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# Cardiovascular System ## Blood Vessels ### Arteries * Carry blood away from the heart * Walls have 3 layers * Tunica intima * Endothelium * Basement membrane * Internal elastic lamina * Tunica media * Smooth muscle * External ela...

# Cardiovascular System ## Blood Vessels ### Arteries * Carry blood away from the heart * Walls have 3 layers * Tunica intima * Endothelium * Basement membrane * Internal elastic lamina * Tunica media * Smooth muscle * External elastic lamina * Tunica adventitia * Connective tissue | Type of artery | Structure | Function | | :------------- | :------------------------------------------------------------------------------- | :----------------------------------------------------------------------------------------------------- | | Elastic | Largest, near heart, more elastic fibers, less smooth muscle | Pressure reservoir, expand and recoil as heart ejects blood | | Muscular | Thickest tunica media, more smooth muscle, less elastic fibers | Vasoconstriction and vasodilation | | Arterioles | Smallest, tunica media only 1-2 layers of smooth muscle, feed into capillaries | Control blood flow into capillary beds via vasodilation and vasoconstriction, also called resistance vessels | ### Capillaries * Smallest blood vessels * Walls have only tunica intima * Endothelium * Basement membrane * Nutrients and gases diffuse across walls * Types | Type | Structure | Function | Locations | | :------------ | :--------------------------------------------------------------------------------------------- | :------------------------------------------------------------------- | :---------------------------------------------------------------------------------------------------------------------------------- | | Continuous | Most common, endothelial cells form complete lining, tight junctions, intercellular clefts | Least permeable | Skin, muscles, lungs, CNS | | Fenestrated | Endothelial cells have pores (fenestrations) | More permeable than continuous | Kidneys, small intestine, endocrine glands | | Sinusoidal | Most permeable, few tight junctions, large intercellular clefts, large lumens, incomplete BM | Most permeable, blood flow sluggish, allow large molecules to pass | Liver, bone marrow, spleen, adrenal medulla | | Capillary Bed | Network of capillaries | | Blood flows from arteriole to venule: Arteriole → metarteriole → capillaries → thoroughfare channel → venule, Precapillary sphincters | ### Veins * Carry blood toward the heart * Walls have 3 layers, thinner than arteries * Tunica intima * Endothelium * Basement membrane * Tunica media * Smooth muscle * Tunica adventitia * Connective tissue * Larger lumens than arteries * Valves prevent backflow of blood | Type of vein | Structure | Function | | :----------- | :------------------------------------------------ | :------------------------------------------------------------------ | | Venules | Smallest, drain capillaries | | | Veins | Thinner walls, larger lumens, less smooth muscle | Blood reservoirs, contain ~65% of blood volume at any given time |Figure of a blood vessel including the tunica intima made of the endothelium, basement membrane and internal elastic lamina; the tunica media made of smooth muscle and external elastic lamina; and the tunica adventitia made of the connective tissue. Also shown are the capillary including the endothelium, basement membrane and intercellular cleft. Showing the flow of blood from the Arteriole to the metarteriole which turns into the capillaries then the thoroughfare channel and finally the venule. The precapillary sphincters are located at the beginning of the capillaries.