Blood is the body’s transport system. It carries food, oxygen, hormones, heat, and other vital resources to every living cell in the body, as well as taking away waste.

Blood circulates endlessly around the body, traveling through thousands of miles of tubes called blood vessels. The largest blood vessels are as thick as a garden hose. The tiniest are a tenth as wide as a hair and too small to see with the naked eye.

As blood flows through the thinnest vessels, it releases oxygen and nutrients to keep the body’s cells alive and functioning. It collects wastes from the same cells and carries them away to be removed from the body. Blood also contains cells that battle against germs and heal wounds. Other roles of blood include transporting chemical messengers called hormones and helping spread heat around the body.

Circulatory System

The heart and blood vessels make up the body’s circulatory system. Blood leaves the heart in vessels called arteries, which divide into finer and finer branches. It then passes through tiny vessels called capillaries, where it releases nutrients and collects waste. Capillaries join to form larger vessels called veins, which take blood back to the heart.

Blood Cells

There are three main types of blood cells. By far the most numerous are red blood cells. These bright red, disc-shaped cells have the sole task of collecting oxygen in your lungs and releasing it everywhere else in your body. White blood cells roam through the body hunting for germs and destroying them. Platelet cells are tiny cell fragments that help blood to clot when the body is injured.


These large blood vessels carry blood away from the heart. They have strong, muscular walls that stretch as blood surges past with each heartbeat. After stretching, arteries shrink back to normal size, which helps push the blood along.


Blood vessels that carry blood back to the heart are called veins. They have thinner walls than arteries. The force of the heartbeat is much weaker in veins, so veins use one-way valves to keep blood flowing.


Microscopic blood vessels called capillaries carry blood between arteries and veins. There are thousands of miles of capillaries running through almost every part of the body. Their very thin walls allow oxygen and nutrients to pass out of the blood into body tissues, as well as allowing waste to enter the blood.

How blood clots

When your skin is cut, a series of chemical reactions causes proteins in blood plasma to form a tangle of threads that trap blood cells. At the same time, the tiny platelet cells in blood change shape, becoming spiky, and then stick together in clumps. These two processes make blood turn solid - it clots. The clotted blood hardens and dries to form a protective scab.