The circulatory system is the body system that moves blood around the body consisting of heart and all blood vessels.
- Blood vessels that take blood away from the heart are arteries. Arteries divide into smaller arteries as they go away from the heart. The smaller arteries that connect to the capillaries, are called arterioles. Arteries are often depicted as red.
- Blood vessels that take blood towards the heart are veins. Veins get bigger as they go towards the heart. The smallest veins are called venules. Veins also have valves, flaps which are used to make sure the blood goes in the correct direction. Veins are often depicted as blue.
- Capillaries go between arteries and veins. Capillaries are quite thin, hence the name which comes from the Latin «capillus» meaning «hair.» Capillaries are often depicted as
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 «lungs»). This is how it works in mammals, including humans. Circulatory systems of other vertebrates differ somewhat. Invertebrates are very different.
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. The 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. 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 merge and 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 anatomy, inferior means below.) The superior vena cava takes blood from the upper part of the body to the heart. (superior means above.)
This same movement of blood goes through the lungs in the pulmonary circulation. There, the air you breath in goes down the trachea into the alveoli, which is where the pulmonary capillaries are. The pulmonary artery carries blood without oxygen, but when the oxygen comes in contact with the blood, the blood goes into the pulmonary veins back into the hear filled with oxygen.