The Cardiovascular System

    The role of the cardiovascular system is to ensure circulation of the blood (about 5 liters, for an average man) throughout the body. This allows the blood to carry oxygen and nutrients to every cells in our body.

    Pumped by the heart at a rate of about 5 liters per minute, the blood flows through a network of blood vessels. It is important to understand that this network is a closed circuit (continuous), divided into two parts: the pulmonary circulation and systemic circulation, also known as peripheral. This means that all the blood pumped to all organs also passes through the lungs at the same rate. It also means that if some of the blood is redirected to a region of our body (for example, the muscles receive more blood during exercise), other regions will be deprived of the same amount of blood.

    By convention, the blood vessels leaving the heart toward the organs are the arteries and the vessels receiving the blood, from the organs to the heart, are the veins. The two figures on the left show the anatomical location of the major arteries and veins (click on them for details).

    We usually denote the arteries in red since they carry oxygenated blood which is red. The veins are usually depicted in blue, since the blood coming back from the organs is depleted in oxygen and darker in color, which gives the skin a bluish color (when the cold decreases blood flow, our lips turn blue). However, the pulmonary circulation is an exception to this rule, because the pulmonary arteries carry non-oxygenated blood from the heart to the lungs, and the pulmonary veins bring back oxygenated blood from the lungs to the heart.

    Here is a diagram of blood flow showing the non-oxygenated blood that enters the right atrium of the heart, continues into the right ventricle, and goes to the pulmonary circulation. In the network of pulmonary capillaries, the blood is oxygenated and returns to the heart, into the left atrium, then passes in the left ventricle where it is pushed toward all tissues of our body to deliver its oxygen content. Then the blood returns to the heart, depleted of oxygen and full of carbon dioxide.

Diagram of the circulatory system.

    The figure on the left shows the proportions of circulating blood flow to the major organs of our body. You will notice that 100% of cardiac output passes through the lungs, and then to the rest of the body. Also, the kidneys, which filter the blood, and brain, which has high metabolic needs, are well perfused for their size, while the skin and bones, which are less active organs, are not perfused as much.