Excretory Products and their Elimination

Metabolism leaves behind substances the body cannot use and must remove: ammonia, urea, uric acid, carbon dioxide, water and ions such as Na⁺, K⁺, Cl⁻ and phosphate. The chapter is mainly about the three nitrogenous wastes, which come from the breakdown of amino acids and nucleic acids. The examiner expects you to know their order of toxicity and the amount of water each needs for its removal, because every other fact in this section follows from that.

Ammonia is the most toxic and needs a large volume of water to be flushed out safely. Urea is less toxic and needs less water. Uric acid is the least toxic and is almost insoluble, so it can be passed out as a pellet or paste with a minimum loss of water. Write the sequence 'ammonia > urea > uric acid' for toxicity and the reverse for water conservation.

Animals are named by the main waste they excrete. Ammonotelic animals excrete ammonia: many bony fishes, aquatic amphibians and aquatic insects. Because ammonia is readily soluble, it is removed by diffusion across the body surface or across the gill surfaces in fishes, as ammonium ions; the kidneys play no significant role in its removal. Ureotelic animals excrete urea: mammals, many terrestrial amphibians and marine fishes. In them the ammonia produced by metabolism is converted into urea in the liver, released into the blood, and then filtered and excreted by the kidneys. Some urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity. Uricotelic animals excrete uric acid: reptiles, birds, land snails and insects.

Two comparisons are asked again and again. First, why are terrestrial animals ureotelic or uricotelic and not ammonotelic? Because ammonia would need far more water than a land animal can afford to lose, so it is converted into the less toxic, water-sparing urea or uric acid. Second, why is the tadpole ammonotelic while the adult frog is ureotelic? The tadpole lives in water where ammonia diffuses away freely; the adult spends much of its life on land and must conserve water, so it switches to urea. Both answers are the same idea: the mode of excretion is set by the availability of water.

Before reaching the human kidney, NCERT gives a short survey of the structures other animals use. It is a match-the-following favourite and a source of one 1-mark question in almost every school paper, so learn the pairs as a list.

• Protonephridia or flame cells: Platyhelminthes (for example planaria), rotifers, some annelids and the cephalochordate Amphioxus. They are primarily concerned with ionic and fluid volume regulation, that is, osmoregulation, rather than with nitrogenous waste as such.
• Nephridia: earthworm and other annelids. They remove nitrogenous wastes and maintain fluid and ionic balance.
• Malpighian tubules: most insects, including cockroaches. They remove nitrogenous wastes and carry out osmoregulation.
• Antennal glands or green glands: crustaceans such as prawns.
• Kidneys: vertebrates, which form the complex tubular organs of the rest of the chapter.

Two traps sit in this list. Amphioxus is a chordate, yet it uses flame cells, and NCERT asks you to name 'a chordate animal having flame cells' in the exercise; the answer is Amphioxus, not any fish. And 'nephridia' belongs to the earthworm while 'Malpighian tubules' belongs to the cockroach; the two are swapped in the exam by tired students more often than you would believe.

Keep in mind also that osmoregulation, the regulation of the volume and ionic concentration of body fluids, is a job shared by all of these organs, not only excretion of nitrogen. The NCERT exercise asks you to define it, and the definition is simply: the process by which an organism regulates the water and solute (ionic) content of its body fluids to keep them at a constant level.

The human excretory system has a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. The kidneys are reddish brown, bean-shaped organs that lie between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity. Each kidney is about 10 to 12 cm long, 5 to 7 cm wide and 2 to 3 cm thick, and weighs on average 120 to 170 g. These numbers are asked as 1-mark items.

Towards the centre of the inner concave surface of the kidney is a notch called the hilum, through which the ureter, blood vessels and nerves enter. Inner to the hilum is a broad funnel-shaped space, the renal pelvis, with projections called calyces. The outer layer of the kidney is a tough capsule. Inside it are two zones: an outer cortex and an inner medulla. The medulla is divided into a few conical masses, the medullary pyramids, which project into the calyces. The cortex extends in between the medullary pyramids as renal columns called the columns of Bertini. The NCERT exercise asks you to name 'cortical portions projecting between the medullary pyramids', and the answer is exactly this phrase.

Each kidney has nearly one million complex tubular structures called nephrons, the functional units of the kidney. Urine formed in the nephrons flows through collecting ducts into the calyces, then into the renal pelvis, down the ureter to the bladder, and out through the urethra. When you draw the longitudinal section of the kidney, this path from pyramid to calyx to pelvis to ureter is what the examiner traces with a finger, so the labels must sit in that order.

A point students miss: the cortex is not only the outer shell. It also dips inward between the pyramids as the columns of Bertini, and the renal corpuscles and convoluted tubules of all nephrons sit in cortical tissue while the loops of Henle and collecting ducts run down into the medulla. Holding that picture makes the nephron section much easier.

Each nephron has two parts: the glomerulus and the renal tubule. The glomerulus is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery; blood is carried away from the glomerulus by the efferent arteriole. Note the unusual arrangement: an arteriole leaves a capillary bed, not a venule. This is what keeps the glomerular blood pressure high enough for filtration.

The renal tubule begins with a double-walled cup-like structure, the Bowman's capsule, which encloses the glomerulus. Glomerulus plus Bowman's capsule together form the Malpighian body or renal corpuscle. The tubule continues as the highly coiled proximal convoluted tubule (PCT), then as a hairpin-shaped Henle's loop with a descending limb and an ascending limb, and the ascending limb continues as another coiled region, the distal convoluted tubule (DCT). The DCTs of many nephrons open into a straight tube, the collecting duct. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.

Where does each part lie? The Malpighian corpuscle, PCT and DCT are in the cortex, while the loop of Henle dips into the medulla. This lets NCERT define two kinds of nephron. In the majority of nephrons the loop of Henle is too short and extends only very little into the medulla; these are cortical nephrons. In some nephrons the loop is very long and runs deep into the medulla; these are juxtamedullary nephrons. The juxtamedullary nephrons are the ones that can build a strong medullary gradient and concentrate urine.

Now the blood supply. The efferent arteriole leaving 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. The vasa recta is absent or highly reduced in cortical nephrons. The exercise question 'a loop of capillary running parallel to the Henle's loop' wants 'vasa recta'.

The nephron diagram (glomerulus, afferent and efferent arterioles, Bowman's capsule, PCT, descending and ascending limbs, DCT, collecting duct, peritubular capillaries, vasa recta) is the single most-drawn figure of this chapter. Practise it until you can draw it in three minutes.

Urine formation involves three main processes: glomerular filtration, reabsorption and secretion, which take place in different parts of the nephron. The first step happens in the renal corpuscle.

Blood is filtered by the glomerular capillaries. The glomerular capillary blood pressure pushes plasma through three layers: (1) the endothelium of the glomerular blood vessels, (2) the epithelium of Bowman's capsule, and (3) the basement membrane between these two layers. The epithelial cells of Bowman's capsule are called podocytes. They are arranged in an intricate manner so as to leave 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 into the lumen of Bowman's capsule. This is why the process is called ultrafiltration, and why the exercise statement 'protein-free fluid is filtered from blood plasma into the Bowman's capsule' is true.

The amount of filtrate formed by the kidneys per minute is called the glomerular filtration rate (GFR). In a healthy person it is approximately 125 mL per minute, that is, 180 litres per day. Check the arithmetic once so you can reproduce it: 125 mL × 60 min × 24 h = 180,000 mL = 180 L.

The kidneys have built-in mechanisms for the regulation of GFR. One 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 activates the JG cells to release renin, which can stimulate the glomerular blood flow and bring the GFR back to normal. This is the 'autoregulatory mechanism of GFR' in exercise question 2; the fuller renin-angiotensin story comes in section 16.5.

Compare the volume of filtrate formed per day, 180 L, with the volume of urine released, about 1.5 L. The difference tells you that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This is tubular reabsorption, and the comparison itself is a 2-mark question: 'GFR is 180 L per day but urine is only 1.5 L per day; give a reason'. The reason is that the tubules reabsorb water, glucose, amino acids, Na⁺ and other useful substances back into the blood of the peritubular capillaries, so only about 1 per cent of the filtrate leaves as urine.

The tubular epithelial cells in different segments of the nephron reabsorb by either active or passive mechanisms. Substances like glucose, amino acids and Na⁺ are reabsorbed actively, against their gradient, using ATP. Nitrogenous wastes are absorbed by passive transport. Reabsorption of water occurs passively in the initial segments of the nephron, following the solutes. The exercise statement 'glucose is actively reabsorbed in the proximal convoluted tubule' is therefore true.

The third process runs in the opposite direction. During urine formation the tubular cells secrete substances like H⁺, K⁺ and ammonia from the peritubular blood into the filtrate. This tubular secretion is an important step because it helps in the maintenance of the ionic and acid-base balance of body fluids. When a question asks 'what is the significance of tubular secretion', that phrase, ionic and acid-base balance, is the marking point.

Put the three steps in one line for a 3-mark answer: filtration takes plasma minus proteins into the capsule (180 L/day); reabsorption returns 99 per cent of it, actively for nutrients and Na⁺, passively for water and wastes; secretion adds H⁺, K⁺ and NH₃ to the filtrate to keep the blood's pH and ions in balance. Then say that the fluid that remains, now called urine, passes to the collecting duct.

This section is the segment-by-segment account of reabsorption and secretion and is asked as a 3-mark or 5-mark question with the NCERT diagram of the nephron showing what moves in and out where. Learn the four segments as four short paragraphs.

Proximal convoluted tubule (PCT). It is lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption. Nearly all the essential nutrients, and 70 to 80 per cent of the electrolytes and water, are reabsorbed here. The PCT also maintains the pH and ionic balance of body fluids by selective secretion of H⁺, ammonia and K⁺ into the filtrate and by absorption of HCO₃⁻ from it.

Henle's loop. Reabsorption is minimum in its ascending limb, but the loop plays a significant role in maintaining the high osmolarity of the medullary interstitial fluid. The descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate becomes more concentrated as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively, so as the concentrated filtrate passes upward it gets diluted because electrolytes leave it for the medullary fluid. This is exercise question 12(a): the ascending limb is impermeable to water whereas the descending limb is permeable to it.

Distal convoluted tubule (DCT). Conditional reabsorption of Na⁺ and water takes place here, 'conditional' because it depends on hormones (aldosterone and ADH). The DCT is also capable of reabsorbing HCO₃⁻ and selectively secreting H⁺, K⁺ and NH₃ to maintain the pH and the sodium-potassium balance in blood.

Collecting duct. This long duct extends from the cortex to the inner parts of the medulla. Large amounts of water can be reabsorbed from it to produce a concentrated urine. It also allows the passage of small amounts of urea into the medullary interstitium to keep up the osmolarity there, and it helps maintain pH and ionic balance by selective secretion of H⁺ and K⁺.

Notice the pattern the examiner rewards: PCT does bulk reabsorption; Henle's loop builds the gradient; DCT does hormone-controlled fine tuning; the collecting duct does final water recovery and urea recycling. If you write only 'reabsorption occurs' for each, you get half marks at best.

Mammals have the ability to produce a concentrated urine, and the Henle's loop and the vasa recta play a significant role in this. The starting observation is about direction. The filtrate flows in the two limbs of Henle's loop in opposite directions, down the descending limb and up the ascending limb; this forms a counter current. The blood in the two limbs of the vasa recta also flows in a counter-current pattern. The proximity between the Henle's loop and the vasa recta, together with the counter-current flow, helps in maintaining an increasing osmolarity towards the inner medullary interstitium: from about 300 mOsmolL⁻¹ in the cortex to about 1200 mOsmolL⁻¹ in the inner medulla.

What creates the gradient? Mainly NaCl and urea. NaCl is transported out by the ascending limb of Henle's loop and is exchanged with the descending limb of the vasa recta. NaCl is returned to the interstitium by the ascending portion of the vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop, and this urea is transported back to the interstitium by the collecting tubule. So salt and urea are kept circulating within the medulla instead of being washed away by the blood, and the medulla stays salty. This special arrangement of the Henle's loop and the vasa recta that keeps the gradient is the counter current mechanism.

Why does the gradient matter? Because the collecting duct passes down through this increasingly salty medulla. As the filtrate moves down the duct, water leaves it by osmosis into the concentrated interstitium (provided ADH has made the duct permeable), and the urine becomes concentrated. Human kidneys can produce urine nearly four times as concentrated as the initial filtrate: 1200 divided by 300 gives that factor of four.

For a 5-mark answer draw the NCERT diagram: a long-looped nephron and a parallel vasa recta with the osmolarity numbers 300, 600, 900, 1200 written down the medulla, arrows showing NaCl leaving the ascending limb and entering the descending vasa recta, urea leaving the collecting duct and entering the thin ascending limb, and water leaving the descending limb and the collecting duct. Then write the four points: counter-current flow, gradient of 300 to 1200, gradient built by NaCl and urea, and water reabsorption from the collecting duct giving urine four times as concentrated. That is the marking scheme almost exactly.

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. The old two-mark question 'do you agree that the heart regulates the kidney?' comes from this sentence, and the answer is yes, through ANF.

ADH and the osmoreceptors. 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 activates these osmoreceptors, which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis (posterior pituitary). ADH facilitates water reabsorption from the latter parts of the tubule (DCT and collecting duct) and so prevents diuresis, the loss of water in urine. An increase in body fluid volume can switch off the osmoreceptors and suppress ADH release, completing the negative feedback. ADH can also affect kidney function by its constrictory effect on blood vessels, which raises blood pressure, increases glomerular blood flow and thereby GFR. Exercise statement 3(b), 'ADH helps in water elimination, making the urine hypotonic', is false for exactly this reason: ADH conserves water and makes the urine hypertonic.

The JGA and the renin-angiotensin mechanism. A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin. Renin converts angiotensinogen in the blood to angiotensin I and further to angiotensin II. Angiotensin II, 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 Na⁺ and water from the distal parts of the tubule, which also increases blood pressure and GFR. This complete pathway is the renin-angiotensin mechanism, and the flow chart (fall in GFR → JG cells → renin → angiotensinogen → angiotensin I → angiotensin II → vasoconstriction and aldosterone → Na⁺ and water reabsorption → BP and GFR restored) is asked as 'complete the flow chart' for 2 or 3 marks.

ANF from the heart. An increase in blood flow to the atria of the heart causes the release of atrial natriuretic factor (ANF). ANF causes vasodilation and thereby decreases the blood pressure. The ANF mechanism thus acts as a check on the renin-angiotensin mechanism. Note the opposition: renin-angiotensin raises BP when it is low; ANF lowers it when the atria are over-filled.

Significance of the JGA, the exercise question 10, is answered in one sentence: it regulates GFR through renin release and the renin-angiotensin mechanism, keeping glomerular blood pressure and hence filtration steady, and it triggers aldosterone-driven Na⁺ reabsorption.

The urine formed by the nephrons is carried to the urinary bladder, where it is stored until a voluntary signal from the central nervous system (CNS) allows its release. The signal is initiated by the stretching of the urinary bladder as it fills with urine. In response, stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages that cause the contraction of the smooth muscles of the bladder and the simultaneous relaxation of the urethral sphincter, and urine is released. This process of release of urine is called micturition, and the neural mechanism causing it is the micturition reflex. Exercise statement 3(a), 'micturition is carried out by a reflex', is true; the reflex is under voluntary control in adults, which is why we can hold on.

Now the numbers and characteristics of urine, all 1-mark facts. An adult human excretes on average 1 to 1.5 litres of urine per day. Urine is a light yellow coloured watery fluid, slightly acidic (pH 6.0), with a characteristic odour. On average, 25 to 30 g of urea is excreted per day; this is exercise question 12(d).

Various conditions affect the characteristics of urine, so urine analysis helps in the clinical diagnosis of many metabolic disorders as well as of malfunctioning of the kidney. The presence of glucose in urine is glycosuria and the presence of ketone bodies is ketonuria; together they are indicative of diabetes mellitus. The one-word questions on these two terms, followed by 'which disorder shows both', appear in nearly every set of school papers.

A short comparison the examiner likes: what part is reflex and what part is voluntary? The stretch-receptor signal and the motor response of bladder muscle and sphincter form the reflex arc through the spinal cord; the brain decides when to let the reflex proceed. Write both halves.

The kidneys are not the only excretory organs. NCERT lists four other routes, and exercise question 5 asks for the role of liver, lungs and skin. Give each organ with its named products.

Lungs. They remove large amounts of CO₂, approximately 200 mL per minute, and also significant quantities of water every day, as water vapour in expired air.

Liver. The largest gland of the body secretes bile, which contains substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out along with the digestive wastes in faeces. Remember too that the liver is where ammonia is converted into urea in ureotelic animals, so it is central to nitrogen excretion even though the kidney does the final removal.

Skin. Sweat and sebaceous glands in the skin eliminate certain substances through their secretions. Sweat is a watery fluid containing NaCl, small amounts of urea, lactic acid and other substances. Its primary function is to cool the body surface by evaporation, but it also helps remove some wastes. Sebaceous glands eliminate substances like sterols, hydrocarbons and waxes through sebum, a secretion that provides a protective oily covering for the skin.

Saliva. Small amounts of nitrogenous wastes can be eliminated through saliva as well.

A 3-mark answer to 'role of liver, lungs and skin in excretion' should have three labelled points with the products named. Writing 'lungs remove CO₂' without the water, or 'skin removes sweat' without the NaCl, urea and lactic acid, costs marks. The 200 mL per minute figure for CO₂ is a common 1-mark item.

Malfunctioning of the kidneys leads to the accumulation of urea in the 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, and the working of the dialysis unit is a 3-mark question you should be able to write from memory.

Haemodialysis. Blood drained from a convenient artery is pumped into a dialysing unit after adding an anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a fluid, the dialysing fluid, which has the same composition as plasma except for the nitrogenous wastes. The porous cellophane membrane allows the passage of molecules on the basis of concentration gradient. As nitrogenous wastes are absent in the dialysing fluid, these substances move freely out of the blood, thereby clearing it. The cleared blood is pumped back into the body through a vein after adding anti-heparin to it. Exercise question 12(c), 'dialysis fluid contains all the constituents as in plasma except ...', is answered by 'nitrogenous wastes'.

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 the chances of rejection by the immune system of the host. Modern clinical procedures have increased the success rate of such a complicated technique.

Renal calculi are stones or insoluble masses of crystallised salts, such as oxalates, formed within the kidney. Glomerulonephritis is the inflammation of the glomeruli of the kidney.

School papers like the 'find the term' format for this section: accumulation of urea in blood (uremia); insoluble mass of crystallised salts in the kidney (renal calculi); surgical correction of renal failure (kidney transplantation); inflammation of glomeruli (glomerulonephritis). Have those four ready as single words.