From the arteries the blood flows into arterioles. Arterioles lead to every organ and tissue in the body. From the arterioles the blood enters a vast network of capillaries. “In the capillaries, the blood cells jostle along in a single file, giving up oxygen and other substances and taking in carbon dioxide and other waste products.”(Weston, 94) Due to the thin walls of capillaries, chemicals can be exchanged between the blood and interstitial fluid. Interstitial fluid is the portion of the extracellular fluid occupying the spaces between cells and tissues.
From the capillaries, blood enters the venous system, first passing through small vessels called venules. Some solutes diffuse across venules walls, which are a bit thicker than capillary walls. From the venules, the blood enters the jugular vein, which receives blood from the brain, tissues of the head, neck, shoulders and arms. All the veins from the various parts of the body eventually merge into two large blood vessels, one called the superior vena cava, the other called the inferior vena cava. The superior vena cava collects blood from the head, arms and neck, and the inferior vena cava receives blood from the lower part of the body. Blood then enters the right side of the heart, the right atria, and from there, the right ventricle. From here it is pumped into the pulmonary artery. The pulmonary circuit is the hearts right half. It pumps blood to the lungs. “There, blood picks up oxygen and gives up carbon dioxide.” (Starr and Taggart, 658) The blood then goes through the pulmonary arteriole, pulmonary capillaries, pulmonary venules, and the pulmonary vein, in which the now oxygen rich blood goes into the left side of the heart. This is called the systematic circuit, in which the hearts left half pumps the oxygenated blood to all regions of the body where oxygen is needed.
There are a few short cuts that the blood takes through its circulation. On leaving the intestines, the blood does not go directly back to the heart, but instead is drained into the hepatic portal system of veins. This allows blood that may be rich in digested food to go directly to the liver. Once it reaches the liver it goes through special capillaries called sinusoids and then goes to the hepatic veins. These eventually lead to the inferior vena cava.
