Master Physiology
for USMLE Step 1

What the USMLE Step 1 Tests in Physiology

USMLE Step 1 Physiology tests the application of homeostatic mechanisms to clinical scenarios. Candidates must predict organ system responses to perturbations like haemorrhage, dehydration, acidosis, or drug effects. Emphasis is on cardiovascular, renal, respiratory, and autonomic physiology, with integrated concepts such as baroreceptor reflexes, renal handling of sodium and water, oxygen-hemoglobin dissociation curve shifts, and acid-base compensation. You must interpret graphs (e.g., pressure-volume loops, flow-volume curves) and calculate values like anion gap, fractional excretion of sodium, or creatinine clearance. Questions often present a patient with a specific condition (e.g., Addisonian crisis, congestive heart failure, diabetic ketoacidosis) and ask for the expected physiological change or the mechanism of a compensatory response. Knowledge of normal ranges (e.g., 135-145 mmol/L Na+, 3.5-5.0 mmol/L K+) and first-line management implications (e.g., IV fluids for hypovolaemia) is essential.

High-Yield Concepts

• Baroreceptor Reflex & Haemorrhage: In acute haemorrhage (e.g., 20% blood loss), decreased arterial pressure reduces baroreceptor firing, increasing sympathetic outflow and decreasing parasympathetic tone. This causes tachycardia, vasoconstriction (increased total peripheral resistance), and renin release. First-line management in trauma is isotonic crystalloid (e.g., Hartmann's or 0.9% saline) 1-2 L bolus. Key cut-off: MAP <65 mmHg defines hypotension requiring pressors.
• Renal Handling of Sodium & Water: In SIADH, hyponatraemia (Na+ <135 mmol/L) with urine osmolality >100 mOsm/kg and urine Na+ >20 mmol/L. First-line: fluid restriction (800-1000 mL/day). In diabetes insipidus, polyuria (>50 mL/kg/day) with dilute urine (specific gravity <1.005). Desmopressin test distinguishes central (responds) from nephrogenic (no response).
• Oxygen-Hemoglobin Dissociation Curve: Right shift (decreased affinity) occurs with acidosis (Bohr effect), increased 2,3-BPG, hyperthermia, and anaemia. Left shift (increased affinity) occurs with alkalosis, hypothermia, carbon monoxide poisoning (P50 <26 mmHg). In CO poisoning, PaO2 is normal but O2 content is low; treatment: 100% O2 or hyperbaric oxygen.
• Acid-Base Disorders: For metabolic acidosis, calculate anion gap: Na+ - (Cl- + HCO3-); normal 8-12 mmol/L. High gap: MUDPILES (methanol, uraemia, DKA, propylene glycol, isoniazid, lactic acidosis, ethanol, salicylates). For respiratory acidosis (PaCO2 >45 mmHg), acute: pH drops 0.08 per 10 mmHg rise in PaCO2; chronic: renal compensation increases HCO3- by 4 mmol/L per 10 mmHg.
• Cardiac Cycle & Pressure-Volume Loops: Preload = end-diastolic volume (EDV); afterload = aortic pressure. In aortic stenosis, increased afterload shifts PV loop right, decreases stroke volume. In dilated cardiomyopathy, EDV increases but ejection fraction (<40%) is reduced. First-line for heart failure with reduced EF: ACE inhibitor (e.g., ramipril) and beta-blocker (e.g., bisoprolol).
• Glomerular Filtration Rate & Clearance: eGFR calculated using CKD-EPI or MDRD formula. Normal GFR >90 mL/min/1.73m2. Creatinine clearance = (urine creatinine × urine volume) / plasma creatinine. Fractional excretion of Na+ (FENa) <1% suggests prerenal azotaemia (e.g., dehydration); >2% suggests intrinsic renal failure (e.g., acute tubular necrosis).
• Pulmonary Function Tests: Obstructive pattern: FEV1/FVC <0.7; e.g., COPD (GOLD criteria: FEV1 <80% predicted). Restrictive pattern: FEV1/FVC normal or increased, TLC <80% predicted; e.g., pulmonary fibrosis. Reversibility: >12% and >200 mL increase in FEV1 after bronchodilator indicates asthma.
• Autonomic Nervous System & Drugs: Sympathetic activation: α1 receptors cause vasoconstriction (phenylephrine); β1 increase heart rate and contractility (dobutamine); β2 cause bronchodilation (salbutamol). Parasympathetic: muscarinic receptors slow heart rate (atropine blocks). In phaeochromocytoma, episodic hypertension, palpitations, and sweating; first-line: alpha-blockade (phenoxybenzamine) before beta-blockade.

Common Traps in Physiology Questions

• Confusing the direction of the oxygen-hemoglobin curve shift: right shift decreases affinity (more O2 offloaded), not left shift.
• Assuming a low urine Na+ always indicates prerenal failure; in diuretic use or adrenal insufficiency, it can be misleading.
• Forgetting that in respiratory alkalosis, renal compensation is slow (hours to days), so acute hyperventilation causes acute alkalemia without immediate HCO3- drop.
• Mixing up preload and afterload: preload is ventricular filling pressure (related to venous return), afterload is resistance to ejection (related to arterial pressure).
• Believing that a normal anion gap excludes metabolic acidosis; consider hyperchloraemic acidosis (e.g., from diarrhoea or renal tubular acidosis).
• Using the wrong formula for cardiac output: CO = HR × SV, not HR × BP.

How to Revise Physiology for the USMLE Step 1

Prioritise cardiovascular and renal physiology as they appear most frequently. Practise interpreting graphs (pressure-volume loops, flow-volume curves, and the oxyhaemoglobin dissociation curve) and calculating anion gap, FENa, and creatinine clearance. Questions often present a clinical vignette with vitals and labs; focus on identifying the primary disturbance (e.g., hypovolaemic shock vs. cardiogenic shock) and the expected compensatory mechanism. Memorise key normal values (e.g., pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3- 22-26 mmol/L) and understand the renal response to acid-base changes. Use first principles: apply Starling forces to oedema, the Nernst equation to membrane potentials, and the Frank-Starling mechanism to heart failure. Review autonomic pharmacology and its physiological effects. Practice with timed blocks to simulate exam pressure.

Practise it: MedLumen has 30 Physiology questions for the USMLE Step 1, each with a full explanation and references.