Cardiovascular System: High-Yield Physiology and Pharmacology

Cardiovascular physiology rewards a small set of relationships that show up again and again: the cardiac cycle, the pressure-volume loop, and the levers of preload, afterload, and contractility. Step 1 pairs these with drug mechanisms, asking you to predict how a given class moves the loop or the reflex arc. Master the frameworks below and most vignettes become predictable.

Core Idea

• Cardiac output equals stroke volume times heart rate (CO = SV x HR). Almost every cardiovascular question is really a question about what is happening to one of these terms.
• Stroke volume is set by three levers: preload, afterload, and contractility. Preload and contractility raise SV; increased afterload lowers it.
• Drugs are just tools that push these levers. Knowing a class's effect on preload and afterload lets you predict its effect on the whole system.

The Cardiac Cycle and Heart Sounds

The cycle alternates systole (contraction) and diastole (filling). Ventricular pressure rises during isovolumetric contraction (all valves closed), then the aortic valve opens for ejection. As the ventricle relaxes, the aortic valve closes and isovolumetric relaxation begins before the mitral valve opens for filling.

• S1 = closure of the mitral and tricuspid (AV) valves, marking the start of systole.
• S2 = closure of the aortic and pulmonic (semilunar) valves, marking the start of diastole.
• S3 is an early-diastolic sound tied to rapid ventricular filling (normal in youth, a sign of volume overload or failure in adults).
• S4 is a late-diastolic sound from atrial contraction against a stiff, non-compliant ventricle.

Pressure-Volume Loops

The PV loop plots ventricular pressure against volume for one beat, moving counterclockwise. Its width is stroke volume.

• Increased preload widens the loop to the right (larger end-diastolic volume), raising SV.
• Increased afterload raises the pressure at which the aortic valve opens, reducing ejection and leaving a larger end-systolic volume, so SV falls.
• Increased contractility shifts the end-systolic pressure-volume relationship up and left, squeezing to a smaller end-systolic volume and raising SV and ejection fraction.

Cardiac Output, Frank-Starling, and Ejection Fraction

The Frank-Starling relationship states that increased ventricular filling stretches sarcomeres and increases the force of contraction, so venous return and cardiac output are matched beat to beat. Ejection fraction (EF = SV / EDV) is the fraction of end-diastolic volume ejected; a normal EF is roughly 55 percent or higher. A reduced EF is the hallmark of systolic (HFrEF) heart failure, while a preserved EF with poor filling suggests diastolic (HFpEF) dysfunction.

Action Potentials

• Ventricular myocytes have a fast response: phase 0 rapid Na+ influx, phase 1 brief K+ efflux, phase 2 plateau from Ca2+ influx balancing K+ efflux, phase 3 repolarization by K+ efflux, phase 4 resting potential. The plateau underlies the long refractory period that prevents tetany.
• SA/AV nodal cells have a slow response: phase 4 spontaneous depolarization (funny current and Ca2+) gives automaticity, and phase 0 upstroke is driven by Ca2+, not Na+. This is why calcium channel blockers slow nodal conduction and heart rate.

The Baroreceptor Reflex

Stretch receptors in the carotid sinus and aortic arch sense arterial pressure. A fall in pressure reduces receptor firing, which withdraws parasympathetic tone and increases sympathetic outflow: heart rate, contractility, and vasoconstriction all rise to restore pressure. A rise in pressure does the opposite. This reflex explains compensatory tachycardia after hemorrhage or vasodilator drugs.

Drug Classes by Mechanism

• Beta-blockers reduce sympathetic drive, lowering heart rate and contractility (negative chronotropy and inotropy); they reduce cardiac work and are cornerstone therapy in chronic HFrEF.
• ACE inhibitors / ARBs block angiotensin II, causing arteriolar vasodilation that lowers afterload and, via less aldosterone, reduce fluid retention and preload.
• Calcium channel blockers vasodilate (dihydropyridines lower afterload) or slow the nodes (non-dihydropyridines lower rate and contractility).
• Diuretics reduce blood volume, lowering preload and relieving congestion.
• Nitrates are primarily venodilators that pool blood peripherally, chiefly lowering preload and myocardial oxygen demand.

Congestive Heart Failure Compensation

A failing heart triggers compensatory mechanisms that become maladaptive: the sympathetic system raises rate and contractility, and the renin-angiotensin-aldosterone system (RAAS) retains salt and water, raising preload. Over time this increases afterload and wall stress, worsening failure. This is why therapy blocks these very systems (ACE inhibitors/ARBs, beta-blockers, and diuretics).

High-Yield Exam Patterns

• Expect a PV loop and be asked which change (wider, taller, shifted ESPVR) matches increased preload, afterload, or contractility.
• A vignette giving a drug and asking for its effect on preload vs. afterload is testing the mechanism table above.
• Reflex tachycardia after a vasodilator is a baroreceptor-reflex question; beta-blockade blunts it.
• Distinguishing S3 vs. S4 by timing and by compliant (volume) vs. stiff (pressure) ventricles is a frequent auscultation item.
• Nodal action potentials being Ca2+-dependent explains why non-dihydropyridine CCBs and beta-blockers slow the heart.
• EF < 40 percent points to systolic failure; normal EF with symptoms points to diastolic failure.

Common Traps to Avoid

• Confusing S1/S2 valve pairs — S1 is AV valve closure, S2 is semilunar valve closure.
• Assuming increased afterload raises stroke volume; it lowers SV by opposing ejection.
• Thinking nitrates mainly reduce afterload — they are chiefly venodilators that reduce preload.
• Forgetting that RAAS and sympathetic activation in heart failure are compensatory but ultimately harmful.
• Mixing up EF and stroke volume; EF is a ratio (SV / EDV), not an absolute volume.