Study guide
This chapter is educational content only, not medical advice, and does not guarantee any exam outcome. It follows the official USMLE content outline's organ-system organization, covering cardiac and vascular physiology and pathology, pulmonary mechanics and disease, renal and acid-base physiology, and the pharmacology tied to each system. Step 1 tests these topics through short clinical vignettes that hinge on a single underlying mechanism, so each section below builds from normal physiology toward the disease and drug logic the exam rewards.
Cardiac Physiology and the Pressure-Volume Relationship
The heart's job is to move blood, and Step 1 questions usually test whether you understand what changes one variable does to the others. Cardiac output equals stroke volume times heart rate, and stroke volume depends on preload, afterload, and contractility. Preload is the stretch on the ventricle before contraction, driven mainly by venous return; the Frank-Starling mechanism describes how more stretch (within limits) produces a stronger contraction, because stretched sarcomeres generate more actin-myosin cross-bridges. Afterload is the resistance the ventricle must overcome to eject blood, closely tied to arterial pressure and vascular resistance; a sudden rise in afterload, as in acute severe hypertension, reduces stroke volume. Contractility is the intrinsic force of contraction independent of preload or afterload, increased by sympathetic stimulation (via beta-1 receptors and increased intracellular calcium) and decreased by conditions like ischemia or negative inotropes. A useful invented case: imagine a patient named Mr. Alvarez with longstanding untreated hypertension. His left ventricle faces chronically elevated afterload, so it responds by adding sarcomeres in parallel, a concentric hypertrophy that increases wall thickness to normalize wall stress (per Laplace's law, wall stress equals pressure times radius divided by wall thickness). Over years this stiff, hypertrophied ventricle relaxes poorly, producing diastolic dysfunction, elevated filling pressures, and eventually symptoms of heart failure with preserved ejection fraction. Contrast this with eccentric hypertrophy from chronic volume overload, such as aortic regurgitation, where sarcomeres are added in series, the chamber dilates, and systolic (not diastolic) dysfunction tends to dominate. Recognizing which stimulus (pressure vs. volume overload) produces which remodeling pattern is a recurring mechanism question.
Vascular Pathology and Shock Physiology
Atherosclerosis begins with endothelial injury, which promotes lipid entry, LDL oxidation, and recruitment of monocytes that become lipid-laden macrophages, or foam cells, forming a fatty streak. Over years, smooth muscle proliferation and collagen deposition create a fibrous cap over a lipid-rich necrotic core. A thin, inflamed cap is prone to rupture, exposing thrombogenic material and triggering platelet aggregation and thrombosis, the event that precipitates most myocardial infarctions. Risk factors (hypertension, diabetes, smoking, dyslipidemia) all act by promoting endothelial dysfunction or accelerating lipid deposition. Shock, defined as inadequate tissue perfusion, is classified by underlying mechanism, and Step 1 loves to test the hemodynamic profile of each type. Hypovolemic shock (from hemorrhage or dehydration) and cardiogenic shock (from pump failure, as after a large myocardial infarction) both show low cardiac output with high systemic vascular resistance, as the body vasoconstricts to compensate; they are distinguished by preload, which is low in hypovolemic shock and high in cardiogenic shock (reflected in a high pulmonary capillary wedge pressure). Distributive shock, including septic, anaphylactic, and neurogenic shock, involves pathologic vasodilation, producing low systemic vascular resistance with a compensatory high cardiac output, at least early on. In septic shock, bacterial components trigger a cytokine cascade (TNF-alpha, IL-1) causing widespread vasodilation and capillary leak. Neurogenic shock, from spinal cord injury above roughly T6, is unique in causing both hypotension and bradycardia, because loss of sympathetic outflow leaves unopposed vagal tone. Matching the hemodynamic pattern (cardiac output, systemic vascular resistance, and wedge pressure) to the clinical scenario is the core skill being tested.
Pulmonary Mechanics, Gas Exchange, and Obstructive vs. Restrictive Disease
Lung disease is organized around two patterns that produce distinct spirometry findings. Obstructive diseases (asthma, chronic bronchitis, emphysema) impair airflow out of the lungs, reducing FEV1 more than FVC, so the FEV1/FVC ratio falls below the normal range of about 0.8. Air trapping increases residual volume and total lung capacity. Emphysema specifically destroys alveolar walls and pulmonary capillaries, driven in most smokers by an imbalance of proteases (elastase from neutrophils) and antiproteases; when alpha-1 antitrypsin, the major antiprotease, is deficient, emphysema develops early and often affects the lower lobes rather than the classic smoking-related upper lobes. Restrictive diseases (pulmonary fibrosis, and chest wall or neuromuscular disorders) reduce lung volumes proportionally, so FVC falls more than FEV1, and the FEV1/FVC ratio is normal or increased. Idiopathic pulmonary fibrosis reflects repeated alveolar injury and aberrant healing with excess collagen deposition, stiffening the lung and impairing diffusion. The A-a gradient, the difference between alveolar and arterial oxygen tension, helps localize the problem: a normal A-a gradient with hypoxemia suggests hypoventilation or high altitude (a ventilation problem without a gas-exchange barrier), while a widened A-a gradient points to a diffusion defect, shunt, or ventilation-perfusion (V/Q) mismatch. Consider an invented patient, Mrs. Okafor, presenting with sudden dyspnea and pleuritic chest pain after a long flight; a pulmonary embolism creates dead space (ventilated but underperfused lung) in the affected segment, causing hypoxemia with a widened A-a gradient. Understanding V/Q relationships (high V/Q approaches dead space, low V/Q approaches shunt) lets you predict oxygenation across scenarios from pneumonia (low V/Q, shunt-like) to embolism (high V/Q, dead-space-like).
Renal Physiology, Acid-Base, and Fluid/Electrolyte Mechanisms
The nephron filters, reabsorbs, and secretes to regulate volume, electrolytes, and pH. The proximal tubule reabsorbs about two-thirds of filtered sodium and water, along with essentially all filtered glucose and the bulk of filtered bicarbonate. The thick ascending limb of the loop of Henle reabsorbs sodium, potassium, and chloride via the Na-K-2Cl cotransporter, the target of loop diuretics, and generates the medullary concentration gradient that allows urine concentration. The distal convoluted tubule reabsorbs sodium and chloride via the Na-Cl cotransporter, targeted by thiazide diuretics, and is also where parathyroid hormone promotes calcium reabsorption. The collecting duct responds to aldosterone (which increases sodium reabsorption and potassium/hydrogen secretion via principal and intercalated cells) and to antidiuretic hormone, ADH, which inserts aquaporin-2 channels to allow water reabsorption. Acid-base disorders are interpreted systematically: check pH first to determine acidemia or alkalemia, then check whether the primary disturbance is respiratory (a PCO2 problem) or metabolic (a bicarbonate problem), then assess compensation. Metabolic acidosis is further split by the anion gap: a high anion gap reflects an unmeasured acid (lactic acidosis, diabetic ketoacidosis, uremia, toxic alcohol ingestion), while a normal anion gap (hyperchloremic) acidosis reflects bicarbonate loss, as in diarrhea or renal tubular acidosis. Consider an invented patient, Mr. Petrov, with poorly controlled type 1 diabetes who presents with rapid deep breathing (Kussmaul respirations); his ketoacidosis produces a high anion gap metabolic acidosis, and the rapid breathing is respiratory compensation blowing off CO2 to partially normalize pH. Winter's formula predicts the expected compensatory PCO2; a measured PCO2 far from the predicted value signals a second, superimposed acid-base disorder, a favorite twist in exam vignettes.
Pharmacology: Cardiovascular, Pulmonary, and Renal-Acting Drugs
Antihypertensive and cardiac drug classes are tested by mechanism far more than by brand name. ACE inhibitors block conversion of angiotensin I to angiotensin II, reducing vasoconstriction and aldosterone release; a hallmark side effect, dry cough, results from decreased breakdown of bradykinin, while angioedema is a rarer but dangerous version of the same mechanism. Angiotensin receptor blockers achieve similar hemodynamic effects by blocking the angiotensin II receptor directly, without affecting bradykinin, so they do not cause the cough. Calcium channel blockers reduce calcium entry into vascular smooth muscle and cardiac myocytes; dihydropyridines (like amlodipine) are more vascular-selective and cause reflex tachycardia and peripheral edema, while non-dihydropyridines (verapamil, diltiazem) also slow AV nodal conduction and reduce heart rate. Beta blockers reduce heart rate, contractility, and renin release by blocking beta-1 receptors, and are cautioned in acute decompensated heart failure and reactive airway disease (via beta-2 blockade causing bronchospasm) unless cardioselective. Diuretics are best learned by their tubular site of action: loop diuretics (furosemide) inhibit the Na-K-2Cl transporter, producing the most potent diuresis along with hypokalemia and ototoxicity at high doses; thiazides inhibit the Na-Cl transporter in the distal tubule, causing hypokalemia, hyponatremia, hyperglycemia, hyperlipidemia, and hyperuricemia, and are useful in hypertension and nephrolithiasis from calcium reabsorption effects; potassium-sparing diuretics (spironolactone, a direct aldosterone receptor antagonist, and amiloride, an epithelial sodium channel blocker) reduce potassium and hydrogen secretion, risking hyperkalemia. Pulmonary drugs include beta-2 agonists (bronchodilation via increased cAMP and smooth muscle relaxation), inhaled corticosteroids (reducing airway inflammation), and muscarinic antagonists (reducing bronchoconstriction and secretions) for chronic obstructive pulmonary disease and asthma. Recognizing a drug's site and receptor, rather than memorizing its name alone, is what the mechanism-based question format rewards.
Key terms
- Preload
- — The stretch on ventricular myocardium before contraction, determined chiefly by venous return, which sets stroke volume via the Frank-Starling mechanism.
- Afterload
- — The resistance or pressure the ventricle must overcome to eject blood, closely linked to arterial pressure and systemic vascular resistance.
- Concentric hypertrophy
- — Thickening of the ventricular wall from added sarcomeres in parallel, typically caused by chronic pressure overload such as hypertension, producing diastolic dysfunction.
- Foam cell
- — A lipid-laden macrophage found in atherosclerotic fatty streaks, formed after macrophages ingest oxidized LDL in the injured arterial intima.
- Distributive shock
- — A shock state (septic, anaphylactic, neurogenic) caused by pathologic vasodilation, producing low systemic vascular resistance and initially high cardiac output.
- FEV1/FVC ratio
- — The spirometric ratio used to distinguish obstructive lung disease (reduced ratio) from restrictive lung disease (normal or increased ratio).
- A-a gradient
- — The difference between alveolar and arterial oxygen tension, widened by diffusion defects, shunt, or V/Q mismatch, but normal in pure hypoventilation.
- Anion gap
- — Na+ minus (Cl- plus HCO3-), used to classify metabolic acidosis as high-gap (unmeasured acids) or normal-gap (bicarbonate loss).
- Aldosterone
- — A mineralocorticoid hormone acting on the collecting duct to increase sodium reabsorption and potassium/hydrogen secretion.
- Antidiuretic hormone (ADH)
- — A posterior pituitary hormone that inserts aquaporin-2 channels in the collecting duct to promote water reabsorption.
- Loop diuretic
- — A drug class (e.g., furosemide) that inhibits the Na-K-2Cl cotransporter in the thick ascending limb, producing potent diuresis and hypokalemia.
- Winter's formula
- — A calculation predicting the expected compensatory PCO2 in metabolic acidosis; deviation suggests a mixed acid-base disorder.
Exam tips
- When a vignette gives you a hemodynamic profile (cardiac output, systemic vascular resistance, wedge pressure), classify the shock type from the numbers before reading further into the stem.
- Pressure overload builds sarcomeres in parallel (concentric hypertrophy, diastolic dysfunction); volume overload builds them in series (eccentric hypertrophy, systolic dysfunction) — keep this pairing straight.
- Check the FEV1/FVC ratio first to sort obstructive from restrictive lung disease, then use the A-a gradient to localize a gas-exchange problem versus pure hypoventilation.
- For acid-base questions, work the algorithm in order: pH, primary disturbance, compensation, and (for metabolic acidosis) the anion gap — do not skip steps even when the diagnosis seems obvious.
- Match diuretics to their tubular segment and cotransporter target; the associated electrolyte disturbance (hypokalemia with loops and thiazides, hyperkalemia with potassium-sparing agents) is almost always the tested detail.