Capillaries go between arteries and veins.
So blood moves: heart->artery->arteriole->capillary->venule->vein->heart. This is called circulation. There are two different circulations in the circulatory system. The systemic circulation is how blood goes to most of the body. The pulmonary circulation is how blood goes through the lungs. (Pulmonary means ¨about the lungs¨.) This is how it works in mammals, including humans. Circulatory systems of other vertebrates differ somehat. Invertebrates are very different.
Systemic circulation[change | change source]
Blood that comes from the left side of the heart is full of oxygen and nutrients. Nutrients are substances that your body needs to live, like protein, fat, carbohydrates, vitamins, and minerals. This blood brings the oxygen and nutrients to your body.
This blood in systemic arteries that is full of oxygen and nutrients is systemic arterial blood. It is sometimes just called arterial blood.
The biggest systemic artery in the body is the aorta. This is the large blood vessel that comes out of the heart. Smaller arteries branch off from the aorta. These arteries have smaller arteries that branch off from them. The smallest arteries turn into arterioles.
The smallest blood vessels are capillaries. Systemic arterioles turn into capillaries. The blood from arterioles goes into the capillaries. There oxygen and nutrients go out of the blood into the tissue around the capillaries. The blood also picks up carbon dioxide and waste from the tissue. The network of capillaries that brings blood to an area is called a capillary bed.
On the other end of the capillary, it turns into a venule. Venules are the smallest veins. Veins take blood back to the heart. As veins go back to the heart, they get bigger. The biggest systemic veins in the body are the vena cava. There are two vena cava. The inferior vena cava takes blood from the lower part of the body to the right side of the heart. (In medicine, inferior means below.) The superior vena cava takes blood from the upper part of the body to the heart. (In medicine, superior means above.)
Pulmonary circulation[change | change source]
This same movement of blood happens in the pulmonary circulation.
The blood that the vena cava takes to the heart is full of carbon dioxide. It has much less oxygen than (systemic) arterial blood. The right side of the heart pushes the venous blood into the pulmonary artery. The pulmonary artery takes blood to the lungs. In the lungs, the blood goes through the pulmonary capillary bed. (The capillaries that are in the lungs.) Here it gets more oxygen. It also drops off carbon dioxide. (This is the opposite of what happens in capillary beds in the rest of the body. In the systemic circulation blood drops off oxygen and picks up carbon dioxide.)
After the pulmonary capillary bed, the blood goes to the pulmonary veins. This pulmonary venous blood now is full of oxygen. The pulmonary veins take blood to the left side of the heart. Then the blood goes to the systemic circulation again.