The series of muscular contractions which squeeze blood through the heart. In Amphibia and higher vertebrates, blood is pumped through a double circulatory system. The right side of the heart pumps blood through the pulmonary circuit to reach the lungs, the left side pumps oxygenated blood through the systemic circuit which supplies the body. Deoxygenated blood enters the right atrium via the vena cava and is pumped into the right ventricle. The right ventricle pumps the blood into the pulmonary artery which divides and leads to both lungs. Oxygenated blood returns from the lungs via the pulmonary vein to the left atrium and in turn to the left ventricle. From the left ventricle, the most muscular of the heart chambers, the blood is pumped around the body where it becomes deoxygenated and returns to the right atrium. Amphibia have only one ventricle, which must pump blood around both circuits; deoxygenated blood is kept separate from oxygenated blood by a combination of timing and complex architecture. During the phase of muscle relaxation (diastole), blood flows freely into the atrium until it is full, whereupon the sino-atrial node sends out an electrical pulse causing the cardiac muscle of the atrium to contract (systole). Blood is forced into the ventricle, which is in diastole, until it is full, whereupon the atrioventricular node sends out an electrical pulse along the Purkinje fibres around the right ventricle and the bundle of His (specialized muscle fibres) around the left ventricle, causing the ventricle to enter systole and force blood out into the arteries of the circulatory system. Nerves of the sympathetic nervous system from the spinal cord connect to the sino-atrial node and can speed up the rate of heartbeat, while the vagus nerve from the medulla also connected to the sino-atrial node slows it down.
Subjects: Zoology and Animal Sciences.