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.
The rewritten readings were written for this dataset.
More to read
Contraction and relaxation during a heartbeat
The cardiac cycle repeats with each heartbeat. Atrial contraction completes ventricular filling. Ventricular contraction then drives blood into the arteries. During general diastole, both atria and ventricles relax, allowing filling before the next beat.
A graph of pressure over time records these changes. Systole raises ventricular pressure; diastole lowers it. Atrial and arterial pressures also change throughout the cycle. Practice identifying the cycle stage associated with each pressure rise or fall.
Pressure differences control the valves
Heart valves respond passively to pressure. Higher pressure upstream pushes a valve open. A reversed difference pushes blood toward the valve, closing its cusps and preventing backflow.
During ventricular systole, rising ventricular pressure first exceeds atrial pressure. The atrioventricular valves close, producing the first heart sound. When ventricular pressure exceeds pressure in the receiving artery, the semilunar valves open and blood is ejected.
During diastole, ventricular pressure falls below arterial pressure, closing the semilunar valves. Once it falls below atrial pressure, the atrioventricular valves open for filling. Pressure differences explain the timing of these valve events.
Two pumps and the volume they deliver
Mammals have double circulation. The heart's right side pumps oxygen-poor blood toward the lungs. Its left side has a thicker muscular wall and pumps oxygen-rich blood through the body. Valves at chamber and arterial openings prevent reverse flow.
Stroke volume measures how much one ventricle ejects per contraction. Calculate it by subtracting the volume remaining after systole from the volume present before systole. These values require volume measurements, not a pressure trace alone.
Cardiac output is the flow per minute, calculated as stroke volume multiplied by heart rate. Pressure changes, chamber volumes, and valve movements provide related descriptions of each heartbeat.
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Where this leads
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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.