The Cardiac Cycle
Think of the heart as a two-storey pump with four rooms. The two upper rooms (atria) are receiving chambers that collect blood returning from the body and lungs. The two lower rooms (ventricles) are the powerful pumping chambers that push blood out to the lungs and the rest of the body.
These four rooms cannot all squeeze at once — if they did, blood would simply slosh around instead of moving in one direction. The heart needs a precise, timed sequence of contraction and relaxation to keep blood flowing forward only. That full sequence, from the start of one heartbeat to the start of the next, is the cardiac cycle.
Systole and Diastole
Every chamber alternates between two states:
- Systole — the chamber contracts, pushing blood out
- Diastole — the chamber relaxes and fills with blood
The atria and ventricles never go through systole and diastole at the same moment — they work in a carefully timed relay.
The Sequence of One Cardiac Cycle
At a resting heart rate of about 72 beats per minute, one full cardiac cycle takes roughly 0.8 seconds, made up of the following stages.
1. Joint Diastole (about 0.4 seconds)
Both atria and both ventricles are relaxed together. Blood flows passively from the veins into the atria, and from the atria through the open atrioventricular valves (the mitral valve on the left, the tricuspid valve on the right) into the ventricles. This passive flow alone fills the ventricles to about 70% of their capacity. The semilunar valves (aortic and pulmonary) stay shut because arterial pressure is still higher than the pressure in the relaxed ventricles.
2. Atrial Systole (about 0.1 seconds)
The atria contract — a gentle squeeze that pushes the remaining 30% of blood into the ventricles, topping them off completely. The ventricles are still relaxed at this point. By the end of atrial systole, the ventricles are full.
3. Ventricular Systole (about 0.3 seconds)
Now the ventricles contract — the powerful squeeze that actually drives blood forward. It happens in two stages:
- Isovolumetric contraction (very brief): The ventricles begin squeezing and pressure inside them rises sharply. The atrioventricular valves slam shut, producing the first heart sound ("lub"). The semilunar valves stay shut too, because ventricular pressure has not yet exceeded arterial pressure — so the ventricles contract against a closed system, and pressure rises while volume stays constant.
- Ejection phase: Once ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is forced into the aorta (from the left ventricle) and the pulmonary artery (from the right ventricle). Throughout this stage the atria are back in diastole, filling again from the veins.
4. Ventricular Diastole (about 0.4 seconds, overlapping the start of the next cycle) …