Cardiac Output and Its Determinants
Core concepts linking heart rate, stroke volume, Frank–Starling mechanics and Guyton's venous return curve.
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Questions Covered in This Set
10 cards to master
What is the basic equation for cardiac output, and typical resting values?
CO = HR × SV ≈ 70 beats/min × 70 mL ≈ 5 L/min — roughly the entire blood volume every minute.
What is cardiac index and its normal range?
Cardiac output divided by body surface area, normally ~3.0–3.5 L/min/m²; it allows comparison between people of different sizes.
Define stroke volume and ejection fraction with typical numbers.
SV = EDV − ESV (≈120 − 50 = 70 mL). EF = SV/EDV ≈ 58%.
What is preload and how is it estimated clinically?
Myocyte stretch at end-diastole; approximated by EDV, right atrial pressure, or pulmonary capillary wedge pressure.
What is the mechanism of the Frank–Starling relationship?
Stretch improves actin–myosin overlap but mainly increases troponin C calcium sensitivity and thin-filament cooperativity, so force rises with filling — the heart auto-matches output to venous return.
How does afterload affect stroke volume, and why does Laplace matter?
Higher afterload (aortic pressure) raises ESV and lowers SV. By Laplace σ ∝ P·r/2h, so a dilated thin-walled ventricle faces greater wall stress — failing hearts are very afterload-sensitive, hence vasodilator therapy.
Trace the β1 signalling pathway that increases contractility.
Noradrenaline → β1 → Gs → cAMP → PKA → phosphorylates L-type Ca²⁺ channels, phospholamban and troponin I → more Ca²⁺ entry, faster SR Ca²⁺ uptake (lusitropy) and faster cross-bridge cycling.
Why does cardiac output fall at heart rates above ~160–180/min?
Diastole shortens so much that ventricular filling and diastolic coronary perfusion fail, so SV drops faster than rate rises.
What is mean systemic filling pressure (P_msf)?
~7 mmHg — the pressure everywhere if the heart stopped and pressures equilibrated; set only by blood volume and venous tone/compliance, not by the heart.
Write the venous return equation and explain the operating point of the circulation.
VR = (P_msf − P_ra)/R_VR. VR falls as P_ra rises, while CO rises with P_ra; at steady state VR = CO, so the circulation operates at the intersection of the two curves.
