Chapter 16: Excretory Products and their Elimination Long Questions | biology Class 11 CBSE

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Chapter 16: Excretory Products and their Elimination Long Questions | biology Class 11 CBSE | ByteNotes.in

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Question 1

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Describe the structure of the human kidney with reference to its external and internal organization.

Kidneys are reddish-brown, bean-shaped structures located between the last thoracic and third lumbar vertebrae, close to the dorsal inner wall of the abdominal cavity; each measures 10-12 cm long, 5-7 cm wide, 2-3 cm thick, weighing 120-170 g.
The inner concave surface has a notch called the hilum, through which the ureter, blood vessels, and nerves pass; inner to the hilum is the funnel-shaped renal pelvis with projections called calyces.
The outer layer is a tough capsule; inside are two zones — an outer cortex and an inner medulla, the latter divided into medullary pyramids projecting into calyces.
The cortex extends between the medullary pyramids as Columns of Bertini; each kidney contains nearly one million nephrons as functional units.
Each nephron consists of a glomerulus and a renal tubule differentiated into PCT, Henle's loop, and DCT; the DCT opens into collecting ducts that converge into the renal pelvis.
The efferent arteriole from the glomerulus forms the peritubular capillaries and the U-shaped vasa recta, which runs parallel to Henle's loop and is important for concentrating urine.

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Question 2

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Explain the three processes involved in urine formation, highlighting the role of each in producing the final urine.

Urine formation involves glomerular filtration, reabsorption, and tubular secretion, each occurring in distinct nephron regions.
Glomerular filtration is a non-selective ultrafiltration driven by glomerular capillary blood pressure; blood is filtered through three layers (glomerular endothelium, basement membrane, podocytes), yielding protein-free filtrate at 125 ml/min (GFR).
Since 180 litres of filtrate are formed daily but only 1.5 litres of urine excreted, nearly 99% of filtrate is reabsorbed; substances like glucose, amino acids, and Na+ are actively reabsorbed, while nitrogenous wastes are passively reabsorbed.
Tubular secretion involves active secretion of H+, K+, and ammonia by tubular cells into the filtrate, helping maintain ionic and acid-base homeostasis of body fluids.
PCT reabsorbs 70-80% of water and electrolytes; Henle's loop creates the medullary osmotic gradient; DCT performs conditional reabsorption of Na+ and water under hormonal control.
The collecting duct reabsorbs large amounts of water to produce final concentrated urine, aided by the osmotic gradient of the medullary interstitium; human kidneys concentrate urine about four times relative to the initial filtrate.

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Question 3

Long Form

Compare the functions of PCT, Henle's loop, DCT, and collecting duct in tubular reabsorption and secretion.

PCT, lined by brush border epithelium, reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water; it secretes H+ and ammonia and absorbs HCO3- to maintain pH and ionic balance.
The descending limb of Henle's loop is permeable to water but impermeable to electrolytes, concentrating the filtrate; the ascending limb is impermeable to water but actively/passively transports electrolytes, diluting the filtrate.
Henle's loop maintains high medullary osmolarity (up to 1200 mOsmol/L) crucial for the counter current mechanism; reabsorption is minimum in the ascending limb.
DCT performs conditional reabsorption of Na+ and water (under aldosterone control), secretes H+, K+, and NH3, and reabsorbs HCO3- to regulate blood pH and sodium-potassium balance.
The collecting duct reabsorbs large amounts of water (ADH-dependent) to produce concentrated urine and allows small amounts of urea into the medullary interstitium to sustain the osmotic gradient.
Collectively, these segments convert 180 litres of initial filtrate into ~1.5 litres of concentrated urine, representing a ~99% efficiency in reabsorption with precise regulation of body fluid composition.

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Question 4

Long Form

Analyse the counter current mechanism and explain how it helps in the production of concentrated urine.

The counter current mechanism involves the opposite-direction flow of filtrate in the two limbs of Henle's loop and the opposite-direction blood flow in the two limbs of vasa recta, all in close proximity within the medulla.
This arrangement maintains an increasing osmolarity gradient from the cortex (300 mOsmol/L) to the inner medulla (1200 mOsmol/L), driven primarily by NaCl and urea.
NaCl transported by the ascending limb of Henle's loop is exchanged with the descending limb of vasa recta; the ascending vasa recta returns NaCl to the interstitium, maintaining the gradient.
Small amounts of urea enter the thin ascending limb from the interstitium and are transported back to the interstitium by the collecting duct, further sustaining medullary osmolarity.
As the filtrate flows down the descending limb, water is lost to the increasingly concentrated interstitium, concentrating the filtrate; as it ascends, electrolytes are lost, diluting it.
The high osmolarity of the medullary interstitium draws water from the collecting duct (especially in the presence of ADH), concentrating the urine to about four times the initial filtrate — an efficient mechanism for water conservation.

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Question 5

Long Form

Describe the hormonal regulation of kidney function, including the roles of ADH, the renin-angiotensin mechanism, and ANF.

Kidney function is regulated by hormonal feedback involving the hypothalamus, JGA, and the heart; osmoreceptors detect changes in blood volume, fluid volume, and ionic concentration.
Excessive fluid loss activates osmoreceptors, stimulating the hypothalamus to release ADH (vasopressin) from the neurohypophysis; ADH promotes water reabsorption from latter tubule parts, preventing diuresis.
ADH also constricts blood vessels, raising blood pressure and GFR; when body fluid volume increases, osmoreceptors switch off ADH release, completing the negative feedback loop.
A fall in GFR activates JG cells to release renin, which converts angiotensinogen to angiotensin I and then angiotensin II; angiotensin II is a potent vasoconstrictor that raises glomerular blood pressure and GFR.
Angiotensin II also triggers aldosterone release from the adrenal cortex; aldosterone increases Na+ and water reabsorption from the distal tubule, raising blood pressure and GFR — this is the renin-angiotensin mechanism.
ANF, released by the heart atria when blood flow increases, causes vasodilation and lowers blood pressure, acting as a counter-check to the renin-angiotensin mechanism and maintaining cardiovascular-renal homeostasis.

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Question 6

Long Form

Explain the process of hemodialysis. How does it mimic the function of a kidney and what are its limitations?

Hemodialysis is used to remove urea and other nitrogenous wastes from the blood when kidneys fail, a condition called uremia; blood is drawn from a convenient artery after adding anticoagulant heparin.
The blood is pumped into a dialysing unit called an artificial kidney, which contains a coiled cellophane tube surrounded by dialysing fluid.
The dialysing fluid has the same composition as plasma except that it lacks nitrogenous wastes; the porous cellophane membrane allows passage of small molecules based on concentration gradient.
Since nitrogenous wastes are absent in the dialysing fluid, they freely diffuse out of the blood across the membrane, effectively cleaning the blood.
The purified blood is pumped back to the body through a vein after adding anti-heparin to counteract the anticoagulant; this method has been a life-saving boon for thousands of uremic patients.
Hemodialysis is a temporary supportive measure, not a cure; kidney transplantation from a donor (preferably a close relative, to minimize immune rejection) is the definitive treatment for acute renal failure.

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Question 7

Long Form

Compare ammonotelism, ureotelism, and uricotelism in terms of the chemical nature of waste, toxicity, water requirement, and examples of animals.

Ammonotelism involves excretion of ammonia, the most toxic nitrogenous waste; it requires large amounts of water for elimination and is thus restricted to aquatic environments.
Ammonotelic animals include bony fishes, aquatic amphibians, and aquatic insects; ammonia is excreted by diffusion through body/gill surfaces as ammonium ions, with no significant renal role.
Ureotelism involves excretion of urea, which is far less toxic than ammonia; it requires moderate water and is suited for terrestrial and semi-aquatic environments; mammals, many terrestrial amphibians, and marine fishes are ureotelic.
Ammonia is converted to urea in the liver of ureotelic animals; urea enters the blood, is filtered by the kidneys, and excreted; some urea is retained in the kidney matrix to maintain osmolarity.
Uricotelism involves excretion of uric acid, the least toxic form; it can be removed with minimal water loss as pellets or paste, making it ideal for water-scarce terrestrial habitats; reptiles, birds, land snails, and insects are uricotelic.
The evolution from ammonotelism to ureotelism and uricotelism reflects increasing terrestrial adaptation — a progressive reduction in water dependence for nitrogenous waste excretion.

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Question 8

Long Form

Describe the structure of a nephron and explain how the arrangement of its parts is suited to their function.

The nephron is the functional unit of the kidney; it consists of a glomerulus (tuft of capillaries from the afferent arteriole) enclosed in the double-walled Bowman's capsule, together forming the malpighian body for ultrafiltration.
The PCT is lined by brush border epithelium, maximizing surface area for reabsorption of 70-80% of electrolytes, water, and all essential nutrients; its active transport mechanisms ensure essential substances are not lost in urine.
The hairpin-shaped Henle's loop dips into the medulla; the descending limb's water permeability concentrates the filtrate, while the ascending limb's impermeability to water but transport of electrolytes dilutes it, creating the osmotic gradient.
The DCT allows conditional, hormone-regulated reabsorption of Na+ and water, and fine-tunes pH and ionic balance through selective secretion, making it the primary site of regulated homeostatic adjustment.
The collecting duct's length and location in the medulla enable large-scale water reabsorption via ADH, urea recycling for sustaining the medullary gradient, and final acid-base regulation by H+/K+ secretion.
The efferent arteriole forms peritubular capillaries and the vasa recta, ensuring close association between tubule and blood supply for efficient substance exchange; juxtamedullary nephrons with long loops are specially adapted for concentrating urine.

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Question 9

Long Form

Discuss the role of lungs, liver, and skin as accessory excretory organs in humans, giving specific substances eliminated by each.

Although the kidneys are the primary excretory organs in humans, the lungs, liver, and skin also play significant roles in eliminating waste products.
The lungs remove approximately 200 mL per minute of CO2, a byproduct of cellular respiration, and also eliminate significant quantities of water vapor through exhalation every day.
The liver, the largest gland in the body, secretes bile containing bilirubin and biliverdin (breakdown products of hemoglobin), cholesterol, degraded steroid hormones, vitamins, and drugs, which are eliminated with digestive wastes.
The skin contains sweat glands that produce sweat containing NaCl, small amounts of urea, and lactic acid; while sweat's primary role is thermoregulation, it also assists in excretion of these substances.
Sebaceous glands in the skin secrete sebum containing sterols, hydrocarbons, and waxes, providing a protective oily covering while also eliminating these substances; small amounts of nitrogenous wastes can also be excreted through saliva.
These accessory organs collectively complement kidney function, ensuring that a wide variety of metabolic wastes — gaseous, lipid-soluble, and ionic — are efficiently cleared from the body through multiple routes.

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Question 10

Long Form

Analyse the significance of reabsorption in the nephron. Why is it essential that 99% of the filtrate is reabsorbed?

The kidneys filter about 180 litres of filtrate daily but excrete only about 1.5 litres of urine; therefore, nearly 99% of the filtrate must be reabsorbed to prevent fatal loss of water, electrolytes, and essential nutrients.
Essential substances such as glucose and amino acids are completely reabsorbed (primarily in PCT) by active transport; their presence in urine indicates kidney malfunction or metabolic disease.
Electrolytes like Na+, K+, and HCO3- are selectively reabsorbed in PCT, DCT, and collecting duct, maintaining blood ionic balance and pH critical for cellular function.
Water is passively reabsorbed across the initial nephron segments following the osmotic gradient created by solute reabsorption; ADH-regulated reabsorption in DCT and collecting duct adjusts water balance based on body needs.
Without reabsorption, the body would lose enormous amounts of valuable molecules and water, causing dehydration, electrolyte imbalance, and loss of metabolic fuels within minutes.
The selectivity of reabsorption — retaining essential substances while allowing nitrogenous wastes to remain in the filtrate for excretion — exemplifies the kidney's role not merely in excretion but in the precise regulation of body fluid composition.

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Chapter 16: Excretory Products and their Elimination | Long Questions

Describe the structure of the human kidney with reference to its external and internal organization.

Explain the three processes involved in urine formation, highlighting the role of each in producing the final urine.

Compare the functions of PCT, Henle's loop, DCT, and collecting duct in tubular reabsorption and secretion.

Analyse the counter current mechanism and explain how it helps in the production of concentrated urine.

Describe the hormonal regulation of kidney function, including the roles of ADH, the renin-angiotensin mechanism, and ANF.

Explain the process of hemodialysis. How does it mimic the function of a kidney and what are its limitations?

Compare ammonotelism, ureotelism, and uricotelism in terms of the chemical nature of waste, toxicity, water requirement, and examples of animals.

Describe the structure of a nephron and explain how the arrangement of its parts is suited to their function.

Discuss the role of lungs, liver, and skin as accessory excretory organs in humans, giving specific substances eliminated by each.

Analyse the significance of reabsorption in the nephron. Why is it essential that 99% of the filtrate is reabsorbed?

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