Morphological Characteristics of Blood Vessels
Comparison of Capillaries, Sinusoids and Lymphatic Capillaries

Region	Function	Layers
		tunica intima	tunica media	tunica adventitia
heart	pump	endocardium (=endothelium, thin subendocardial connective tissue, fibroelastic connective tissue)	myocardium (=cardiac muscle)	epicardium (=visceral pericardium and subepicardial areolar connective tissue with coronary vessels)
elastic arteries	conducting	endothelium with Weibel Palade granules, thick subendothelial connective tissue, a few longitudinal smooth muscle fibers	40–60 spiraling fenestrated sheets of elastic laminae alternating with layers of smooth muscle/collagen/elastic fibers (=lamellar units)	thin to well-developed fibrous connective tissue, vasa vasorum, nervi vascularis
muscular arteries	distributing	endothelium, subendothelial areolar connective tissue, some longitudinal smooth muscle [internal elastic lamina]	smooth muscle layers with some elastin fibers (not laminae) (large art: 20–40 layers; small art: 3–20 layers)	[external elastic lamina only in largest arteries] mostly collagen merges with surrounding connective tissue, vasa vasorum, nervi vascularis
arterioles	regulating	endothelium, thin or no subendothelial connective tissue [internal elastic lamina]	fewer than three layers of smooth muscle cells	very thin connective tissue, merges with surrounding connective tissue
capillaries	exchange	endothelium (occasional pericytes)	none	none
venules	exchange, capacitance	endothelium, some pericytes	very thin; 0–3 layers of smooth muscle	very thin: mostly collagen
veins	capacitance, returning	endothelium, little subendothelial connective tissue, valves	thin and loosely packed: 1–3 layers smooth muscle with associated reticular and elastic fibers	relatively thick longitudinal collagen, elastic fibers, fibroblasts
large vein	returning	endothelium, thick subendothelial connective tissue, longitudinal smooth muscle	poorly developed; 3–12 layers smooth muscle loosely arranged with associated connective tissue	relatively thick longitudinal bundles of smooth muscle and collagen fibers, vasa vasorum
heart	pump	endocardium	myocardium	epicardium

	Continuous capillaries	Fenestrated capillaries	Sinusoidal capillaries	Lymphatic capillaries
common name	somatic capillary	visceral capillary	sinus	lymphatic
typical location	CNS, muscle, lung, many connective tissues	glomerulus of kidney, endocrine glands, most viscera	liver, spleen, bone marrow, lymph node	in most tissues except brain, bone, cartilage, placenta
endothelial continuity	continuous	continuous	discontinuous	continuous
fenestrations	none (caveolae present)	many pores, 60–100 nm in diameter; pores (fenestrations) may be in patches	may be present	only in lacteals
lumen diameter and profile	small (5–12 mm), regular profile	small (5–12 mm), regular profile	large (30–50 mm), fits in tissue spaces	large (10–50 mm), irregular profile
basement membrane	well-developed, continuous	well-developed, continuous	scanty or absent	scanty or absent
intercellular spaces	none	none	present, up to 0.5 mm	none
intercellular junctions	fascia or zonula occludens	fascia occludens	absent, except in spleen	usually absent
pericytes	present	present	absent (macrophages may be associated with the abluminal side)	absent

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