The heart is a large muscle that keeps the circulatory system flowing. Blood from the body enters the heart, is pumped to the lungs for oxygenation, and then is returned back to the heart. From there the blood is pumped out to the rest of the body. The heart has four chambers which must coordinate their contractions to work efficiently.
Atrial Contraction
The Texas Heart Institute explains that the first part of the heartbeat is the contraction of the two upper chambers, known as the atria. The atria receive blood from the veins. The blood that is in the two atria is different: the right atrium receives deoxygenated blood (blood that is low in oxygen) from the veins throughout the body, whereas the left atrium receives oxygen-filled blood from the lungs (via the pulmonary veins). As the two atria contract, valves between the atria and the ventricles (known as the tricuspid and bicuspid valves) are forced open by the increased pressure in the atria. That allows the blood to move into the other two chambers of the heart, known as the ventricles.
Ventricular Contraction
Once the ventricles have filled with blood from the atria, the second part of the heartbeat can occur. As the National Heart Lung and Blood Institute explains, the blood-filled ventricles contract to pump blood out of the heart. This process is called ventricular systole. The right ventricle (which has received deoxygenated blood from the right atrium) pumps blood to the lungs where it can acquire oxygen and release carbon dioxide. The left ventricle, on the other hand, pumps oxygen-rich blood into the aorta where it can be propelled throughout the body. As the ventricles contract, the tricuspid and bicuspid valves stay closed. The valves on the aortic and pulmonary arteries, on the other hand, open briefly to allow blood to leave the heart.
Relaxation
Once the blood has been pumped out of the heart, there is a period of time during which the atria are at rest which allows them to fill up with blood again. The length of time of this relaxing phase is variable, because it is controlled by a small part of the heart tissue called the sinoatrial node. The sinoatrial node generates the electrical signal that stimulates the pumping action of the heart. The node is modulated by signals coming from the brain and the rest of the body and is responsible for controlling the rate at which the heart beats.
Member Comments