Schematic illustration of the human blood circulation, vessels that transport low-oxygen blood to the heart are depicted in blue and are called veins, those that transport high-oxygen blood away from the heart are depicted in red and are called arteries. Nerves and nervous plexus are depicted in yellow, a so called Rete mirabile, a second capillary network to slow down the bloodstream, in purple. The blood circulation is the physiological process of transporting arterial or venous blood through the vessel system. It's fueled by the pumping function of the heart. One differentiates between the big and small blood circulation. The big blood circulation begins in the left ventricle of the heart. From there, the high-oxygen (oxygenated) blood is pumped by contractions into the aorta, then into subordinated arteries and arterioles and finally into the capillaries of the body. From the capillary bed, the blood flows into the venoles and veins and finally via the superior and inferior hollow vein into the right atrium of the heart. The small blood circulation begins in the right ventricle of the heart. From there, the low-oxygen (desoxygenated) blood is pumped through the pulmonary arteries into the lung capillaries. From the capillary bed of the lung it flows through the pulmonary veins and ends up in the left atrium of the heart, where it can begin anew its journey through the big blood circulation. Some of the most important functions of the blood circulation are oxygen supply, supply of the cells with nutrients, removal of metabolic waste products and heat transport.