Mammals have a closed blood circulatory system. The heart is four chambered and acts as a double pump for a double circulatory system with the pulmonary circulation completely separated from the systematic circulation.
Blood that has circulated round the body in arteries, arterioles and capillaries return to the right auricle of the heart by the vein.
Blood from the lower parts of the body returns to the right auricle by the inferior vena cava.
The tricuspid valve relaxes and the blood flows into the right ventricle. The right ventricle contracts to force the blood into pulmonary artery which branches into two to take blood into the right and left lungs.
In the lung the deoxygenated blood drops its carbon (iv) oxide content and picks oxygen in the alveoli. This is called gaseous exchange.
Oxygenated blood returns by the pulmonary veins to the left autricle of the heart.
The distribution of the oxygenated blood throughout the body is called SYSTEMIC CIRCULATION. As oxygenated blood enters the left auricle the bicuspid valves relax and the blood flows into the left ventricle.
The left ventricle contracts to force the blood into the main artery, the aorta which circulates oxygenated blood through arteries, arterioles and capillaries to all parts of the body. As the blood distributes essential materials such as oxygen, glucose, water, hormones it also pick up waste products such as urea, uric acid, water plus dissolved mineral salts.