The Cardiac Cycle and Pressure in the Heart
Valves open and shut because of pressure differences, not because of nerve signals, and a pressure trace shows it. Reading the trace explains the order of atrial and ventricular contraction.
What a learner can do afterwards
- Identifies systole and diastole on a pressure against time trace.
- Explains each valve opening or closing from the pressure either side of it.
- Relates the trace to the volume of blood leaving the ventricle in one beat.
1 · Read
One cardiac cycle is a repeating sequence with a fixed order. The atria contract first in atrial systole, pushing the last blood into the ventricles. Then the ventricles contract powerfully in ventricular systole, ejecting blood into the arteries. Finally the whole heart relaxes in general diastole while the chambers refill. Systole means contraction and diastole means relaxation and filling.
Valves never decide anything themselves. Each one simply obeys the pressure difference across it. When pressure behind a valve exceeds pressure ahead, blood pushes it open. When the gradient reverses, blood tries to flow back and the cusps slam shut. As the ventricle contracts, its pressure passes atrial pressure, so the atrioventricular valves shut with the first heart sound. Pressure keeps climbing until it passes artery pressure, so the semilunar valves open and ejection begins.
A pressure against time trace shows these stages as peaks and dips. Ventricular pressure shoots up during systole and falls during diastole. When ventricular pressure falls back below artery pressure, blood tries to re-enter and the semilunar valves close. Learn to point at each wave and name the stage, because trace questions always test that skill.
The trace also tells you how much blood each beat delivers. Stroke volume is the volume leaving a ventricle in one contraction: the volume before systole minus the volume after. Say a ventricle holds 130 ml before systole and 60 ml after, giving 70 ml per beat. Multiply stroke volume by heart rate to get cardiac output per minute.
Pressure differences open and shut each valve, and the trace lets you read every stage.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.