1. 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

2. 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

3. 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

4. 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

5. Capillary Exchange Model