The heart and blood vessels make up the body’s circulatory system. With each beat, the heart pumps blood out through thick-walled vessels called arteries. Blood returns to the heart in veins. Between the arteries and veins is a vast network of tiny vessels called capillaries, which have thin walls that let oxygen, food molecules, and other chemicals pass freely across.
The human body contains thousand of miles of blood vessels, from arteries as thick as your thumb to capillary vessels finer than hairs. Like a network of roads that reach every house in a country, blood vessels deliver vital supplies to every living cell in the body, as well as carrying away waste.
Blood is a liquid tissue, made up of billions of cells suspended in a watery liquid called plasma. By volume, blood is about 54 percent liquid and 46 percent cells. Oxygen is carried by a protein called hemoglobin in red blood cells. Food molecules, hormones, salt, wastes, and various other chemicals are carried dissolved in plasma.
Red blood cells: These disc-shaped cells pick up oxygen in the lungs and release it everywhere else in the body.
White blood cells and platelets: Germs that get into the body are destroyed by white blood cells. Platelet cells help blood to clot.
Blood flows around your body in two loops, driven by the heart. The shorter loop runs from the heart to the lungs, where blood picks up oxygen. The longer loop takes oxygen-rich blood to the rest of the body. Blood rich in oxygen is bright red, while blood with little oxygen is dark red.