Let's start by taking a closer look at what blood is, and what it does.

Blood flows throughout your entire body, carrying oxygen and nutrients to all of your cells, and taking away the waste products that they produce. Blood is a fluid that carries several different kinds of cells. The three types that we will talk about are red blood cells, white blood cells, and special cell fragments called platelets.

Red blood cells carry oxygen to your organs, and take carbon dioxide away from them.

White blood cells help your body fight infection from bacteria and viruses. They play a very important role in your body's immune system.

Platelets help to seal off a blood vessel when it has been cut or torn, so they act like your own natural bandage. They clump together to plug a wound inside the body or on the skin, and stop blood from flowing out of the damaged area. This is the first step in the formation of a "clot," which we'll talk more about later.

Because red and white blood cells and platelets are so important, they are constantly flowing throughout your body, in a network of blood vessels.