The basic circulatory system of vertebrate animals, which are those with a backbone, consists of a heart, the blood and the arteries and system of blood vessels. The heart pumps the blood first to the arteries and then throughout the body via the system of blood vessels.
The heart is a muscular pump organ with two upper chambers, called atria, where the blood enters. After passing through a valve, the blood then enters one of the two lower chambers, called ventricles. The muscles of the ventricle contract to send the blood out of the heart and into the arteries. About the size of a fist, the heart muscle is extraordinarily durable, pumping about 3 billion times throughout the average human lifespan.
The blood carries nutrients, oxygen, water and waste products to and from the cells and organs through the system of blood vessels that exist throughout the body, sometimes traveling thousands of miles. There are different types of blood vessels.
- Arteries: connect directly to the heart and carry blood from the heart to the body
- Capillaries: tiny blood vessels where gases and nutrients pass through the thin walls of capillaries and into the body
- Veins: carry the blood back to the heart.