Acid-base homeostasis Practice Questions
19 free Acid-base homeostasis practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
What is the normal physiological arterial blood pH range maintained by acid–base homeostasis in humans?
- A 7.25–7.35
- B 7.30–7.40
- C 7.35–7.45
- D 7.40–7.50
Correct answer: 7.35–7.45
Normal arterial pH is tightly regulated and typically falls between 7.35 and 7.45; values below 7.35 are considered acidemia, above 7.45 alkalemia.
Which buffer system acts as the principal extracellular (plasma) buffer against acid–base disturbances in the body?
- A Phosphate buffer system
- B Plasma protein buffer system
- C Bicarbonate–carbonic acid system
- D Hemoglobin buffer system
Correct answer: Bicarbonate–carbonic acid system
The bicarbonate–carbonic acid buffer system is the primary extracellular buffer in blood, efficiently resisting pH changes by shifting the equilibrium between CO₂, H₂CO₃ and HCO₃⁻.
According to acid–base physiology, if bicarbonate concentration increases or PaCO₂ decreases, what happens to blood pH (assuming other factors constant)?
- A pH decreases
- B pH increases
- C pH remains unchanged
- D pH becomes strongly acidic
Correct answer: pH increases
An increase in bicarbonate or decrease in CO₂ shifts the bicarbonate buffer equilibrium toward less H⁺, raising pH (alkalemia).
Which organ is primarily responsible for long-term (hours to days) regulation of acid–base balance by modifying bicarbonate reabsorption and hydrogen ion excretion?
- A Liver
- B Heart
- C Kidney
- D Skin
Correct answer: Kidney
The kidneys regulate acid–base balance in the long term by reabsorbing bicarbonate and secreting H⁺, thus adjusting plasma bicarbonate concentration.
Which part of the nephron reabsorbs approximately 70–80% of filtered bicarbonate under normal conditions?
- A Distal convoluted tubule
- B Proximal convoluted tubule
- C Collecting duct
- D Loop of Henle
Correct answer: Proximal convoluted tubule
Around 70–80% of the filtered bicarbonate is reabsorbed in the proximal tubule; the remainder is reclaimed farther down the nephron.
What compensatory mechanism does the respiratory system provide in response to a primary metabolic acidosis?
- A Decrease alveolar ventilation to retain CO₂
- B Increase alveolar ventilation to blow off CO₂
- C Activate renal bicarbonate reabsorption immediately
- D Increase hepatic acidogenesis
Correct answer: Increase alveolar ventilation to blow off CO₂
In metabolic acidosis (low HCO₃⁻ / high H⁺), the respiratory system increases ventilation to lower PaCO₂, reducing carbonic acid and helping partially normalize pH.
Which of the following is the fastest acting mechanism to buffer acute changes in blood pH?
- A Renal compensation
- B Respiratory compensation
- C Chemical buffer systems
- D Bone buffering
Correct answer: Chemical buffer systems
Chemical buffer systems (like bicarbonate, phosphate, protein buffers) act within seconds to minimize pH changes, preceding respiratory or renal compensation.
In metabolic alkalosis, which renal adaptation helps restore normal pH over time?
- A Increased H⁺ secretion and HCO₃⁻ reabsorption
- B Decreased H⁺ secretion and increased HCO₃⁻ excretion
- C Increased ammonium production
- D Increased phosphate reabsorption
Correct answer: Decreased H⁺ secretion and increased HCO₃⁻ excretion
During alkalosis, kidneys decrease hydrogen ion secretion and increase bicarbonate excretion to lower plasma pH back toward normal.
What is the major non-volatile acid elimination pathway in kidneys during chronic acid loads (e.g., metabolic acidosis)?
- A Excretion of CO₂ by lungs
- B Ammonium (NH₄⁺) excretion
- C Excretion of bicarbonate
- D Sweat acidification
Correct answer: Ammonium (NH₄⁺) excretion
Excretion of ammonium (NH₄⁺), generated from glutamine metabolism in renal cells, is quantitatively the most important mechanism for eliminating fixed acids in chronic acid load.
Which condition describes a primary respiratory alkalosis on arterial blood gas analysis?
- A Low PaCO₂, high HCO₃⁻
- B High PaCO₂, high HCO₃⁻
- C Low PaCO₂, low HCO₃⁻
- D High PaCO₂, low HCO₃⁻
Correct answer: Low PaCO₂, low HCO₃⁻
In primary respiratory alkalosis, hyperventilation lowers PaCO₂, shifting equilibrium to reduce H⁺, causing pH rise; HCO₃⁻ falls later due to compensatory renal excretion.
Which acid–base disturbance is most likely in a patient with prolonged vomiting (gastric acid loss)?
- A Respiratory acidosis
- B Metabolic acidosis
- C Metabolic alkalosis
- D Respiratory alkalosis
Correct answer: Metabolic alkalosis
Loss of gastric acid (HCl) leads to net loss of H⁺ and chloride, raising blood bicarbonate relative to acid — producing metabolic alkalosis.
Why is the bicarbonate buffer system considered “open” in human physiology?
- A Because it neutralizes only fixed acids
- B Because CO₂ is exhaled by the lungs, restoring buffer components
- C Because bicarbonate is permanently lost from plasma
- D Because it operates only within the kidneys
Correct answer: Because CO₂ is exhaled by the lungs, restoring buffer components
It's called an open buffer because CO₂ (volatile acid) can be removed by the lungs, enabling the system to continuously restore buffer balance rather than reaching equilibrium like a closed system.
What happens to intracellular hydrogen ion concentration when extracellular pH drops (acidemia), if buffers and cellular mechanisms cannot fully compensate?
- A Intracellular pH rises sharply
- B Intracellular pH falls, impairing enzymes
- C Intracellular pH remains unchanged
- D Intracellular pH becomes strongly alkaline
Correct answer: Intracellular pH falls, impairing enzymes
A drop in extracellular pH increases extracellular H⁺; if buffering is overwhelmed, H⁺ enters cells, lowering intracellular pH and potentially disrupting enzyme activity and cellular function.
In chronic respiratory acidosis, what renal adaptation helps mitigate the acid load over days?
- A Decreased HCO₃⁻ reabsorption in tubules
- B Increased HCO₃⁻ generation and reabsorption
- C Reduced ammonium excretion in urine
- D Increased phosphate reabsorption
Correct answer: Increased HCO₃⁻ generation and reabsorption
In chronic respiratory acidosis (elevated CO₂), kidneys increase generation and reabsorption of bicarbonate and excrete more H⁺ (e.g., via ammonium), helping restore pH toward normal.
Which intracellular buffer system contributes significantly to pH regulation inside cells?
- A Bicarbonate buffer system
- B Hemoglobin buffer system
- C Phosphate and protein buffers
- D Ammonia buffer system
Correct answer: Phosphate and protein buffers
Inside cells, phosphate and proteins (with ionizable side chains) serve as major intracellular buffers to resist changes in intracellular H⁺ concentration.
Which of the following best describes metabolic acidosis on arterial blood gas analysis?
- A pH high, HCO₃⁻ high, PaCO₂ low
- B pH low, HCO₃⁻ low, PaCO₂ normal or low
- C pH high, HCO₃⁻ low, PaCO₂ high
- D pH low, HCO₃⁻ high, PaCO₂ normal
Correct answer: pH low, HCO₃⁻ low, PaCO₂ normal or low
Metabolic acidosis is evidenced by low bicarbonate (primary disturbance), resulting in reduced pH; respiratory compensation may lower CO₂ (low PaCO₂), but pH remains acidic.
Why are small changes in plasma H⁺ concentration physiologically significant even though absolute H⁺ concentration is tiny (≈ 40 nmol/L)?
- A Because H⁺ ions transport oxygen in blood
- B Because enzymes and proteins are highly pH-sensitive
- C Because H⁺ ions contribute to blood viscosity
- D Because H⁺ ions determine plasma volume
Correct answer: Because enzymes and proteins are highly pH-sensitive
Enzymes and structural proteins are very sensitive to pH changes; even slight variations in H⁺ concentration can alter protein conformation and impair cellular functions.
Which of the following represents a primary disturbance in respiratory acidosis?
- A Low PaCO₂ due to hyperventilation
- B High HCO₃⁻ due to renal compensation
- C High PaCO₂ due to hypoventilation
- D Low HCO₃⁻ due to metabolic loss
Correct answer: High PaCO₂ due to hypoventilation
Respiratory acidosis begins with elevated PaCO₂ (hypoventilation), increasing carbonic acid and lowering pH; renal compensation (increased HCO₃⁻) may follow if chronic.
Which renal tubular cell type is primarily responsible for H⁺ secretion during chronic acid–base regulation?
- A Proximal tubule principal cells
- B Loop of Henle thin segment cells
- C Collecting duct type A intercalated cells
- D Glomerular mesangial cells
Correct answer: Collecting duct type A intercalated cells
Type A intercalated cells in the distal nephron and collecting duct actively secrete H⁺ into the urine, facilitating acid excretion and bicarbonate generation.