Circulatory System

The circulatory system is the body system that moves blood through your body. It has three main parts: the heart, the blood vessels, and the blood itself. The heart is the pump. The blood vessels are the pipes. The blood is the liquid that carries supplies to every cell. Together, these three parts work without stopping for your whole life.

Your blood carries many things. It carries oxygen from your lungs to your cells. It carries food from your stomach to the rest of your body. It also picks up waste, like carbon dioxide, and carries it away. Blood even helps spread heat around your body to keep your temperature steady.

There are three main types of blood vessels. Arteries carry blood away from the heart. They have thick walls because the blood inside them is moving with a lot of force. Veins carry blood back toward the heart. Their walls are thinner. Between arteries and veins are tiny vessels called capillaries. Capillaries are so thin that blood cells have to squeeze through them in single file. This is where oxygen and food pass into the cells, and where waste passes out.

Blood travels in a loop. It leaves the heart, picks up oxygen in the lungs, returns to the heart, and then gets pumped out to the rest of the body. After dropping off oxygen and picking up waste, the blood comes back to the heart again. One full loop takes less than a minute when you are resting.

An adult body holds about five quarts of blood, a little more than a gallon of milk. The heart pumps all of that blood through the body once every minute or so. When you exercise, your heart speeds up, and your blood can make a full loop in only about 20 seconds. That is why you breathe harder during a hard run. Your muscles need more oxygen, fast.

The English doctor William Harvey figured out how the circulatory system works in 1628. Before that, even smart doctors had it wrong. Many thought blood was made fresh in the liver and used up by the body. Harvey showed that the same blood travels in a loop, over and over. Today doctors can fix many circulatory problems with medicine, surgery, or even artificial valves. Some patients live for years with mechanical pumps that help a weak heart keep that endless loop going.