Microcirculation Structure and Function

The microcirculation is comprised of arterioles, capillaries, venules, and terminal lymphatic vessels.

Arterioles
· Small precapillary resistance vessels (10-50 ) composed of an endothelium surrounded by one or more layers of smooth muscle cells.
· Richly innervated by sympathetic adrenergic fibers and highly responsive to sympathetic vasoconstriction via both a 1 and a 2 postjunctional receptors.
· Represent a major site for regulating systemic vascular resistance.
· Rhythmical contraction and relaxation of arterioles sometimes occurs (i.e., spontaneous vasomotion).
· Primary function within an organ is flow regulation, thereby determining oxygen delivery and the washout of metabolic by-products.
· Regulate, in part, capillary hydrostatic pressure and therefore influence capillary fluid exchange.









Capillaries
· Small exchange vessels (6-10 ) composed of highly attenuated (very thin) endothelial cells surrounded by basement membrane – no smooth muscle.
· Three structural classifications:
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.
· In some organs, precapillary sphincters (a circular band of smooth muscle at entrance to capillary) can regulate the number of perfused capillaries.
Venules
· Small exchange vessels (10-50 ) composed of endothelial cells surrounded by basement membrane (smallest postcapillary venules) and smooth muscle (larger venules).
· Fluid and macromolecular exchange occur most prominently at venular junctions.
· Sympathetic innervation of larger venules can alter venular tone which plays a role in regulating capillary hydrostatic pressure.
Terminal Lymphatics
· 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.
· Larger lymphatics also have smooth muscle cells.
· Spontaneous and stretch-activated vasomotion is present which serves to "pump" lymph.
· Sympathetic nerves can modulate vasomotion and cause contraction.
· 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).


Vessel Sizes

Vessel Diameter in Microns
Arterioles 20-50
Capillaries 5-10
Sinusoids 30-40
Venules 30-40

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