Part 2. Circulation of Blood

Blood vessels together with the heart form a circulatory system for the flow of blood.

Blood deficient in oxygen (deoxygenated) flows through two large veins, the venae cavae, on its way from the tissue capillaries to the heart. The blood became deoxygenated at the tissue capillaries when oxygen left the blood and entered the body cells.

Deoxygenated blood enters the right side of the heart and travels through that side and into the pulmonary artery, a vessel which divides in two, one branch leading to the left lung, the other to the right lung. The arteries continue dividing and subdividing within the lungs, forming smaller and smaller vessels (arterioles) and finally reaching the lung capillaries. The pulmonary artery is the only artery in the body which carries deoxygenated blood.

While passing through the lung capillaries, blood absorbs the oxygen. The newly oxygenated blood next immediately returns to the heart through the pulmonary vein, the only vein in the body which carries oxygen-rich blood. The circulation of blood through the vessels from the heart to the lungs and then back to the heart again is known as the pulmonary circulation.

Oxygenated blood enters the left side of the heart from the pulmonary veins. The muscles in the left side of the heart pump the blood out of the heart through the largest artery in the body, the aorta. The aorta moves up at first (ascending aorta) but then arches over dorsally and runs downward (descending aorta) just in front of the vertebral column. The aorta branches into numerous arteries which carry the oxygenated blood to all parts of the body. The names of some of these arterial branches are: brachiocephalic, intercostal, esophageal, celiac, renal, and iliac arteries.

Arterial vessels branch further to form the smaller arterioles. The oxygen-rich arterioles branch into smaller tissue capillaries which are near the body cells. Oxygen leaves the blood as it passes through the capillary walls to enter the body cells. There it combines with food to release needed energy. Waste product of this process is carbon dioxide. It must pass out of the cells and into the capillary bloodstream, at the same time that oxygen is entering the cell. As the blood makes its way back from the tissue capillaries toward the heart in venules and veins, it is full of carbon dioxide and deoxygenated.

The circuit is thus completed when deoxygenated blood enters the heart from the venae cavae. This circulation of blood from the body organs (except the lungs) to the heart and back again is called the systemic circulation.

Date: 2014-12-29; view: 1020