The circulatory system is the movement of blood through the body. It flows through a network of many veins. At the center is a pump, the heart. The whole thing is called the cardiovascular system. Through the circulatory system, nutrients and fresh oxygen reach the individual parts of the body. At the same time, waste products are picked up to be taken out of the body.
The blood circulation of vertebrates, including humans, has the shape of an 8. The heart is in the middle. At the top is the "small circulation." There the blood goes to the lungs and back to the heart. It is therefore also called the pulmonary circulation. In the "large circulation" the blood reaches the muscles, the digestive organs and the brain. It is therefore also called the systemic circulation. However, the pulmonary circulation and the systemic circulation are not two separate circuits, but the blood always flows through both of them in succession.
The heart is the pump of these circuits. Blood from the lungs flows into the left half of the heart and from there through the arteries, or arteries, into the body and head. Through another system of veins, the veins, the blood flows from the body back to the right half of the heart and then to the lungs. This is how the circulatory system closes.
The arteries branch out like branches of a tree to the smallest branches. These are called capillaries or hair vessels. Through them, substances are exchanged with the cells, the building blocks that make up our body. Other hair vessels lead away from the cells and then, like small streams, meet to form ever wider rivers, veins that carry the blood back to the heart. They are called veins.
When the circulatory system stops working, a person can survive only a few minutes: The body parts would starve and suffocate in waste products. There are many reasons for the heart to stop working: It may simply be due to old age. The heart can also shut down due to a disease, such as a heart attack. The circulation can also collapse if an animal loses too much blood or is dying of thirst.