RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Metabolism never stops producing waste, and this chapter is about the machinery that keeps that waste from poisoning the very cells that made it. It compares ammonia, urea and uric acid as different solutions animals have evolved to the same problem, then zooms into the human kidney and nephron to show, step by step, how blood is filtered, useful substances reclaimed, and urine concentrated to whatever degree the body's water balance demands.
Birds and reptiles excrete nitrogenous waste mainly as uric acid because this strategy chiefly serves to:
Answer
Conserve water is correct — uric acid is nearly insoluble and can be excreted as a semi-solid paste with minimal water loss, an important adaptation for animals that lay shelled eggs or live in water-scarce environments and cannot rely on constant dilution as ammonotelic aquatic animals do.
The tuft of capillaries enclosed within the Bowman's capsule, responsible for the initial filtration of blood, is called the:
Answer
Glomerulus is correct — it is a knot of fine capillaries fed by the wider afferent arteriole and drained by the narrower efferent arteriole, and the resulting pressure difference forces plasma components through its walls into the surrounding Bowman's capsule as filtrate.
The hormone that increases the permeability of the distal convoluted tubule and collecting duct to water, producing concentrated urine, is:
Answer
Antidiuretic hormone (ADH) is correct — released from the posterior pituitary in response to dehydration or rising blood osmotic pressure, it makes the DCT and collecting duct more permeable to water, so more water is reabsorbed and a smaller volume of concentrated urine is produced.
Most reabsorption of glucose, amino acids and water from the glomerular filtrate occurs in the:
Answer
Proximal convoluted tubule is correct — the PCT reabsorbs nearly all of the glucose and amino acids and the bulk of the filtered water and ions back into the blood, leaving the loop of Henle and later segments to fine-tune the final concentration of urine.
Assertion (A): A fall in glomerular filtration rate activates the juxtaglomerular apparatus to release renin. Reason (R): Renin ultimately promotes formation of angiotensin II, which raises blood pressure and restores glomerular filtration rate toward normal.
Answer
Both A and R are true and R is the correct explanation of A — renin released by the JGA converts angiotensinogen to angiotensin I, which is converted to angiotensin II; this raises blood pressure both by vasoconstriction and by stimulating aldosterone release, and the resulting rise in blood pressure restores the glomerular filtration rate that had fallen.
Differentiate between ammonotelism, ureotelism and uricotelism, giving one example of an animal that shows each.
Answer
Ammonotelic animals, such as most bony fish and aquatic amphibians, excrete nitrogenous waste directly as ammonia, which is highly toxic but is safely and rapidly diluted away by the large volume of surrounding water. Ureotelic animals, such as mammals including humans, convert the more toxic ammonia into the far less toxic urea in the liver before excretion, allowing it to be safely transported in blood and stored temporarily in the body without immediate elimination. Uricotelic animals, such as birds, reptiles and insects, convert nitrogenous waste into non-toxic uric acid, which can be excreted as a semi-solid paste with minimal loss of water, a valuable adaptation for animals with limited water access or those developing inside shelled eggs.
Explain the counter-current mechanism of the loop of Henle and its role in producing concentrated urine.
Answer
The loop of Henle is a long, hairpin-shaped tubule with a descending limb that is permeable to water but not very permeable to salts, and an ascending limb that is impermeable to water but actively pumps out Na+ and Cl- ions into the surrounding medullary tissue. As filtrate flows down the descending limb, water is progressively drawn out into the increasingly salty interstitium, concentrating the filtrate; as it then flows up the ascending limb, salts are actively removed while water is retained, diluting the filtrate again but leaving the surrounding medullary tissue increasingly concentrated toward the inner medulla. This flow of filtrate in opposite directions in the two limbs, working against blood flowing in a similarly opposite pattern in the neighbouring vasa recta capillaries, is called the counter-current mechanism, and it builds up a strong osmotic gradient in the medulla that the collecting duct can then exploit, under the influence of ADH, to draw water out of the urine and produce a small volume of highly concentrated final urine.
Describe the structure of a nephron and explain the three sequential processes involved in the formation of urine.
Answer
A nephron, the structural and functional unit of the kidney, consists of two main parts. The Bowman's capsule is a cup-shaped structure that encloses a tuft of capillaries, the glomerulus, fed by an afferent arteriole and drained by a narrower efferent arteriole; together this is called the malpighian body. Leading away from the Bowman's capsule is a long, coiled tubule differentiated into the proximal convoluted tubule (PCT), the loop of Henle with its descending and ascending limbs, the distal convoluted tubule (DCT), and finally the collecting duct, which receives filtrate from many nephrons and carries it toward the renal pelvis. Urine formation proceeds in three stages. First, glomerular filtration: the narrower efferent arteriole compared with the afferent arteriole maintains high pressure in the glomerular capillaries, forcing water and small dissolved solutes through the filtration membrane into the Bowman's capsule as filtrate, while blood cells and large proteins are retained in the blood. Second, tubular reabsorption: as the filtrate passes especially through the PCT, but also through later segments, useful substances such as glucose, amino acids, and the great majority of water, sodium and other ions are actively or passively reabsorbed back into the surrounding blood capillaries, since almost all of the roughly 180 litres of filtrate produced daily would otherwise be lost. Third, tubular secretion: as filtrate moves through the DCT, the tubule cells actively secrete additional substances such as hydrogen ions, potassium ions, ammonia and certain drugs from the blood into the filtrate, which helps maintain the ionic and acid-base balance of the blood. The fluid that remains after all three processes, now called urine, passes through the collecting ducts into the renal pelvis and out via the ureter to the bladder.
Explain how hormones regulate kidney function, and describe the principle behind haemodialysis used to treat kidney failure.
Answer
Kidney function is closely regulated by several hormones acting on the nephron to maintain the body's water and ion balance. Antidiuretic hormone (ADH), also called vasopressin, is released from the posterior pituitary when the body is dehydrated or blood osmotic pressure rises; it increases the permeability of the DCT and collecting duct to water, promoting greater water reabsorption and the production of a smaller volume of concentrated urine, helping to conserve body water. Aldosterone, secreted by the adrenal cortex, acts mainly on the DCT to stimulate reabsorption of sodium ions (with water following osmotically) and secretion of potassium ions, which helps raise blood pressure and blood volume when these fall too low; its release is itself triggered by the renin-angiotensin mechanism, in which a fall in glomerular filtration rate causes the juxtaglomerular apparatus to release renin, which converts angiotensinogen to angiotensin I and then, via a converting enzyme, to angiotensin II, a hormone that both constricts blood vessels directly and stimulates aldosterone release. When kidney function fails and these regulatory processes can no longer keep nitrogenous wastes and excess fluid from accumulating in the blood, a condition called uremia results, and patients require haemodialysis. In this procedure, blood is withdrawn from the patient and passed through a dialysing unit containing a selectively permeable membrane immersed in a dialysing fluid of similar composition to normal plasma but free of nitrogenous wastes; because these wastes are present at a higher concentration in the patient's blood than in the dialysing fluid, they diffuse out across the membrane down their concentration gradient, while useful substances at similar concentrations on both sides are not lost, and the purified blood is then returned to the patient's body.
A patient presents with severe flank pain that comes in waves, and imaging reveals a small, hard, crystalline mass lodged in the ureter. Blood tests in a separate patient, who has long-standing untreated hypertension and diabetes, show steadily rising blood urea levels over several years, and the doctor recommends regular dialysis sessions. (a) Name the condition affecting the first patient and identify what such masses are usually composed of. (b) Explain, in terms of kidney structure, why a mass lodged in the ureter causes such severe pain. (c) Name the condition affecting the second patient's blood chemistry and the general term for kidney failure of this kind. (d) Briefly explain the principle by which dialysis would help the second patient.
Answer
(a) The first patient has a renal calculus, commonly called a kidney stone, and such stones are usually composed of crystallised deposits of substances such as calcium oxalate that have precipitated out of the urine instead of remaining dissolved. (b) The ureter is a narrow, muscular tube that normally transports urine by rhythmic contractions; a hard mass lodged within it obstructs this flow and stretches and irritates the muscular wall as it is forced against the mass during these contractions, producing the intense, wave-like pain characteristic of a stone attempting to pass. (c) The rising blood urea level in the second patient indicates uremia, the accumulation of nitrogenous waste in the blood due to inadequate kidney filtration, and this progressive loss of kidney function over years of untreated hypertension and diabetes is a form of chronic kidney failure. (d) Dialysis works because the patient's blood, high in urea and other wastes, is passed alongside a dialysing fluid across a selectively permeable membrane; since the wastes are far more concentrated in the blood than in the fluid, they diffuse out of the blood down their concentration gradient into the dialysing fluid, and the cleaned blood is then returned to the body, substituting for the filtering role the patient's own kidneys can no longer adequately perform.
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