Cardiovascular System Health Article

The capillaries

The arteries and the veins are connected by the vessel web of the capillaries. The lumen of these vessels is very small, to the extent that blood cells must line up single file to pass through the thinnest of them. Capillary walls are also very thin, allowing the passage of gases and nutrients between the blood cells and the cells of the body.

The exact role of the capillaries varies depending on the part of the body in which they are located. The capillaries of the pulmonary circulation are found in the air sacs of the lungs, called alveoli, and it is there that the exchange of oxygen into the blood and carbon dioxide out of the blood occurs. In the kidneys, the capillaries in the organ's tubules are the point where waste products are taken out of the blood to be excreted in the urine. The capillaries of the intestine are the location where nutrients from digested food are absorbed into the bloodstream. Capillaries serving the muscles bring in oxygen and nutrients and take away carbon dioxide and waste products.

Function

For reference, at any particular point in time, about 9% of the body's blood is located in the pulmonary circulation and about 7% is in the heart's circulation. The remaining 84% is located in the systemic circulation, with 64% in the veins, 13% in the arteries, and 7% in the arterioles and capillaries. The greater percentage in the veins is due to the less elastic nature of the vessels and the tendency of the blood to pool there.

As the pulmonary circulation has a relatively smaller network of vessels when compared to the systemic circulation, the right side of the heart doesn't have to work as hard as the left side to move the blood. Accordingly, the left side of the heart is larger and more muscular. The passive-filling nature of the heart keeps the unequal balance in blood volume between the pulmonary and systemic circulation. Without active filling, the physical differences between the systemic and pulmonary capillaries such as relative size of the vessel bed and relative elasticity determine the blood distribution. If the heart was a different kind of pump, cardiac characteristics, such as rate or stroke volume (amount of blood pumped by one contraction of the left ventricle) would govern the relative volumes.

One way to visualize the function of the cardiovascular system is to follow the movement of one blood cell throughout the body. The path can begin at the left ventricle, where an oxygenated blood cell is pumped out by contraction of the myocardium, through the aortic valve into the aorta. The cell follows the curve into the abdominal artery and into the axillary artery into the arm. The artery subdivides into smaller and smaller branches, small enough to be called arterioles. Blood is needed at a muscle in the arm, so the arterioles are open to keep a large quantity of blood flowing in that direction. The blood cell continues through smaller vessels until it is in a capillary bed next to a muscle cell.

There the cell gives up its oxygen cargo, takes up carbon dioxide waste produced by the muscle, and begins the journey back to the heart. Travelling through the capillaries to the venules and then into the axillary vein, the cell goes into the superior vena cava and into the right atrium. The right atrium contracts, and the cell moves through the tricupsid valve into the right ventricle. On the next systole, the cell rushes out of the right ventricle, through the pulmonary valve into the pulmonary artery to the lungs. The branches of vessels grow smaller and smaller, until the cell is in the capillaries of the alveoli where it releases the carbon dioxide to the lung space to be exhaled, and picks up another load of oxygen.

Travelling back to the heart through the veins of the pulmonary circulation system, the cell enters the left atrium through the pulmonary vein. When the atrium contracts, the cell goes through the mitral valve into the left ventricle, having made one cycle through the cardiovascular system. In this way, the cardiovascular system supplies all the cells of the body with oxygen and nutrients and carries away carbon dioxide and other wastes.