Cardiovascular System BLOOD VESSELS There are three types of blood vessels: arteries and arterioles - carry blood away from heart veins and venuoles - carry blood to heart capillaries - exchange materials with all cells

Arteries have a thick wall of three layers (inner, outer connective, and thick middle elastic muscle allows expansion of artery with each heartbeat so thick that artery walls have blood vessels ARTERIOLES small branches of arteries muscle fibres in walls relax/contract to make artery larger/ smaller in diameter if many are larger in diameter - blood pressure decreases if many are smaller in diameter - blood pressure increases usually carry oxygenated blood

CAPILLARIES arterioles branch into very small vessels only one cell thick capillary bed = network of capillaries; present in all body tissues oxygen and nutrients diffuse out of capillaries into the fluid surrounding cells then into cells carbon dioxide and ammonia diffuse into capillaries then into lungs and kidney after eating - capillary bed open to digestive tract; closed to muscles during exercise, capillary bed open to muscles of legs and arms; closed to digestive tract the larger the number of capillaries open; the more the decrease in blood pressure

VEINS AND VENULES venules carry blood from capillary beds to veins, back to the heart walls are thinner than arteries but major veins have valves which allow the blood to flow only toward the heart when open and prevent back flow when closed skeletal muscles contraction (i.e., calf muscles) help move blood against gravity usually carry deoxygenated blood

Capillary Exchange Model