Excretory System — Set 1

Correct Answer: B. Nephron

• **Nephron** = The nephron is the microscopic tubular unit of the kidney that filters blood, reabsorbs useful substances, and produces urine — each kidney contains about one million nephrons. • **Two zones** — The renal corpuscle (glomerulus + Bowman's capsule) handles filtration, while the tubular system handles reabsorption and secretion. • 💡 Option A (Neuron) is wrong because neurons are nerve cells that transmit electrical signals, not filtration units; Option C (Alveoli) is wrong because alveoli are tiny air sacs in the lungs for gas exchange; Option D (Villi) is wrong because villi are finger-like projections in the small intestine that absorb nutrients.

Correct Answer: A. Urea

• **Urea** = Humans are ureotelic organisms — the liver converts toxic ammonia (from amino acid breakdown) into less toxic urea via the ornithine (urea) cycle, which is then excreted by the kidneys. • **Solubility advantage** — Urea is highly water-soluble and relatively non-toxic, making it safe to transport in blood until excreted in urine. • 💡 Option B (Ammonia) is wrong because ammonia is the primary waste of aquatic animals like fish — it is far too toxic for humans to accumulate; Option C (Uric acid) is wrong because uric acid is the primary waste of birds and reptiles (uricotelic); Option D (Guanine) is wrong because guanine is found as the nitrogenous waste only in some spiders and insects.

Correct Answer: D. Bowman's capsule

• **Bowman's capsule** = Bowman's capsule is the double-walled, cup-shaped structure that encloses the glomerulus and collects the glomerular filtrate — it marks the very beginning of the nephron tubule. • **Filtration barrier** — The inner (visceral) layer of Bowman's capsule contains specialised cells called podocytes whose foot-processes create tiny filtration slits through which only small molecules pass. • 💡 Option A (Renal pelvis) is wrong because the renal pelvis is the funnel-shaped space where the calyces drain and urine collects before entering the ureter; Option B (Henle's loop) is wrong because the loop of Henle is a U-shaped segment that concentrates urine in the medulla; Option C (Glomerulus) is wrong because the glomerulus is the capillary tuft inside Bowman's capsule, not the capsule itself.

Correct Answer: B. Liver

• **Liver** = The liver synthesises urea through the ornithine cycle (urea cycle), converting highly toxic ammonia — produced from amino acid catabolism — into a safer, water-soluble form for excretion. • **Detoxification role** — This conversion is critical: liver failure leads to ammonia accumulation in blood (hyperammonemia), causing neurological damage. • 💡 Option A (Spleen) is wrong because the spleen destroys old red blood cells and is part of the immune system, not involved in urea production; Option C (Pancreas) is wrong because the pancreas produces digestive enzymes and hormones like insulin, not urea; Option D (Kidney) is wrong because the kidney excretes urea but does not synthesise it.

Correct Answer: C. Ureter

• **Ureter** = The ureter is a paired muscular tube (one per kidney) that carries urine from the renal pelvis down to the urinary bladder using rhythmic peristaltic contractions. • **Length and structure** — Each ureter is about 25–30 cm long and has three muscular layers that prevent back-flow of urine. • 💡 Option A (Urethra) is wrong because the urethra carries urine from the bladder to the outside of the body — it is the final exit tube, not the kidney-to-bladder tube; Option B (Vasa recta) is wrong because vasa recta are straight capillaries inside the kidney that maintain the medullary osmotic gradient; Option D (Distal tubule) is wrong because the distal convoluted tubule is a segment of the nephron inside the kidney.

Correct Answer: C. Osmoregulation

• **Osmoregulation** = Osmoregulation is the homeostatic process by which organisms actively regulate the osmotic pressure of their body fluids, controlling water and ion balance through the kidneys, gills, or skin. • **Kidney's central role** — In humans, the kidneys adjust urine concentration and the hypothalamus monitors blood osmolality, releasing ADH when water conservation is needed. • 💡 Option A (Hemolysis) is wrong because hemolysis is the rupture of red blood cells due to osmotic imbalance, not the regulatory process itself; Option B (Diffusion) is wrong because diffusion is the passive movement of molecules from high to low concentration — it is not an active regulatory process; Option D (Phagocytosis) is wrong because phagocytosis is the engulfing of particles by cells like macrophages, unrelated to osmotic regulation.

Correct Answer: B. Antidiuretic Hormone

• **Antidiuretic Hormone (ADH)** = ADH, also called vasopressin, is released by the posterior pituitary when blood osmolality rises; it inserts aquaporin-2 water channels into the distal tubule and collecting duct cells, dramatically increasing water reabsorption. • **Negative feedback** — When body fluids are diluted (e.g., after drinking water), osmoreceptors in the hypothalamus suppress ADH secretion, allowing more dilute urine to be produced. • 💡 Option A (Thyroxine) is wrong because thyroxine is a thyroid hormone that regulates the basal metabolic rate, not kidney water reabsorption; Option C (Oxytocin) is wrong because oxytocin stimulates uterine contractions and milk ejection, though it shares structural similarity with ADH; Option D (Insulin) is wrong because insulin regulates blood glucose uptake by cells and has no direct role in renal water reabsorption.

Correct Answer: B. Hilum

• **Hilum** = The hilum is the concave notch on the medial border of the kidney where the renal artery enters, the renal vein and lymphatics exit, and the ureter emerges — it is the gateway to the kidney. • **Renal sinus** — Behind the hilum lies the renal sinus, a fat-filled cavity that houses the renal pelvis, calyces, and major vessels. • 💡 Option A (Calyx) is wrong because calyces are cup-shaped extensions of the renal pelvis that collect urine from the pyramids — they are inside the kidney, not the entry point; Option C (Cortex) is wrong because the cortex is the outer granular layer of the kidney containing most of the glomeruli; Option D (Medulla) is wrong because the medulla is the inner striated region containing the renal pyramids and loops of Henle.

Correct Answer: C. Loop of Henle

• **Loop of Henle** = The loop of Henle creates a hypertonic osmotic gradient in the renal medulla through its counter-current multiplier system — the descending limb is permeable to water but not salt, while the ascending limb actively pumps out salt without losing water. • **Concentration power** — This gradient can rise to 1200 mOsm/kg in the deep medulla, allowing humans to produce urine up to four times more concentrated than blood plasma. • 💡 Option A (Proximal tubule) is wrong because the proximal tubule performs bulk reabsorption of nutrients and ions but does not directly concentrate urine; Option B (Glomerulus) is wrong because the glomerulus is where ultrafiltration of blood occurs — it produces filtrate, not concentrated urine; Option D (Bowman's capsule) is wrong because Bowman's capsule simply collects the filtrate draining from the glomerular capillaries.

Correct Answer: A. 6.0

• **pH 6.0** = Normal human urine is slightly acidic with a pH of approximately 6.0 (range 4.5–8.0), primarily because the kidneys excrete excess hydrogen ions and acidic metabolic waste products. • **Diet dependence** — A high-protein diet lowers urine pH (more acidic) while a vegetarian diet tends to raise it toward neutral or slightly alkaline. • 💡 Option B (2.0) is wrong because a pH of 2.0 is extremely acidic — similar to stomach acid — and would damage the urinary tract; Option C (10.0) is wrong because a pH of 10 is strongly alkaline, far outside the normal physiological range for urine; Option D (8.5) is wrong because pH 8.5 is alkaline and only occurs in specific conditions such as urinary tract infections caused by urease-producing bacteria.