In this topic you will learn about how your body moves important materials to and from each of your cells.

Blood travels from all parts of your body to the heart. Then it is pumped to your lungs and back to the heart to be pumped out to the body again. Blood is a mixture of liquid and cells. The pale-yellow liquid, called plasma, is mostly water. Plasma contains dissolved sugars, proteins, and gases.

Most cells in blood are red blood cells, which are shaped like a doughnut without the hole. They contain the chemical hemoglobin, which carries oxygen around your body. Blood also contains germ-fighting white blood cells and platelets. Platelets are cell fragments in the blood that help to form clots. When you have a cut, platelets stop the bleeding.

Blood travels through three kinds of blood vessels—arteries, veins, and capillaries. An artery is a blood vessel that carries blood away from the heart. A vein is a blood vessel that carries blood to the heart. The smallest veins and arteries are connected by capillaries. A capillary is the smallest type of blood vessel, where materials are exchanged between blood and body cells. Capillaries are too tiny to see without a microscope. Red blood cells move through them in single file.

A transfusion is blood taken from a healthy person and given to another person who needs it. The person giving the blood is the donor. The person receiving the blood is the recipient.

Scientists discovered two chemicals, called antigens, on the surface of red blood cells. These antigens are named A and B. People with only type A antigens in their blood have type A blood. People with only type B antigens have type B blood. AB blood contains both antigens. Blood without A or B antigens is called type O blood.

Certain antibodies affect these blood types. An antibody is a protein in blood that helps find and destroy harmful materials in the body. Type O blood is special because it has no A or B antigens, so antibodies won't clump around it when it is given to a recipient having A or B antibodies. That is why it is called the "universal donor" type.

Your heart constantly pumps blood to every part of your body. Each contraction and relaxation makes one heart beat. Every time your heart pumps, it squeezes blood out of your heart. The heart uses a strong force to push blood out. That force is your blood pressure. The pushed blood presses against the artery walls from the inside. You can feel an artery wall stretch when you take your pulse.