The circulatory system is a network that consists of blood, blood vessels, the heart, and the lymphatic system. Blood vessels are tubular structures that circulate blood between the heart and the body. If we measure the length of the blood vessels stretched out in a line, it would be enough to encircle Earth more than twice. The heart is a vital organ in the circulatory system that circulates blood throughout the body. The heart muscles relax and contract cyclically to pump blood. The heart consists of four chambers: two atria and two ventricles. The blood circulation process is divided into systemic and pulmonary circulation:
(systemic circulation) blood flows from the left ventricle – main artery – throughout the body (capillary vessels) – the main vein – into right atrium;
(pulmonary circulation) blood flows from the right ventricle – the pulmonary artery – lungs – the pulmonary vein – into the left atrium.