Excretory System — Set 2

Correct Answer: D. Glomerulus

• **Glomerulus** = The glomerulus is a specialised tuft of fenestrated capillaries within Bowman's capsule that operates under high hydrostatic pressure to force small molecules (water, urea, glucose, ions) from blood into the capsular space — a process called ultrafiltration. • **High-pressure design** — The efferent arteriole is narrower than the afferent arteriole, which maintains high glomerular pressure (~55 mmHg) necessary for efficient filtration. • 💡 Option A (Vein) is wrong because veins carry blood away from capillaries at low pressure and do not perform filtration; Option B (Capillary bed) is wrong because while the glomerulus is a capillary bed, this generic term does not identify the specific renal structure asked; Option C (Artery) is wrong because arteries are thick-walled vessels that supply blood under pressure but do not themselves filter blood.

Correct Answer: B. Renal Medulla

• **Renal Medulla** = The renal medulla is the inner zone of the kidney consisting of 8–18 cone-shaped renal pyramids; each pyramid contains straight segments of nephron tubules and collecting ducts that converge at the renal papilla. • **Urine flow** — Urine drips from the papilla of each pyramid into the minor calyx, then the major calyx, and finally into the renal pelvis before entering the ureter. • 💡 Option A (Renal Cortex) is wrong because the cortex is the outer layer that contains the glomeruli, Bowman's capsules, and convoluted tubules — not the pyramids; Option C (Renal Capsule) is wrong because the capsule is merely the outermost fibrous covering of the kidney; Option D (Renal Pelvis) is wrong because the pelvis is the funnel-shaped cavity that collects urine and funnels it into the ureter.

Correct Answer: A. Glycosuria

• **Glycosuria** = Glycosuria is the abnormal presence of glucose in urine, occurring when blood glucose exceeds the renal threshold of approximately 180 mg/dL — beyond this level the proximal tubule transporters become saturated and cannot reabsorb all filtered glucose. • **Diabetes indicator** — Glycosuria is a classic sign of uncontrolled diabetes mellitus, though it can also occur in renal glycosuria where the reabsorption threshold is genetically lower. • 💡 Option B (Pyuria) is wrong because pyuria is the presence of pus (white blood cells) in urine, indicating a urinary tract infection; Option C (Anuria) is wrong because anuria means the complete absence of urine production (less than 100 mL/day), signalling severe kidney failure; Option D (Hematuria) is wrong because hematuria is blood in the urine, caused by kidney stones, infection, or injury.

Correct Answer: D. Dialysis

• **Dialysis** = Dialysis is an artificial kidney process that removes metabolic waste products (urea, creatinine), excess electrolytes, and surplus water from the blood when the kidneys can no longer perform this function adequately. • **Two types** — Haemodialysis routes blood through an external machine with a semipermeable membrane, while peritoneal dialysis uses the patient's own peritoneal membrane as the filter. • 💡 Option A (Electrocardiogram) is wrong because an ECG records the electrical activity of the heart and is a diagnostic tool, not a blood-cleaning treatment; Option B (Stethoscope) is wrong because a stethoscope is a listening device used to hear heart and lung sounds; Option C (Endoscopy) is wrong because endoscopy uses a camera to visually inspect internal organs and has no role in blood filtration.

Correct Answer: C. Micturition

• **Micturition** = Micturition (also called urination or voiding) is the coordinated physiological process of emptying the urinary bladder — the detrusor muscle contracts and both the internal (involuntary) and external (voluntary) urethral sphincters relax to release urine. • **Nervous control** — The micturition reflex is initiated by stretch receptors in the bladder wall when volume reaches ~300 mL; the cerebral cortex can override the reflex voluntarily until a convenient time. • 💡 Option A (Peristalsis) is wrong because peristalsis is the wave-like muscle contraction that moves food through the digestive tract and urine through the ureters — not the act of voiding; Option B (Deglutition) is wrong because deglutition means swallowing; Option D (Mastication) is wrong because mastication means chewing food in the mouth.

Correct Answer: D. Red blood cells

• **Red blood cells** = Red blood cells (erythrocytes) are too large (~8 µm diameter) to pass through the glomerular filtration barrier, which has pores of only about 8 nm — along with large plasma proteins, they are retained in the bloodstream. • **Three-layer barrier** — The glomerular membrane consists of endothelial fenestrae, the basement membrane, and podocyte filtration slits — this combination blocks cells and large proteins while allowing water and small solutes through. • 💡 Option A (Urea) is wrong — urea is a small molecule that freely passes through the filtration barrier into the filtrate; Option B (Glucose) is wrong — glucose is filtered but normally completely reabsorbed in the proximal tubule; Option C (Water) is wrong — water is the primary component of the glomerular filtrate, constituting about 99% of the 180 L filtered daily.

Correct Answer: C. Urochrome

• **Urochrome** = Urochrome (urobilin) is the yellow pigment that gives urine its characteristic colour — it is derived from the breakdown of urobilinogen, which itself comes from the degradation of haemoglobin in old red blood cells. • **Concentration clue** — Dark yellow or amber urine indicates dehydration (concentrated urochrome), while very pale or colourless urine indicates high fluid intake or high ADH suppression. • 💡 Option A (Melanin) is wrong because melanin is the brown-black pigment responsible for skin and hair colour and is not excreted in urine; Option B (Hemoglobin) is wrong because haemoglobin is the red iron-containing protein in red blood cells — haemoglobin in urine (haemoglobinuria) causes red-brown colouration and is abnormal; Option D (Chlorophyll) is wrong because chlorophyll is the green pigment in plants used for photosynthesis and has no role in human urine colour.

Correct Answer: A. Renal Capsule

• **Renal Capsule** = The renal capsule is a tough, smooth layer of fibrous connective tissue that tightly encloses the entire kidney, providing structural support and protecting it from physical trauma and infection. • **Layers outside** — Outside the capsule lie the perirenal fat (cushioning) and the renal fascia (which anchors the kidney to the abdominal wall). • 💡 Option B (Renal Pelvis) is wrong because the renal pelvis is the funnel-shaped collecting space inside the kidney where urine gathers before entering the ureter; Option C (Cortex) is wrong because the cortex is the outer functional layer of kidney tissue containing nephron corpuscles — it lies inside the capsule; Option D (Renal Medulla) is wrong because the medulla is the inner zone of the kidney that contains the renal pyramids.

Correct Answer: C. Pelvic cavity

• **Pelvic cavity** = The urinary bladder is a hollow, muscular, distensible organ situated in the pelvic cavity, just behind the pubic symphysis — in females it lies anterior to the uterus; in males, anterior to the rectum. • **Capacity** — The bladder wall is lined with transitional epithelium (urothelium) that allows it to stretch from nearly empty (~50 mL) to full (~400–600 mL) without rupturing. • 💡 Option A (Cranial cavity) is wrong because the cranial cavity houses the brain; Option B (Thoracic cavity) is wrong because the thoracic cavity contains the lungs and heart, separated from the abdomen by the diaphragm; Option D (Abdominal cavity) is wrong because while the kidneys and ureters are in the abdomen, the bladder descends into the pelvic cavity below the pelvic brim.

Correct Answer: C. Skin

• **Skin** = The skin acts as an accessory excretory organ through its sweat glands (eccrine glands), which secrete sweat containing water, sodium chloride, small amounts of urea, lactic acid, and ammonia — helping remove wastes while also regulating body temperature. • **Other accessory organs** — The lungs excrete CO₂ and water vapour; the liver excretes bile pigments; together with the kidneys these form the complete excretory system. • 💡 Option A (Heart) is wrong because the heart is a pumping organ for circulation and has no excretory function; Option B (Stomach) is wrong because the stomach is a digestive organ that secretes acid and enzymes, not an organ of excretion; Option D (Brain) is wrong because the brain is the central nervous system organ responsible for cognition and body regulation — it does not excrete metabolic waste.