Arteries have thick, muscular walls which can expand and contract. Capillaries also help control the amount of heat lost through the skin. Veins - blood vessels that carry blood back to the heart. Average person has 5-6 liters of blood. Platelets - aid in blood clotting 3.
Blood is a transport system.
O 2 from the lungs to body cells b. food substances from the digestive tract to body cells. CO 2 from body cells to the lungs. Blood clots can form on an outer wound or internally. Blood types . Rh factor - refers to the presence of certain proteins in the blood.