What is the fastest system involved in control of [H⁺]?

Correct Answer: B

Rationale: The fastest system controlling [H⁺] is CO₂ excretion by the lungs, acting within seconds to minutes via increased ventilation, reducing PCO₂ and thus H₂CO₃ (H⁺ source). Buffers (C) act instantly but don't eliminate H⁺, kidney H⁺ excretion (A) takes hours, and carbonic anhydrase (D) is a mechanism, not a system, making B correct.

Correct Answer: B

Rationale: Erythropoietin (EPO) release by kidney interstitial cells is triggered by low blood oxygen levels (B) (hypoxia), stimulating red blood cell production in bone marrow to improve oxygen delivery. High oxygen (A) or RBCs (C) suppress it, and COâ‚‚ (D) isn't the trigger, making B correct.

Correct Answer: D

Rationale: Solute uptake and oncotic pressure drive water reabsorption (A), Na/H antiports facilitate sodium and water movement (B), and aquaporin-1 enables water flow (C). All are true—solutes and pressure pull water, Na/H links sodium to water, and aquaporins allow passage—making E (all are correct), reassigned as D, correct.

Correct Answer: A

Rationale: Antifreeze (ethylene glycol) causes metabolic acidosis. Kidneys primarily excrete excess H⁺ via ammonium (NH₄⁺) excretion (A), buffering and eliminating acid in urine. Phosphates (B) help, hyperventilation (C) is respiratory, and free H⁺ (D) is minimal, making A the main mechanism.

Correct Answer: B

Rationale: The urinary bladder is lined with transitional epithelium (B), allowing stretch during filling. Stratified squamous (A) is in skin, ciliated columnar (C) in airways, and cuboidal (D) in tubules, making B the correct lining.