USMLE Physiology Practice Questions with Answers

Concept Explanation

USMLE Physiology is the study of how organ systems function and interact within the human body to maintain homeostasis during health and disease. This core discipline requires a deep understanding of the mechanical, physical, and biochemical functions of humans, their organs, and the cells of which they are composed. On the USMLE Step 1 and Step 2 exams, physiology is rarely tested in isolation; instead, it is integrated with pathology and pharmacology to assess clinical reasoning. Key areas include cardiovascular mechanics, renal clearance, respiratory gas exchange, and endocrine feedback loops. Students must transition from memorizing facts to applying principles like the Frank-Starling law or the Henderson-Hasselbalch equation to solve complex clinical vignettes. For those looking to streamline their study process, using a USMLE Prep hub can help organize these high-yield concepts into manageable units.

Solved Examples

Reviewing worked examples helps clarify how mathematical formulas translate into clinical physiological assessments.

1. Calculating Cardiac Output: A patient has a heart rate of 70 beats per minute and a stroke volume of 70 mL. What is the cardiac output?
   1. Identify the formula: $$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$$
   2. Substitute the values: $$\text{CO} = 70 \text{ bpm} \times 70 \text{ mL}$$
   3. Calculate the final result: $$\text{CO} = 4,900 \text{ mL/min} \text{ or } 4.9 \text{ L/min}$$
2. Determining Renal Clearance: Calculate the clearance of a substance if its urinary concentration is 100 mg/dL, the urine flow rate is 2 mL/min, and the plasma concentration is 2 mg/dL.
   1. Identify the formula: $$C = \frac{U \times V}{P}$$
   2. Substitute the values: $$C = \frac{100 \text{ mg/dL} \times 2 \text{ mL/min}}{2 \text{ mg/dL}}$$
   3. Simplify: $$C = \frac{200}{2} = 100 \text{ mL/min}$$
3. Calculating Alveolar Ventilation: A person has a tidal volume of 500 mL, a respiratory rate of 12 breaths/min, and an anatomic dead space of 150 mL. What is the alveolar ventilation?
   1. Identify the formula: $$\dot{V}_A = (V_T - V_D) \times RR$$
   2. Substitute the values: $$\dot{V}_A = (500 \text{ mL} - 150 \text{ mL}) \times 12 \text{ breaths/min}$$
   3. Calculate: $$\dot{V}_A = 350 \text{ mL} \times 12 = 4,200 \text{ mL/min}$$

Practice Questions

1. A 65-year-old male with a history of hypertension is found to have a plasma glucose concentration of 300 mg/dL. If his Glomerular Filtration Rate (GFR) is 100 mL/min, what is the filtered load of glucose?
2. During an experimental study, a subject's left ventricular end-diastolic volume is measured at 140 mL and the end-systolic volume is 50 mL. Calculate the ejection fraction.
3. An individual is trekking at high altitudes where the barometric pressure is lower than at sea level. Which physiological shift in the oxygen-hemoglobin dissociation curve would most likely occur to facilitate oxygen unloading at the tissues?

4. A patient presents with severe diarrhea. Which of the following changes in extracellular fluid (ECF) volume and osmolarity would you expect to see?
5. If the resistance of an arteriole is increased by a factor of 2, by what factor does the blood flow through that vessel decrease, assuming the pressure gradient remains constant?
6. A researcher is studying the effects of a new drug on the cardiac cycle. The drug is found to increase the slope of phase 4 depolarization in the SA node. What is the most likely effect on the heart rate?
7. Calculate the net filtration pressure given the following values: Capillary hydrostatic pressure = 35 mmHg, Interstitial hydrostatic pressure = 2 mmHg, Capillary oncotic pressure = 25 mmHg, and Interstitial oncotic pressure = 1 mmHg.
8. In the context of respiratory physiology, how does the presence of 2,3-bisphosphoglycerate (2,3-BPG) affect hemoglobin's affinity for oxygen?

Answers & Explanations

1. Answer: 300 mg/min. The filtered load is calculated by multiplying the GFR by the plasma concentration of the substance: $$\text{Filtered Load} = GFR \times P_x$$. Here, $$100 \text{ mL/min} \times 3 \text{ mg/mL} (300 \text{ mg/dL} = 3 \text{ mg/mL}) = 300 \text{ mg/min}$$.
2. Answer: 64.3%. Ejection fraction (EF) is the stroke volume divided by the end-diastolic volume. $$SV = EDV - ESV = 140 - 50 = 90 \text{ mL}$$. $$EF = \frac{90}{140} \approx 0.643 \text{ or } 64.3\%$$.
3. Answer: Rightward shift. High altitude triggers an increase in 2,3-BPG production, which shifts the oxygen-hemoglobin dissociation curve to the right, decreasing affinity and facilitating oxygen unloading. You can practice similar logic using the AI Question Generator to test different physiological stressors.
4. Answer: Decreased ECF volume, no change in osmolarity. Diarrhea is an isotonic fluid loss, meaning volume is lost from the ECF, but the concentration of solutes remains the same.
5. Answer: Factor of 16. According to Poiseuille's Law, resistance is inversely proportional to the radius to the fourth power, but flow is inversely proportional to resistance. If resistance increases by 2, flow decreases by 2. (Note: If the radius were halved, resistance would increase by 16).
6. Answer: Increased heart rate. A steeper slope in phase 4 depolarization means the threshold for an action potential is reached faster, leading to more frequent firing of the SA node.
7. Answer: 9 mmHg. Net filtration pressure is calculated as $$(P_c - P_i) - (\pi_c - \pi_i)$$. Thus, $$(35 - 2) - (25 - 1) = 33 - 24 = 9 \text{ mmHg}$$.
8. Answer: Decreases affinity. 2,3-BPG binds to the center of the hemoglobin tetramer, stabilizing the T (taut) state, which encourages the release of oxygen. Understanding these molecular interactions is vital for biology-related science sections across various medical exams.

1. Which of the following factors will cause a rightward shift in the oxygen-hemoglobin dissociation curve?

• ADecreased temperature
• BIncreased pH
• CIncreased \( P_{CO2} \)
• DDecreased 2,3-BPG

Frequently Asked Questions

What is the Frank-Starling Law of the heart?

The Frank-Starling Law states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end-diastolic volume). This occurs because stretching the cardiac muscle fibers increases the force of contraction.

How do the kidneys compensate for respiratory acidosis?

The kidneys compensate for respiratory acidosis by increasing the reabsorption of bicarbonate ions and increasing the secretion of hydrogen ions into the urine. This process typically takes 3 to 5 days to reach maximal effectiveness.

What is the difference between anatomical and physiological dead space?

Anatomical dead space refers to the volume of the conducting airways where no gas exchange occurs, whereas physiological dead space includes both anatomical dead space and functional dead space in alveoli that are ventilated but not perfused.

How does the renin-angiotensin-aldosterone system (RAAS) regulate blood pressure?

RAAS is activated by low blood pressure or low sodium, leading to the production of Angiotensin II, which causes vasoconstriction and stimulates aldosterone release. Aldosterone increases sodium and water retention, effectively raising blood volume and pressure.

What is the role of surfactant in pulmonary physiology?

Surfactant, produced by Type II pneumocytes, reduces surface tension at the air-liquid interface of the alveoli. This prevents alveolar collapse (atelectasis) at the end of expiration and increases lung compliance.