The heart together with the two networks of blood vessels, namely the systemic circulation and the pulmonary circulation. The vascular circulatory system between the right side of the heart, the lungs, and the left atrium is called the pulmonary circulation. The circuit of vessels between the left side of the heart, most of the body, and the right atrium is called the systemic circulation. It serves to provide the supply of nutrients and the removal of waste products from all parts of the body. Deoxygenated blood flows through the two largest veins (the superior and inferior venae cavae) into the right atrium; it then passes through the tricuspid valve into the right ventricle which then contracts forcing blood into the pulmonary arteries through the pulmonary valve to deliver it to the capillaries of the lungs. In the lungs the blood absorbs oxygen and gives up carbon dioxide via the capillaries surrounding the air sacs. Oxygen-rich blood from the lungs flows through the pulmonary veins into the left atrium of the heart; the blood then passes through the mitral valve into the left ventricle. On contraction of the left ventricle blood is forced through the aortic valve into the aorta and thence via the systemic arteries to all parts of the body except the lungs.
Schematic representation of the cardiovascular system