MCAT Physiology Practice Questions with Answers

Mastering human physiology is a cornerstone of success on the Biological and Biochemical Foundations of Living Systems section of the MCAT. This guide provides high-yield MCAT Physiology Practice Questions with Answers designed to test your understanding of organ systems, homeostasis, and cellular mechanisms. By integrating active recall and retrieval practice for medical students, you can bridge the gap between passive reading and exam-day performance.

Concept Explanation

MCAT Physiology is the study of how living systems—ranging from individual cells to complex organ systems—function and interact to maintain a stable internal environment known as homeostasis. The MCAT focuses heavily on how these systems respond to stressors, the feedback loops that regulate them, and the underlying biochemical principles that drive physiological processes. Key areas include the nervous system, endocrine regulation, cardiovascular dynamics, renal filtration, and respiratory gas exchange. Understanding these concepts requires more than memorization; you must be able to predict how a change in one variable, such as blood pH, triggers a cascade of responses across multiple systems like the lungs and kidneys. For a deeper dive into effective learning strategies for these complex topics, check out our guide on retrieval practice for STEM subjects.

Solved Examples

Review these solved examples to understand the logic required for multi-step physiology problems.

Example 1: The Renin-Angiotensin-Aldosterone System (RAAS)

A patient presents with low blood pressure. Describe the physiological cascade that leads to the release of aldosterone.

1. Detection: Low blood pressure is detected by the juxtaglomerular cells in the kidney, which release the enzyme renin into the bloodstream.
2. Activation: Renin cleaves angiotensinogen (produced by the liver) into angiotensin I.
3. Conversion: Angiotensin-converting enzyme (ACE), primarily in the lungs, converts angiotensin I into the potent vasoconstrictor angiotensin II.
4. Response: Angiotensin II stimulates the adrenal cortex to release aldosterone, which increases sodium reabsorption in the distal tubule and collecting duct, leading to water retention and increased blood pressure.

Example 2: Oxygen-Hemoglobin Dissociation Curve

What happens to the affinity of hemoglobin for oxygen when blood pH decreases (acidosis)?

1. The Bohr Effect: A decrease in pH (increase in $[H^+]$) and an increase in $PCO_2$ reduce hemoglobin's affinity for oxygen.
2. Curve Shift: This physiological change results in a "right-shift" of the oxygen-hemoglobin dissociation curve.
3. Unloading: Reduced affinity facilitates the unloading of oxygen to tissues that are metabolically active and producing $CO_2$.

Example 3: Action Potential Propagation

Calculate the net change in membrane potential if a neuron at a resting potential of $-70 \text{ mV}$ experiences a stimulus that allows a net influx of $10^6$ sodium ions, assuming the membrane capacitance is $1 \text{ pF}$. (Note: Use the formula $Q = CV$).

1. Charge Calculation: The charge of one ion is approximately $1.6 \times 10^{-19} \text{ C}$. Total charge $Q = 10^6 \times 1.6 \times 10^{-19} = 1.6 \times 10^{-13} \text{ C}$.
2. Voltage Change: Using $V = \frac{Q}{C}$, where $C = 1 \times 10^{-12} \text{ F}$, the change in voltage $\Delta V = \frac{1.6 \times 10^{-13}}{1 \times 10^{-12}} = 0.16 \text{ V}$ or $160 \text{ mV}$.
3. Final Potential: The membrane would depolarize significantly, likely reaching a positive potential and triggering an action potential.

Practice Questions

1. A researcher inhibits the $Na^+/K^+ \text{-ATPase}$ pump in a group of neurons. What is the most likely immediate effect on the resting membrane potential?
2. During heavy exercise, the body produces lactic acid. How does the respiratory system respond to maintain blood pH within a narrow range?
3. In the nephron, which segment is primarily responsible for the majority of reabsorption of glucose, amino acids, and vitamins?

4. Compare the roles of calcitonin and parathyroid hormone (PTH) in calcium homeostasis. Which one increases bone resorption?
5. A patient has a condition that prevents the closure of the bicuspid (mitral) valve. During which phase of the cardiac cycle would blood most likely flow backward into the left atrium?
6. What is the primary physiological mechanism by which the loop of Henle establishes a concentration gradient in the renal medulla?
7. If the partial pressure of oxygen ($PO_2$) in the alveoli is $100 \text{ mmHg}$ and in the pulmonary capillary blood is $40 \text{ mmHg}$, in which direction will oxygen diffuse?
8. Explain the difference between the absolute and relative refractory periods in terms of ion channel states.
9. How does an increase in plasma osmolarity affect the secretion of Antidiuretic Hormone (ADH) from the posterior pituitary?
10. Which hormone, secreted by the S-cells of the duodenum, stimulates the pancreas to release bicarbonate-rich juice?

Answers & Explanations

1. Answer: The resting membrane potential will become less negative (depolarize). Explanation: The $Na^+/K^+ \text{-ATPase}$ pump maintains the concentration gradient by pumping 3 $Na^+$ out and 2 $K^+$ in. Without it, the gradients dissipate, and the cell loses its negative resting potential.
2. Answer: Hyperventilation (increased breathing rate). Explanation: Lactic acid increases $[H^+]$. The bicarbonate buffer system shifts: $$H^+ + HCO_3^- \rightarrow H_2CO_3 \rightarrow H_2O + CO_2$$ To remove the excess acid, the body increases the rate of $CO_2$ exhalation.
3. Answer: The Proximal Convoluted Tubule (PCT). Explanation: The PCT is the site where nearly all glucose and amino acids are reabsorbed via secondary active transport with sodium. For more on maximizing your study efficiency, see our medical education retrieval practice guide.
4. Answer: Parathyroid hormone (PTH). Explanation: PTH increases blood calcium levels by stimulating osteoclasts to break down bone (resorption). Calcitonin, secreted by the thyroid, acts to "tone down" blood calcium by inhibiting resorption.
5. Answer: Ventricular systole. Explanation: During ventricular systole, the ventricles contract to push blood into the aorta. If the mitral valve fails to close, the high pressure in the left ventricle will force blood back into the low-pressure left atrium.
6. Answer: Countercurrent multiplication. Explanation: The descending limb is permeable to water but not salt, while the ascending limb actively pumps out salt but is impermeable to water. This creates an osmotic gradient in the medulla.
7. Answer: From the alveoli to the pulmonary capillaries. Explanation: Gases diffuse down their partial pressure gradients. Since $100 \text{ mmHg} > 40 \text{ mmHg}$, oxygen moves into the blood.
8. Answer: During the absolute refractory period, voltage-gated $Na^+$ channels are inactivated. During the relative refractory period, they have reset to the closed state, but the membrane is hyperpolarized due to open $K^+$ channels, requiring a stronger stimulus to reach threshold.
9. Answer: ADH secretion increases. Explanation: High osmolarity is detected by osmoreceptors in the hypothalamus, triggering the release of ADH to increase water reabsorption in the collecting ducts, thus diluting the plasma.
10. Answer: Secretin. Explanation: Secretin is released in response to acidic chyme entering the duodenum. It signals the pancreas to release bicarbonate to neutralize the acid, protecting the intestinal lining.

1. Which of the following occurs during the "fight or flight" response triggered by the sympathetic nervous system?

• AIncreased digestion
• BDilation of the pupils
• CConstriction of the bronchioles
• DDecreased heart rate

Frequently Asked Questions

What is the most high-yield physiology topic for the MCAT?

The renal system and the endocrine system are consistently cited as high-yield because they integrate biochemical signaling with organ-level function. Understanding how the kidneys regulate blood pressure and pH is essential for a top score.

How is blood pH regulated by the respiratory system?

The respiratory system regulates pH by adjusting the rate of carbon dioxide exhalation to shift the bicarbonate buffer equilibrium. Increasing ventilation decreases $PCO_2$, which reduces $[H^+]$ and raises blood pH.

What is the difference between peptide and steroid hormones?

Peptide hormones are water-soluble, bind to surface receptors, and act via secondary messengers like cAMP. Steroid hormones are lipid-derived, cross the cell membrane, and bind to intracellular receptors to directly alter gene expression.

How does the heart ensure one-way blood flow?

Heart valves, including the atrioventricular and semilunar valves, open and close in response to pressure changes during the cardiac cycle. This mechanical gating prevents backflow (regurgitation) and ensures blood moves from the atria to the ventricles and out to the arteries.

What is the role of the surfactant in the lungs?

Surfactant is a lipoprotein complex that reduces surface tension at the air-water interface within the alveoli. This prevents the small air sacs from collapsing during exhalation and significantly reduces the work required for breathing.

How does the body respond to low blood glucose?

The alpha cells of the pancreas secrete glucagon, which stimulates the liver to perform glycogenolysis and gluconeogenesis. These processes release glucose into the bloodstream to restore normal levels.