Excretory System — Set 4

Correct Answer: D. Kidney

• **Kidney** = Erythropoietin (EPO) is a glycoprotein hormone secreted by peritubular fibroblasts in the renal cortex when oxygen levels in the blood fall. • **Mechanism** — EPO travels to the bone marrow and stimulates stem cells to differentiate into red blood cells, restoring oxygen-carrying capacity. • The kidneys act as oxygen sensors; high-altitude climbers naturally produce more EPO due to lower O₂ levels. • 💡 Option A (Spleen) is wrong because the spleen filters and destroys old RBCs, not produce EPO; Option B (Heart) is wrong because the heart secretes ANP for blood pressure, not EPO; Option C (Bone marrow) is wrong because it is the production site of RBCs but does not secrete EPO.

Correct Answer: B. Carbon dioxide

• **Carbon dioxide** = CO₂ is the primary gaseous waste produced during cellular respiration; it diffuses from tissues into blood and is expelled by the lungs during exhalation. • **pH balance** — Lungs also regulate blood pH by controlling CO₂ levels; excess CO₂ lowers pH, triggering faster breathing to expel more. • Water vapour is also excreted by the lungs alongside CO₂, making the lungs an excretory organ for gaseous wastes. • 💡 Option A (Uric acid) is wrong because it is excreted in urine via the kidneys; Option C (Ammonia) is wrong because it is converted to urea in the liver and eliminated by kidneys; Option D (Urea) is wrong because it is the major nitrogenous waste dissolved in urine, not a gas.

Correct Answer: C. Renal vein

• **Renal vein** = After filtration, the cleaned blood exits the kidney through the renal vein, which drains directly into the inferior vena cava and returns blood to the right atrium. • **Blood composition** — Renal venous blood has lower urea, creatinine, and excess ions compared to the blood that entered via the renal artery. • Each kidney has one renal vein; the left renal vein is longer than the right as it must cross the midline. • 💡 Option A (Renal artery) is wrong because it carries unfiltered blood from the aorta to the kidney, not away; Option B (Ureter) is wrong because it carries urine from the kidney to the bladder, not blood; Option D (Aorta) is wrong because the aorta is the main artery distributing oxygenated blood from the heart to the body.

Correct Answer: D. External urethral sphincter

• **External urethral sphincter** = It is composed of skeletal (striated) muscle and is under somatic (voluntary) nervous control, allowing a person to consciously delay urination. • **Toilet training** — Children learn to control this sphincter during toilet training; damage to the pudendal nerve eliminates this voluntary control. • The internal urethral sphincter (smooth muscle) contracts involuntarily to prevent leakage, while the external sphincter provides the final conscious override. • 💡 Option A (Internal urethral sphincter) is wrong because it is made of smooth muscle under autonomic control, not voluntary; Option B (Pyloric sphincter) is wrong because it controls food passage from stomach to small intestine; Option C (Cardiac sphincter) is wrong because it is located between the oesophagus and stomach, unrelated to urination.

Correct Answer: C. Urinary bladder

• **Urinary bladder** = It is a hollow, muscular, and distensible organ lined with transitional epithelium that can expand to hold urine until micturition is convenient. • **Capacity** — The adult bladder holds 300–500 mL comfortably; the urge to urinate typically begins around 150–200 mL filling. • The detrusor muscle in the bladder wall contracts during urination; its coordination with sphincters is controlled by the micturition centre in the sacral spinal cord. • 💡 Option A (Urethra) is wrong because it is the final passage for urine to exit the body, not for storage; Option B (Ureter) is wrong because it is the tube that transports urine from kidney to bladder; Option D (Renal pelvis) is wrong because it is the funnel-shaped collecting space inside the kidney, not a storage organ.

Correct Answer: D. Regulating blood pressure

• **Regulating blood pressure** = RAAS is a cascade-hormonal system activated when blood pressure or sodium levels fall; it ultimately increases blood pressure and fluid retention. • **Cascade steps** — Renin (kidney) → cleaves angiotensinogen to angiotensin I → converted to angiotensin II (lungs) → stimulates aldosterone (adrenal cortex) → kidneys retain sodium and water, raising blood pressure. • ACE inhibitors (common antihypertensive drugs) work by blocking the conversion of angiotensin I to II, disrupting this system. • 💡 Option A (Lowering blood sugar) is wrong because that is insulin's role, secreted by the pancreas; Option B (Memory storage) is wrong because that is a brain function involving the hippocampus; Option C (Digestion of proteins) is wrong because that involves pepsin and trypsin in the GI tract.

Correct Answer: B. Urethra

• **Urethra** = The urethra is the terminal duct of the urinary system through which urine is expelled from the bladder to the exterior during micturition. • **Sexual dimorphism** — In males it is approximately 20 cm long (also serves reproductive function); in females it is only 3–4 cm, explaining the higher UTI susceptibility in women. • The urethra is guarded by two sphincters — internal (involuntary) and external (voluntary) — that coordinate to control voiding. • 💡 Option A (Vas deferens) is wrong because it is the male reproductive duct that carries sperm, not urine; Option C (Ureter) is wrong because it connects the kidney to the bladder, not the bladder to outside; Option D (Collecting duct) is wrong because it is an internal nephron segment within the kidney, not a connecting structure.

Correct Answer: D. Increase production

• **Increase production** = Both caffeine and alcohol act as diuretics — they increase urine output by inhibiting ADH (antidiuretic hormone), preventing the kidneys from reabsorbing water. • **ADH suppression** — Without adequate ADH, the collecting ducts remain impermeable to water, causing dilute, high-volume urine; this leads to dehydration if fluids are not replaced. • Excessive alcohol consumption causes thirst and headaches largely due to this diuretic-induced dehydration. • 💡 Option A (Decrease production) is wrong because diuretics increase, not decrease, urine; Option B (No effect) is wrong because both substances have a well-documented pharmacological diuretic effect; Option C (Stop production) is wrong because anuria (no urine) results from kidney failure, not caffeine or alcohol.

Correct Answer: B. Renal cortex

• **Renal cortex** = It is the granular-appearing outer zone of the kidney, located just beneath the fibrous renal capsule, and contains approximately one million renal corpuscles (glomeruli + Bowman's capsules) and convoluted tubules. • **Filtration zone** — Most glomerular filtration and proximal/distal tubular reabsorption occurs in the cortex; it also receives the richest blood supply of any organ per gram of tissue. • Cortical nephrons (located in the cortex) outnumber juxtamedullary nephrons; the latter are crucial for concentrating urine. • 💡 Option A (Renal sinus) is wrong because it is the central fat-filled cavity housing vessels and the renal pelvis; Option C (Renal calyx) is wrong because it is a cup-shaped passage that collects urine from medullary pyramids; Option D (Renal pelvis) is wrong because it is the funnel-shaped basin where urine collects before entering the ureter.

Correct Answer: D. Uremia

• **Uremia** = Uremia (or uraemia) is the toxic accumulation of urea and other nitrogenous waste products in the blood due to severe kidney dysfunction or failure. • **Symptoms** — Uremia causes nausea, vomiting, fatigue, mental confusion, and pericarditis; if untreated it is fatal; dialysis or kidney transplant is required. • Normal blood urea nitrogen (BUN) is 7–20 mg/dL; uremia is diagnosed when levels are dangerously elevated alongside other symptoms. • 💡 Option A (Anemia) is wrong because it refers to low haemoglobin or red blood cell count, not urea; Option B (Leukemia) is wrong because it is a cancer of blood-forming tissues involving white blood cells; Option C (Insomnia) is wrong because it is a sleep disorder with no direct relation to urea levels.