The rhythmic contraction and relaxation of the heart muscle is what drives blood flow throughout your body. This coordinated process is called the cardiac cycle and has two main phases: systole (contraction) and diastole (relaxation).
  • Contraction Trigger:The electrical impulse originates in the SA node and travels through the atria, causing them to contract and push blood into the ventricles.
  • Ventricular Filling:As the atria contract, the ventricles are also momentarily relaxed and filling with blood.
  • Closing the Doors:Once the ventricles are sufficiently filled, the atria relax. Simultaneously, special valves between the atria and ventricles (atrioventricular valves) snap shut, preventing blood from flowing back into the atria.
  • The Big Push:Now comes the powerful contraction of the ventricles (ventricular systole). The thickened muscular walls of the ventricles squeeze inward, forcefully pushing blood out of the heart.
  • One-Way Streets:During ventricular systole, valves on the exits of the ventricles (semilunar valves) open. The aortic valve allows oxygen-rich blood from the left ventricle to flow into the aorta, the largest artery in the body, for distribution throughout the body. The pulmonary valve allows oxygen-depleted blood from the right ventricle to flow into the pulmonary artery for transport to the lungs for oxygenation.
Refilling the Chambers: Ventricular Diastole
  • Muscular Relaxation:After forcefully contracting, the ventricles relax (ventricular diastole). This decrease in pressure allows the semilunar valves to snap shut, preventing blood from flowing back into the ventricles.
  • As the ventricles relax, the pressure drops below that in the atria. This pressure difference causes the atrioventricular valves to open again, allowing blood to flow passively from the atria into the ventricles.
  • The Cycle Repeats: The cycle then begins again with atrial contraction, and the continuous pumping of blood is maintained.
  • Valves ensure blood flows in one direction only, preventing backflow.
  • Ventricular systole is responsible for propelling blood throughout the circulatory system.
  • Diastole allows the heart to refill with blood in preparation for the next contraction.
The heart’s intricate interplay between contraction, relaxation, and valve function is essential for maintaining a healthy and efficient circulatory system.

Heart Contraction and Blood Flow