More than half of your blood is a light yellow watery liquid called plasma. Plasma contains nutrients and waste products, along with chemicals and matter needed for clotting, or sealing a wound before too much blood escapes. The rest of blood is made of tiny cells. Most are red blood cells, which distribute oxygen throughout your body and carry away the waste gas carbon dioxide, which is released from your lungs. The remaining cells are white blood cells, which protect you from infection by attacking and destroying disease-causing germs that enter your body. Red blood cells are the smallest cells in your body. But what they lack in size they make up for in number: in a drop of blood the size of the head of a pin there are 5 million red blood cells. In that same drop are 10,000 white blood cells and 250,000 platelets, small ovals of matter that gather wherever a blood vessel is injured to plug the hole and help form a clot.