The most important blood buffer is

Correct Answer: C

Rationale: Carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻) are the key blood buffer e.g., H₂CO₃ ↔ H + HCO₃⁻ (H-H equation). Water alone doesn't buffer neutral. CO₂/HCO₃⁻ lacks acid form partial. HCO₃⁻/water omits dynamic weak. H₂CO₃/HCO₃⁻ pair distinguishes it, critical for pH control, unlike incomplete sets.

Correct Answer: D

Rationale: Osmosis moves water not ions into interstitium, diluting osmolality (e.g., passive water flow). Active ion transport (ascending Loop) increases osmolality true. Urea diffusion from ducts adds solutes correct. Na /Cl⁻ transport boosts gradient valid. Osmosis's dilution distinguishes it, key to countercurrent dynamics, unlike solute-adding mechanisms.

Correct Answer: A

Rationale: No ADH makes distal tubules/collecting ducts impermeable to water e.g., no aquaporins, dilute urine. H sensitivity is pH not ADH-related. K /Na swap in PCT is false Na dominant. Hypothalamus/Oâ‚‚ is respiratory unrelated. Impermeability distinguishes it, critical for diuresis, unlike sensitivity or swap errors.

Correct Answer: B

Rationale: Aldosterone increases Na reabsorption e.g., distal tubule ENaC, raises BP. It's adrenal not JGA (renin triggers). K is secreted not reabsorbed. H secretion is separate pH effect indirect. Na reabsorption distinguishes it, key to volume control, unlike origin or ion errors.

Correct Answer: A

Rationale: JG cells are adjacent to macula densa e.g., JGA, secrete renin. Urethra is exit tube not cells. Zona glomerulosa produces aldosterone not JG. Stimulation is angiotensin not cells. Proximity distinguishes it, key to renin release, unlike tubes or hormones.