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A ventricle is a chamber of the heart that pumps blood out of the organ. It collects blood from an atrium, the other type of chamber in the heart. Ventricles are larger and more muscular than atria, and can withstand greater blood pressure. Humans have four chambered hearts, with a right and left ventricle, and a right and left atrium.
The left ventricle is larger than the right ventricle, as it is responsible for pumping blood throughout the body, while the right ventricle pumps blood only into the lungs, where is is oxygenated. The path of blood through the heart begins with deoxygenated blood from the body entering the right atrium. It then flows into the right ventricle and is pumped into the lungs. Oxygenated blood from the lungs enters the heart through the left atrium and is pumped into the aorta through the left ventricle. The aorta is the largest artery in the body, and from there, oxygenated blood is distributed to the entire body through the circulatory system.
The cardiac muscle tissue of the ventricles is distinct from all other muscle tissue in the body. It combines features of the skeletal muscle responsible for voluntary body movement and the involuntarily controlled smooth muscles of the organs. Like skeletal muscle, cardiac muscle is striated, or formed of bands called sarcomeres that give it a striped appearance under the microscope. It also has multiple nuclei per cell, like skeletal muscle, but unlike smooth muscle. At the same time, cardiac muscle is like smooth muscle in that it is involuntarily controlled by the autonomic nervous system.
The heartbeat is caused by the contraction and relaxation of the ventricles. When they relax, they allow blood to enter from the atria, and when they contract, they pump blood out of the heart. The contraction is known as systole, while the relaxation is called diastole. Cardiology measures the performance of the ventricles through their blood volume at the end of systole and diastole, as well as through the volume and percentage of blood pumped out with each beat.