• tunica intima (interna): endothelium; thin basement membrane; longitudinal subendothelial CT; internal elastic membrane (varies in thickness)
• elastic membrane appears as wavy line in transverse section (when thick)
• tunica media:
• smooth muscle circular (spiral)
• elastic fibers as fibrous network or concentric tubes of fenestrated elastic membrane
• elastic fibers and SM often found in alternating layers
• quantity of SM proportional to caliber of vessel
• tunica externa (adventitia)
• external elastic layer (predominantly collagen): between media and externa
• moderately compact CT that grades into surrounding CT
• also contains vasa vasorum (blood vessels of blood vessels; nutrient vessels)
• supply vessels > 1mm in diameter
• in adventitia of arteries; adventitia and media of veins (O₂ content – ß in veins)
• also lymph vessels in larger vessels
• nerves: unmyelinated form networks in externa and innervate SM in media (vasomotor fibers); myelinated sensory fibers may reach intima

Capillaries – 3-12 mm; RBC 7 mm

• usually accompany artery – increase size as approach heart; blood is under less pressure, so travels more slowly
• irregular shape; thin walls; large lumen
• little circular SM; little elastic tissue; more collagenous tissue
• valves: folds of media and interna with elastic tissue
• not in venules; in large and medium veins
• prevent backflow; most found in limbs (arms and legs)
• venules (15-20 mm): gradual transition from caps; long. CT
• intima: endothelium
• media: a few layers of circular SM Þ continuous layer of smooth muscle as diameter approaches 200 m m
• externa: long. collagen and CT; some elastic fibers
• medium veins: 2 mm - 1 cm
• most of superficial veins of extremities and vicera
• intima: thin, circ SM separated by collegen
• media: circ SM, collagen, elastic, ground subst.
• externa: thickest layer; long. SM, bundles of collegen and elastic tissue
• vasa vasorum may be present
• large veins:
• intima – thin, similar to medium veins
• media – similar to medium veins, but with more layers
• externa – thickest layer; longitudinal smooth muscle, bundles of collagen and elastic tissue
• vasa vasorum may be present

• large: act as compression chambers, shock absorbers
• heartbeat expands artery, then walls retract like second pump: more uniform blood flow
• intima: usually thick; endothelial cells not as flat; networks of collagen; ground subst; scattered SM; some elastic fibers continuous with denser bands of media
• media: alternating sheets of SM and elastic tissue mixed with collagen; SM cells have many processes that connect them to the elastic sheets; SM cells parallel in a sheet, sheets are oriented in dif directions
• externa: thick; collagen, elastic tissue, loose connective tissue; lots of blood vessels; fat
• medium arteries, arterioles:
• intima: flattened endothelium; thin internal elastic membrane (sheet); very little else
• media: SM perpendicular to long axis of vessel; appear as zebra stripes or line of circles
• externa: collagen; few elastic fibers; merges with surrounding CT
• arteriole-capillary junction
• arterio-venous anastomoses, metarterioles, pre-capillary sphincters help regulate circulation and bp
• arterioles and venules can be directly connected by sphincters (connections are called anastamoses)
• when closed, blood can fill capillary bed, when open, bypasses capillaries (precapillary sphincters are closed)
• thermoregulation (lips, nose); tissues of intermittent metabolic acitvity (GI tract)

• endocardium: three layers - endothelium and CT; CT and SM; subendocardial layer (Purkinje fibers)
• myocardium: bulk of heart; muscle, reticular, collagen, elastic fibers; highly vascularized
• epicardium: visceral layer of pericardial sac (pericardium is parietal); single layer mesothelium and CT
• CT has many elastic fibers, blood vessels, nerves, fat
• mesothelium – secretes liquid into pericardial space, also used to create plural cavity in lungs