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Microcirculation Structure and Function
The microcirculation is comprised of arterioles,
capillaries, venules, and terminal lymphatic vessels.
Arterioles
Capillaries
Continuous (found in muscle, skin, lung, central
nervous system) basement membrane is continuous and intercellular clefts are tight
(i.e., have tight junctions); these capillaries have the lowest permeability.
Fenestrated (found in exocrine glands, renal glomeruli, intestinal
mucosa) perforations (fenestrae) in endothelium result in relatively high
permeability.
Discontinuous (found in liver, spleen, bone marrow)
large intercellular gaps and gaps in basement membrane result in extremely high
permeability.
Large surface area and relatively high permeability (especially at
intercellular clefts) to fluid and macromolecules make capillaries the primary site of
exchange for fluid, electrolytes, gases, and macromolecules.
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In some organs, precapillary sphincters (a circular band of smooth muscle at entrance to
capillary) can regulate the number of perfused capillaries.
Venules
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Small exchange vessels (10-50 µ) composed of endothelial cells surrounded by basement
membrane (smallest postcapillary venules) and smooth muscle (larger venules).
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Fluid and macromolecular exchange occur most prominently at venular junctions.
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Sympathetic innervation of larger venules can alter venular tone which plays a role in
regulating capillary hydrostatic pressure.
Terminal
Lymphatics
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Composed of endothelium with intercellular gaps surrounded by highly permeable basement
membrane and are similar in size to venules terminal lymphatics end as blind
sacs.
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Larger lymphatics also have smooth muscle cells.
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Spontaneous and stretch-activated vasomotion is present which serves to "pump"
lymph.
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Sympathetic nerves can modulate vasomotion and cause contraction.
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One-way valves direct lymph away from the tissue and eventually back into the systemic
circulation via the thoracic duct and subclavian veins (2-4 liters/day returned).
RK Revised
04/16/2007
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