The cardiac cycle and circulation

MCAT content library / Biology & biochemistry / The cardiac cycle and circulation

Connect four-chamber anatomy, valve pressure differences and volume conservation to explain how blood circulates.

About 17 minutes, including the worked examples and checks.

Useful foundations

• Autonomic control of organs

On this page

• Trace a complete circuit before interpreting a pressure
• Concept: a valve follows its local pressure difference
• Worked application: infer valve states from snapshots
• Calculate output without counting the same blood twice
• Pattern: vessel structure changes pressure and velocity
• Application: distinguish pumping from resistance
• Check your reasoning
• Continue learning
• Sources

Trace a complete circuit before interpreting a pressure

A useful circulation model has two pumps arranged in series. The right heart supplies the pulmonary circuit; the left heart supplies the systemic circuit. Each pump generates pressure, and valves favor forward flow. The AAMC asks you to connect this anatomy with pressure, vascular properties and physiological regulation.¹

Blood returning from systemic tissues enters the right atrium, crosses the tricuspid valve into the right ventricle, and leaves through the pulmonary valve toward the lungs. It returns through pulmonary veins to the left atrium, crosses the mitral valve into the left ventricle, and exits through the aortic valve into the systemic circulation. A septum separates the right and left sides.²

An artery carries blood away from the heart; a vein returns it. These names describe direction, so pulmonary arteries can carry blood with less oxygen than pulmonary veins. The left ventricle normally generates a greater pressure than the right because the systemic circuit presents greater resistance. Greater pressure does not mean that its sustained volume output can simply exceed the other pump’s output.²⁵

Concept: a valve follows its local pressure difference

For an ideal valve with negligible opening threshold, forward upstream pressure opens it; a reversed pressure difference closes it. The myocardium creates the changing pressures. The valve itself does not actively propel blood. The atrioventricular valves connect atria to ventricles, whereas the semilunar valves connect ventricles to their outflow arteries.³

Systole means contraction, and diastole means relaxation; specify the chamber because atria and ventricles do not contract simultaneously. During ventricular filling, an atrioventricular valve is open. After ventricular pressure rises above atrial pressure but before it exceeds outflow-artery pressure, both valves are closed: isovolumetric contraction. Ejection follows when the outflow valve opens. Early relaxation again closes both valves until ventricular pressure falls below atrial pressure.³

Worked application: infer valve states from snapshots

Consider an original ideal left-heart model. The mitral valve permits left atrium-to-ventricle flow; the aortic valve permits ventricle-to-aorta flow. Assume no leak, shunt or opening delay. Pressures share one reference. The values are separate snapshots within the named phases, not a full measured pressure waveform or equal-duration time intervals.

Phase	Left atrium	Left ventricle	Aorta
Filling	8	5	75
Isovolumetric contraction	8	40	80
Ejection	8	110	95
Isovolumetric relaxation	8	30	85

During filling, 8 > 5 opens the mitral valve, but 5 < 75 keeps the aortic valve closed. During ejection, 110 > 95 opens the aortic valve, while 110 > 8 closes the mitral valve against backward flow. Both valves are closed in each isovolumetric snapshot, even though one occurs during contraction and the other during relaxation.

The rule therefore identifies valve state from pressures, but a single both-closed snapshot does not tell you whether ventricular pressure is rising or falling. That requires the time sequence or another measurement. Likewise, a closed aortic valve does not mean all systemic blood flow stops: stretched arterial walls can recoil between ventricular ejections.⁶

Calculate output without counting the same blood twice

Stroke volume is the amount ejected by one ventricle per beat: SV = end-diastolic volume − end-systolic volume. Cardiac output is its volume per unit time: CO = heart rate × SV. Heart rate alone is insufficient because filling and the volume ejected per beat can also change.⁴

For an original example, end-diastolic volume is 120 mL and end-systolic volume is 45 mL. Thus SV = 75 mL/beat. At 72 beats/min, CO = 72 × 75 = 5400 mL/min, or 5.4 L/min. If rate rises to 90 beats/min while SV falls to 60 mL/beat, output remains 5.4 L/min. A faster rate has not increased volume delivery in that supplied case.

In a steady, closed, ideal circuit without shunts or fluid accumulation, average right- and left-ventricular outputs must match. Adding two 5.4 L/min outputs to claim 10.8 L/min systemic delivery counts the same circulating stream twice. If inflow to a compartment temporarily exceeds outflow, its volume increases by the difference multiplied by elapsed time. This conservation argument explains why a persistent mismatch cannot describe an unchanged steady state.

Pattern: vessel structure changes pressure and velocity

Large elastic arteries stretch with ejection and recoil afterward. Muscular arteries distribute flow, and arterioles adjust resistance through smooth-muscle contraction. Capillaries provide thin exchange surfaces; venules collect their output, and veins return blood while accommodating substantial volume at relatively low pressure. Venous valves and surrounding muscle activity can support return without supplying a second ventricular pump.⁵

Pressure falls as blood moves through a resistive circuit; a substantial systemic drop occurs across arterioles. The large combined cross-sectional area of parallel capillaries permits relatively low blood velocity. Do not confuse a single tiny capillary’s resistance with the effective resistance of the entire capillary network, or blood speed with the total volume passing through all branches per minute.⁶

Application: distinguish pumping from resistance

For a defined circuit, use time-averaged values in Q = ΔP/R. The relevant ΔP is inlet pressure minus outlet pressure, not automatically the arterial pressure alone. If Q is 5.4 L/min and ΔP is 90 mmHg, effective R is 90/5.4 = 16.7 mmHg·min/L. At the same measured flow, a rise to 108 mmHg gives R = 20.0: a 20% increase. These values describe the model’s effective resistance, not a diagnosis.

Sympathetic activity can increase cardiac rate and contractile force, while parasympathetic activity prominently slows the heart. Circulating catecholamines can affect both heart and vessels; the vascular result depends on receptors and tissue. Other hormonal pathways influence blood volume or vascular tone, and local tissue signals redistribute flow. Therefore, predict a final pressure only after identifying which quantities are actually held constant or measured.⁴⁷

For a new passage, trace the route, compare pressures across each valve, calculate flow with units, and check whether the proposed behavior conserves volume. These steps connect anatomy to a testable physical model without requiring a detailed clinical waveform.

Check your reasoning

Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.

Continue learning

Browse biology & biochemistry · MCAT content library

MCAT preparation and roadmap · Aim High, Score Higher

Sources

1. AAMC — B/B Content Category 3B

2. OpenStax Anatomy and Physiology 2e — 19.1 Heart Anatomy

3. OpenStax Anatomy and Physiology 2e — 19.3 Cardiac Cycle

4. OpenStax Anatomy and Physiology 2e — 19.4 Cardiac Physiology

5. OpenStax Anatomy and Physiology 2e — 20.1 Structure and Function of Blood Vessels

6. OpenStax Anatomy and Physiology 2e — 20.2 Blood Flow, Blood Pressure, and Resistance

7. OpenStax Anatomy and Physiology 2e — 20.4 Homeostatic Regulation of the Vascular System