NCERT Solutions for Class 11 Biology Chapter 16: Excretory Products and their Elimination

Concept. The first step in urine formation is the filtration of blood, carried out by the glomerulus and called glomerular filtration. On an average 1100–1200 mL of blood is filtered by the kidneys per minute, which is roughly one-fifth (1/5th) of the blood pumped out by each ventricle of the heart in a minute. The glomerular capillary blood pressure causes filtration of blood through three layers: the endothelium of the glomerular blood vessels, the epithelium of Bowman’s capsule (formed by podocytes with filtration slits) and a basement membrane between these two layers. Since almost all constituents of plasma except the proteins pass into the lumen of Bowman’s capsule, the process is called ultrafiltration.

Definition. The amount of the filtrate formed by the kidneys per minute is called the glomerular filtration rate (GFR).

GFR in a healthy individual = about 125 mL per minute = about 180 litres per day

Extra depth. Comparing 180 litres of filtrate per day with only 1.5 litres of urine shows that nearly 99% of the filtrate has to be reabsorbed by the renal tubules. GFR is not constant: the kidneys have built-in mechanisms for its regulation, the most efficient being carried out by the juxtaglomerular apparatus (JGA). A fall in GFR activates JG cells to release renin, which helps restore the glomerular blood flow and thereby the GFR to normal. GFR is a measure of kidney function; a very low GFR indicates kidney failure.

Conclusion: GFR is the amount of filtrate formed by the kidneys per minute, about 125 mL per minute or 180 litres per day in a healthy person, of which nearly 99% is reabsorbed to produce 1–1.5 litres of urine.

Concept. The kidneys have built-in mechanisms for the regulation of the glomerular filtration rate. The most efficient of these is carried out by the juxtaglomerular apparatus (JGA). The JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. Because the DCT and afferent arteriole touch each other here, the JGA can sense the blood flow and the composition of the fluid and adjust the glomerular blood flow accordingly — a self-regulating (autoregulatory) system.

Mechanism (the Renin–Angiotensin mechanism)

1. Fall in GFR. A fall in glomerular blood flow, glomerular blood pressure or GFR is detected by the JGA.
2. Release of renin. The juxtaglomerular (JG) cells are activated and release the enzyme renin into the blood.
3. Formation of angiotensin. Renin converts angiotensinogen (a plasma protein) into angiotensin I and further to angiotensin II.
4. Action of angiotensin II. Being a powerful vasoconstrictor, angiotensin II increases the glomerular blood pressure and thereby the GFR, bringing it back to normal.
5. Action of aldosterone. Angiotensin II also activates the adrenal cortex to release aldosterone, which causes reabsorption of Na⁺ and water from the distal parts of the tubule. This increases blood volume, blood pressure and GFR.

Check on the mechanism — ANF. An increase in blood flow to the atria of the heart causes release of atrial natriuretic factor (ANF), which causes vasodilation (dilation of blood vessels) and thereby decreases blood pressure; the ANF mechanism acts as a check on the renin–angiotensin mechanism and prevents over-correction.

Extra depth. The system is a negative feedback loop: when GFR returns to normal the stimulus for renin release disappears. Other regulators include ADH, whose constrictory effect on blood vessels also raises blood pressure and increases glomerular blood flow and GFR.

Conclusion: The JGA, formed at the contact of the DCT and afferent arteriole, autoregulates GFR: a fall in GFR makes JG cells release renin, which through angiotensin II (vasoconstriction) and aldosterone (Na⁺ and water reabsorption) restores the glomerular pressure and GFR, while ANF acts as a check.

Concept. Each statement tests a specific point from Sections 16.2 to 16.6. A True/False answer needs a clear justification; for a False statement the correct form should be written.

Detailed justification

1. (a) True. Urine is stored in the urinary bladder until a voluntary signal is given by the CNS. As the bladder fills, its stretch receptors send signals to the CNS, which passes motor messages to contract the smooth muscles of the bladder and simultaneously relax the urethral sphincter, releasing urine. This is called micturition and the neural mechanism is the micturition reflex.
2. (b) False. ADH (vasopressin) is released from the neurohypophysis when osmoreceptors detect excess loss of fluid. It increases water reabsorption from the latter parts of the tubule and prevents diuresis (large urine volume). So it conserves water, and the urine becomes hypertonic, not hypotonic.
3. (c) True. The glomerular filtrate is plasma without proteins because the filtration membrane (endothelium, basement membrane, podocyte slits) does not allow large protein molecules to pass.
4. (d) True. The Henle’s loop and vasa recta create and maintain the osmotic gradient in the medullary interstitium, which lets water be drawn out of the collecting duct, producing a urine about four times more concentrated than the filtrate.
5. (e) True. The PCT reabsorbs nearly all essential nutrients; glucose, amino acids and Na⁺ are reabsorbed actively (using energy), whereas nitrogenous wastes are absorbed passively.

Extra depth. If the glucose concentration of the filtrate exceeds the tubular capacity to reabsorb it (as in diabetes mellitus), glucose appears in urine (glycosuria).

Conclusion: Statements (a), (c), (d) and (e) are true, while (b) is false because ADH promotes water reabsorption and makes the urine hypertonic (concentrated), not hypotonic.

Concept. Mammals have the ability to produce a concentrated urine, which is an excellent mechanism of conservation of water. The Henle’s loop and the vasa recta play the significant role. 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. The special arrangement of these two U-shaped structures, running close to each other, is called the counter current mechanism.

1. Opposite flows. Filtrate flows down the descending limb and up the ascending limb of Henle’s loop; blood in the vasa recta flows down one limb and up the other, in the opposite direction to the adjacent limb.
2. Building the gradient. The proximity of the loop and vasa recta and the counter current maintain an increasing osmolarity towards the inner medullary interstitium — from about 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla. The gradient is mainly caused by NaCl and urea.
3. Role of NaCl. NaCl is transported out by the ascending limb of Henle’s loop and is exchanged with the descending limb of the vasa recta; it is returned to the interstitium by the ascending portion of the vasa recta.
4. Role of urea. Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.
5. Water movement. The descending limb is permeable to water but almost impermeable to electrolytes, so water leaves and the filtrate becomes concentrated as it moves down; the ascending limb is impermeable to water but lets electrolytes out, so the filtrate is diluted as it moves up.
6. Concentration of urine. The medullary interstitial gradient allows easy passage of water from the collecting tubule (under ADH), so the filtrate is concentrated; human kidneys can produce urine nearly four times more concentrated than the initial filtrate.

Extra depth. Because the vasa recta also runs as a counter current, it carries away the water and reabsorbed solutes without washing out the gradient — blood flow to the medulla does not dissipate the high osmolarity. Juxtamedullary nephrons with long loops of Henle contribute most to this concentration.

Conclusion: The counter current mechanism, based on opposite flows in the limbs of Henle’s loop and vasa recta, maintains an osmotic gradient from 300 to 1200 mOsmol L⁻¹ in the medulla (by NaCl and urea), enabling water reabsorption from the collecting duct and the formation of urine four times more concentrated than the filtrate.

Concept. Besides the kidneys, which are the main excretory organs, the lungs, liver and skin also help in the elimination of excretory wastes. They remove substances that the kidneys do not handle, and together they ensure the removal of gases, bile pigments, salts and heat.

1. Role of the lungs. Our lungs remove large amounts of CO₂ (approximately 200 mL per minute) produced during cellular respiration and also significant quantities of water every day (as water vapour in exhaled air). Thus the lungs excrete the gaseous waste CO₂ (a major waste) and water.

2. Role of the liver. The liver, the largest gland in the body, secretes bile containing substances such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out along with the digestive wastes (faeces). The liver is also the organ in which ammonia is converted into urea in ureotelic animals (including humans), which is then excreted by the kidneys.

3. Role of the skin. The skin has two kinds of glands that help in excretion:

• Sweat glands produce sweat, a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. The primary function of sweat is to facilitate a cooling effect on the body surface, but it also helps in the removal of some wastes.
• Sebaceous glands eliminate substances like sterols, hydrocarbons and waxes through sebum; this secretion also gives a protective oily covering to the skin.

Extra depth. NCERT also mentions that small amounts of nitrogenous wastes can be eliminated through saliva. These organs are complementary: the kidneys regulate water and ion balance and eliminate urea, but they cannot remove CO₂ in quantity, bile pigments or sebum — those are handled by the lungs, liver and skin.

Conclusion: The lungs excrete CO₂ and water, the liver excretes bile pigments, cholesterol, hormone derivatives, vitamins and drugs through bile, and the skin eliminates NaCl, urea and lactic acid through sweat and sterols, hydrocarbons and waxes through sebum.

Concept. The urine formed by the nephrons is passed through the collecting ducts into the renal pelvis, then through the ureters to the urinary bladder, where it is stored. The bladder stores urine until a voluntary signal is given by the central nervous system (CNS).

Micturition is the process of release of urine from the urinary bladder, and the neural mechanism causing it is called the micturition reflex.

1. Filling and stretching. As the bladder fills with urine, its walls are stretched.
2. Sensory signal. The stretch receptors on the walls of the bladder send signals to the CNS.
3. Motor response. The CNS passes on motor messages to initiate the contraction of the smooth muscles of the bladder and the simultaneous relaxation of the urethral sphincter.
4. Release. Urine is expelled from the bladder through the urethra to the outside.

Extra depth. The reflex is partly under voluntary control: in adults the CNS can delay urination until it is convenient, which is why the signal from the CNS is called “voluntary”. Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney.

Conclusion: Micturition is the release of urine from the bladder by the micturition reflex: stretching of the bladder activates stretch receptors, the CNS sends motor signals that contract the bladder muscles and relax the urethral sphincter, and urine is expelled.

Note. In the printed question the last item is also lettered (d); it is the fifth item, i.e. (e) ADH. The terms are explained below.

1. (a) Ammonotelism → (iii) Bony fish. The process of excreting ammonia is ammonotelism; ammonia is the most toxic nitrogenous waste and needs a large amount of water, which is readily available to aquatic animals such as many bony fishes, aquatic amphibians and aquatic insects. Ammonia is excreted by diffusion across body surfaces or gill surfaces as ammonium ions; the kidneys play no significant role.
2. (b) Bowman’s capsule → (v) Renal tubule. The renal tubule begins with the double-walled cup-like Bowman’s capsule that encloses the glomerulus; it then continues as PCT, Henle’s loop and DCT.
3. (c) Micturition → (iv) Urinary bladder. Micturition is the voluntary release of urine stored in the urinary bladder by the micturition reflex.
4. (d) Uricotelism → (i) Birds. Reptiles, birds, land snails and insects excrete uric acid, the least toxic form, with a minimum loss of water; they are called uricotelic.
5. (e) ADH → (ii) Water reabsorption. ADH (vasopressin) from the neurohypophysis facilitates the reabsorption of water from the distal tubule and collecting duct, preventing diuresis.

Conclusion: The correct matches are (a)–(iii), (b)–(v), (c)–(iv), (d)–(i) and ADH–(ii).

Concept. The fluids in the bodies of animals contain water, ions and dissolved solutes, and their concentration (osmolarity) has to be kept within narrow limits for cells to function. Animals live in different environments (fresh water, sea water, land) that tend to disturb this balance, so special mechanisms are required.

Definition. Osmoregulation is the regulation of the osmotic pressure (osmolarity) of body fluids by controlling the water and ionic (salt) content of the body, i.e. the active maintenance of a proper balance of water and electrolytes (ionic and fluid volume regulation) in the body fluids.

• Why needed. A constant internal concentration is necessary for the normal working of cells, tissues and enzymes; loss or gain of water would cause cells to shrink or swell.
• In invertebrates. Protonephridia (flame cells) of flatworms, rotifers, some annelids and Amphioxus are primarily concerned with ionic and fluid volume regulation, i.e. osmoregulation. Nephridia of annelids and Malpighian tubules of insects help in the removal of nitrogenous wastes and in osmoregulation.
• In humans. The kidneys do it. The loop of Henle and vasa recta (counter current mechanism) build the medullary osmotic gradient; the DCT and collecting duct reabsorb water and electrolytes, and ADH controls water reabsorption on feedback from osmoreceptors in the hypothalamus; aldosterone regulates Na⁺ reabsorption; secretion of H⁺, K⁺ and NH₃ maintains ionic and acid-base balance.

Conclusion: Osmoregulation is the maintenance of the proper osmotic concentration of body fluids through control of water and ion balance, performed by structures such as protonephridia, nephridia, Malpighian tubules and, in humans, the kidneys under hormonal control.

Concept. The nature of the nitrogenous waste excreted by an animal depends mainly on its habitat, i.e. the availability of water. Ammonia, urea and uric acid are the three major nitrogenous wastes. They differ in toxicity and in the amount of water required to remove them.

1. Ammonia is highly toxic and highly soluble. It is the most toxic form and cannot be allowed to accumulate in the body; it must be eliminated immediately by diffusing out of body surfaces or gills, which requires a large volume of water to dilute it.
2. Water is scarce on land. Terrestrial animals have to conserve water to prevent dehydration. Ammonotelism would require so much water that it would be impossible on land; terrestrial adaptation therefore necessitated the production of less toxic nitrogenous wastes like urea and uric acid for conservation of water.
3. Ureotelism. In mammals and many terrestrial amphibians the ammonia produced by metabolism is converted into urea in the liver, released into the blood, filtered and excreted by the kidneys; urea is far less toxic, so it can be stored temporarily and excreted in a more concentrated form with less water, and some urea even helps maintain osmolarity in the kidney matrix.
4. Uricotelism. In reptiles, birds, land snails and insects uric acid is excreted as a pellet or paste with a minimum loss of water; this is the best adaptation for extremely dry environments, and for animals with shelled eggs (general knowledge).
5. Ammonia removal needs a medium. Ammonia is excreted by diffusion across body surfaces or gills, which is possible only in water; terrestrial animals have no such continuous watery contact.

Extra depth. The cost of conserving water is energy: conversion of ammonia to urea in the liver, and especially to uric acid, requires ATP. Terrestrial animals accept this energy cost to save water. Marine fishes retain urea to balance the osmolarity of sea water, which is another reason for being ureotelic.

Conclusion: Terrestrial animals are ureotelic or uricotelic and not ammonotelic because ammonia is highly toxic and needs large quantities of water for its removal, whereas urea and uric acid are less toxic and can be excreted with little water, which conserves water on land.

Concept. The juxtaglomerular apparatus (JGA) is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. Its cells include the juxtaglomerular (JG) cells in the afferent arteriole wall, which can sense blood pressure and flow. The JGA is a specialised portion of the nephron which plays a significant role in the regulation of GFR.

1. Regulation of GFR. A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin, which stimulates the glomerular blood flow and thereby brings the GFR back to normal.
2. Renin–angiotensin mechanism. Renin converts angiotensinogen to angiotensin I and then to angiotensin II, a powerful vasoconstrictor, which raises the glomerular blood pressure and hence the GFR.
3. Release of aldosterone. Angiotensin II activates the adrenal cortex to release aldosterone, which causes reabsorption of Na⁺ and water from the distal parts of the tubule, raising blood volume, blood pressure and GFR.
4. Regulation of blood pressure and body fluid volume. By these effects the JGA indirectly regulates blood pressure and the water and sodium balance of the body.
5. Autoregulation. Because it works at the nephron itself, the JGA makes the kidney partly self-regulating, keeping filtration steady in spite of changes in the general blood pressure.

Conclusion: The JGA, formed by the DCT and afferent arteriole, senses a fall in GFR and releases renin, which through angiotensin II and aldosterone restores glomerular blood flow, GFR, blood pressure and sodium and water balance.

Concept. The three parts ask for specific terms from Sections 16.1 and the introduction of the chapter.

1. (a) Amphioxus. It is the only chordate in the NCERT list of animals with protonephridia (flame cells), which are primarily concerned with ionic and fluid volume regulation, i.e. osmoregulation. The other animals having flame cells (flatworms, rotifers, some annelids) are non-chordates.
2. (b) Renal columns or Columns of Bertini. Inside the kidney there are an outer cortex and an inner medulla; the medulla is divided into conical masses called medullary pyramids projecting into the calyces, and the cortex extends between them as renal columns (Columns of Bertini).
3. (c) Vasa recta. The efferent arteriole forms the peritubular capillaries around the renal tubule; a minute vessel of this network runs parallel to Henle’s loop as the U-shaped vasa recta, which is absent or highly reduced in cortical nephrons and works with the loop in the counter current mechanism.

Conclusion: (a) Amphioxus, (b) renal columns (Columns of Bertini), (c) vasa recta.

Concept. The four blanks test facts from the sections on the functions of tubules, regulation of kidney function, haemodialysis and micturition/urine composition.

1. (a) Impermeable; permeable. The ascending limb of Henle’s loop is impermeable to water but allows active or passive transport of electrolytes, so the filtrate is diluted as it moves up; the descending limb is permeable to water but almost impermeable to electrolytes, which concentrates the filtrate as it moves down.
2. (b) ADH (vasopressin). Released from the neurohypophysis when osmoreceptors detect body fluid loss, it increases the water reabsorption from the DCT and collecting duct and prevents diuresis.
3. (c) Nitrogenous wastes (urea). The dialysing fluid in the artificial kidney has the same composition as plasma except for nitrogenous wastes; because these are absent in the fluid, they move freely out of the blood across the cellophane membrane along their concentration gradient, thus clearing the blood.
4. (d) 25–30 gm. On an average, 25–30 g of urea is excreted per day by an adult, along with 1–1.5 L of urine.

Extra depth. In the dialysis unit heparin is added to prevent clotting before blood enters the tube, and anti-heparin is added to the cleaned blood before it returns to the body through a vein.

Conclusion: (a) impermeable, permeable; (b) ADH; (c) nitrogenous wastes (urea); (d) 25–30 g.