When you're injured and bleeding, your blood forms a clot to seal damaged blood vessels and stop the bleeding. Blood has special clotting substances and platelets (very small blood cells) in order to make the clot.
Blood is the red fluid in your arteries and veins. It provides the oxygen, water, and nutrients that your tissues and organs need to survive. You have about 5 liters (a little more than 1 gallon) of blood in your body. Your heart constantly pumps blood throughout your body.