cardiac cycle
Summary
The cardiac cycle is the sequence of mechanical and electrical events that occur during one complete heartbeat, lasting approximately 0.8 seconds at rest. It consists of systole (ventricular contraction and ejection) and diastole (ventricular relaxation and filling), coordinated by electrical conduction and regulated by preload, afterload, and contractility.
Detail
The cardiac cycle consists of seven distinct phases: (1) Atrial systole - atrial contraction contributes final 10-15% of ventricular filling; (2) Isovolumetric ventricular contraction - all valves closed, pressure rises without volume change; (3) Ventricular ejection - aortic/pulmonary valves open, stroke volume ejected; (4) Isovolumetric ventricular relaxation - all valves closed, pressure drops without volume change; (5) Early ventricular filling - rapid passive filling as AV valves open; (6) Diastasis - slow ventricular filling; (7) Late ventricular filling - atrial kick. Key pressure relationships: ventricular pressure exceeds atrial pressure (AV valves close), ventricular pressure exceeds aortic/PA pressure (semilunar valves open). Heart sounds correlate with valve closure: S1 (mitral/tricuspid closure at onset of systole), S2 (aortic/pulmonary closure at end of systole). Frank-Starling mechanism ensures stroke volume matches venous return. Abnormalities in timing, pressure relationships, or valve function lead to murmurs, heart failure, or arrhythmias.
Sources
- First Aid for the USMLE Step 1
- Costanzo Physiology
- Braunwald's Heart Disease
- Guyton and Hall Textbook of Medical Physiology
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