The Aorta carries oxygenated blood away from the heart to the rest of the body. Pulmonary arteries carry de-oxygenated blood from the heart to the lung.
The Superior Vnea Cava brings de-oxygenated blood back from the body to the heart. They serve the head, neck arms and upper trunk/chest. The Inferior Vena Cava serves the legs, pelvis and lower trunk.
Capillaries are very fine blood vessels with thin walls to allow substances to pass into and out of the blood. These extremely thin walls allow oxygen and nutrients to pass easily from the blood into the tissues and carbon dioxide and other waste products to pass in the opposite direction. The capillaries deliver deoxygenated blood into the venules.
On its journey frommthe heart to the tissues, blood is forced along the arteries at high pressure from the left ventricle-a powerful pump. On its return journey through the veins and back to the heart the blood is at low pressure and is kept moving by the muscles in the arms and legs compressing the walls of the veins, and by the valaves in the veins preventing the blood from flowing backwards.
Bllod is the sticky red fluid that circulates in our veins, arteries and capillaries. An average-sized human has about five litres of blood. At rest, roughly this amount is pumped each minute by the heart via the arteries to the lungs and all other tissues, and then returned to the heart in the veins, in a continuous circuit. During exercise the heart may pump blood at a rate of 30 litres or more a minute. Almost half the volume of blood consists of blood cells; these include red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (Thrombocytes.) The remainder of the blood volume is a watery, straw-colouredfluid called plasma, which contains dissolved proteins, sugars, fats, salts and minerals.