or GFR activates them. The adrenal cortex is the source of ALDOSTERONE, and it releases it only when angiotensin II activates it. Learn the chain in that order and the two cannot change places - JG cells give renin, renin makes angiotensin II, angiotensin II calls on the adrenal cortex, the adrenal cortex gives aldosterone.
5. Confusing Malpighian tubules with the Malpighian body. They are not related structures and they are not in the same animal. MALPIGHIAN TUBULES are the excretory structures of most of the insects including cockroaches, and they help in the removal of nitrogenous wastes and osmoregulation. The MALPIGHIAN BODY, also called the renal corpuscle, is the glomerulus along with Bowman's capsule in a vertebrate nephron. Tubules belong to an insect; the body belongs to a nephron.
6. Confusing protonephridia with nephridia. Again, different structures in different animals with different jobs. PROTONEPHRIDIA, or FLAME CELLS, are found in Platyhelminthes such as Planaria, in rotifers, in some annelids and in the cephalochordate Amphioxus, and they are primarily concerned with ionic and fluid volume regulation, that is osmoregulation. NEPHRIDIA are the tubular excretory structures of EARTHWORMS and other annelids, and they help to remove nitrogenous wastes and maintain a fluid and ionic balance. Note the one overlap that the paper likes - some annelids appear on BOTH lists, and the chordate on the flame-cell list is Amphioxus.
7. Forgetting that the proteins do not enter Bowman's capsule. Almost all the constituents of the plasma EXCEPT THE PROTEINS pass onto the lumen of the Bowman's capsule, and that exclusion is exactly why glomerular filtration is called ULTRA FILTRATION. Glucose, amino acids, ions, urea and water all pass; the plasma proteins are held back. An option that lists a protein among the constituents of the filtrate is always wrong, and an option that explains ultra filtration by anything other than the retention of the proteins is wrong too.
8. Forgetting that the kidneys play no significant role in removing ammonia. In an ammonotelic animal - many bony fishes, aquatic amphibians and aquatic insects - ammonia, as it is 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. The kidneys become the route only once the animal is ureotelic, when ammonia is converted into urea IN THE LIVER, released into the blood, and filtered and excreted out by the kidneys.
9. Calling sweating a primarily excretory process. It is not. The PRIMARY function of sweat is to facilitate a COOLING EFFECT on the body surface; it also helps in the removal of some wastes, and that secondary role is the only excretory credit it gets. Sweat is a watery fluid containing $\mathrm{NaCl}$, small amounts of urea, lactic acid and so on. Keep it apart from sebum, which the sebaceous glands use to eliminate sterols, hydrocarbons and waxes, and which provides a protective oily covering for the skin.

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. This chapter's exercise set has an unusual mix - alongside the ordinary short-answer questions it carries a true-or-false exercise with five parts, a "name the following" exercise with three parts, and a fill-in-the-gaps exercise with four parts. Each of those three has its own way of being written, and getting the format right is worth as much as knowing the fact.

For a true-or-false item, write the verdict first and then the corrected statement if it is false. Open with the single word - True or False - so the marker sees your decision before anything else, and then, if the verdict is False, write out the corrected version of the whole statement, not just the word that was wrong. "False. The ascending limb of Henle's loop is impermeable to water, not permeable to it; it is the descending limb that is permeable to water and almost impermeable to electrolytes." That answer earns the verdict mark and the correction mark together. A bare "False" earns half of what was on offer, and a correction that only names the wrong word - "not permeable" - leaves the marker to write your answer for you. When the verdict is True, stop there or add one confirming clause; do not hedge a true statement, because a hedge reads as uncertainty and can cost the mark you had already earned.

For a fill-in-the-gaps item, restate the whole sentence with the gap filled. Do not write the missing word on its own next to the letter of the part. Copy the sentence out and put the word in. "The Bowman's capsule along with the glomerulus is called the MALPIGHIAN BODY or RENAL CORPUSCLE." "A cup like structure surrounding the glomerulus is called the BOWMAN'S CAPSULE." There are two reasons this matters and both are practical. The first is that a restated sentence proves you understood the sentence rather than pattern-matching a word, which is what the marker is checking. The second is that a restated sentence lets you write the whole of the answer where the chapter gives an alternative name - Malpighian body OR renal corpuscle - and a marking scheme that accepts either will always accept both.

For a "name the following" item, give the name and one identifying clause, not the name alone. The name by itself is a guess; the name with its identifier is an answer. "The vasa recta - a minute vessel of the peritubular network that runs parallel to Henle's loop, forming a U shape." "The juxta glomerular apparatus - a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact." "The columns of Bertini - the renal columns formed where the cortex extends in between the medullary pyramids." The clause takes six extra words and it does two jobs - it shows the marker that the name was not a lucky guess, and it protects you when your name is slightly off, because a correct description beside an imperfect name usually still scores. Where the chapter gives a structure two names, give both - Malpighian body or renal corpuscle, protonephridia or flame cells, antennal glands or green glands, ADH or vasopressin.

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 - 10-12 cm, 120-170 g, nearly one million, 1100-1200 mL per minute, approximately 125 mL per minute, 180 litres per day, nearly 99 per cent, 70-80 per cent, 300 and about 1200 mOsmol per litre, 1 to 1.5 litres, pH 6.0, 25-30 g, approximately 200 mL per minute - 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 tubule from Bowman's capsule to renal pelvis, the renin chain from the fall in GFR to aldosterone, the micturition reflex from the stretching of the bladder to the release of urine. And when an exercise asks you to draw, describe what you draw in the labels - for the nephron, label the glomerulus, Bowman's capsule, PCT, the descending and ascending limbs of Henle's loop, DCT, collecting duct, afferent and efferent arterioles, peritubular capillaries and vasa recta, and mark which parts lie in the cortex and which dip into the medulla. 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 filtration figures first - 1100-1200 mL of blood filtered per minute, roughly one-fifth of the blood pumped out by each ventricle; 3 layers; approximately 125 mL per minute, that is 180 litres per day - because those are the ones the paper offers you in the same option list. Then the reabsorption figures - nearly 99 per cent by the whole tubule, 70-80 per cent of electrolytes and water by the PCT alone. Then the gradient - 300 mOsmol per litre in the cortex, about 1200 mOsmol per litre in the inner medulla, nearly four times concentrated. Then the kidney and the urine - 10-12, 5-7, 2-3 cm, 120-170 g, nearly one million nephrons, 1 to 1.5 litres, pH 6.0, 25-30 g of urea, 200 mL of carbon dioxide per minute. Write them out from memory and check that every range is still a range.

2. The tube, walked out loud from the top. Fifteen minutes. Bowman's capsule, PCT, descending limb, ascending limb, DCT, collecting duct, renal pelvis - and at each station say the one job that station has and no other station has. Then walk it a second time saying only cortex or medulla at each station. Then walk the blood - renal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries and vasa recta. If you can do all three walks without stopping, every segment question in the paper is already answered.

3. The hormone sheet, rebuilt from the sources. Ten minutes. Name the source, then the action, then up or down - renin from the JG cells, angiotensin II from the blood, aldosterone from the adrenal cortex, ADH from the neurohypophysis, ANF from the atria of the heart. Then say the one line that collapses the sheet - four of them raise the blood pressure and the GFR, and ANF alone lowers it.

4. The nine 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. Spend the longest on the two limbs of Henle's loop and the vasa recta in the two kinds of nephron.

5. The counter current mechanism, as two routes. Ten minutes. The sodium chloride route - transported by the ascending limb of Henle's loop, exchanged with the descending limb of the vasa recta, returned to the interstitium by the ascending portion of the vasa recta. The urea route - small amounts enter the thin segment of the ascending limb of Henle's loop and are transported back to the interstitium by the collecting tubule. Then say why it works at all - two streams running past each other in opposite directions, so a small difference held at every level adds up along the length of the loop into a very large one.

6. The invertebrate structures and the disorders. Five minutes each. Protonephridia in Planaria, rotifers, some annelids and Amphioxus; nephridia in earthworms and other annelids; Malpighian tubules in insects including cockroaches; antennal or green glands in crustaceans like prawns. Then uremia, haemodialysis in its fixed order, transplantation, renal calculi and glomerulonephritis, one line each.

If you have ten minutes and no more, read the number sheet and walk the tube once. This chapter rewards exact figures and correctly placed segments over everything else, and a student who can name what each segment does and reproduce the volumes will out-score one who has read the whole chapter through again.