Blood Vessels  Arteries- strong and elastic Fx: carry blood away from heart under high pressure  Arterioles- thinner arteries Surface is made of endothelium which secretes biochemicals to prevent clotting within vessels

Blood Vessels  Capillaries- smallest in diameter Connect smallest arterioles and venules Walls are thin; allow for substance exchange Openings are thin slits and vary in size Increase in # of capillaries = increase in metabolism rate (lots in nerve and muscle)

Blood Vessels  Venules- continue from capillaries, merge to form veins  Veins- carry blood back to heart under low pressure Roughly follow pathway parallel to arteries Thinner walls than arteries (less pressure) Many have valves to prevent backflow Serve as blood reservoirs

Veins  Flap-like valves close if blood begins to flow backwards (aid in blood return to heart)  Varicose veins = faulty valves

Venous Return  Blood flow to heart depends on: Heart action (a little) Skeletal muscle contraction (massaging action moves blood from one valved region to the next) Respiratory movements (diaphragm acts like skeletal muscle on thoracic and abdominal veins) Venoconstriction