The microcirculation is comprised of arterioles, capillaries, venules, and terminal lymphatic vessels.
Arterioles
Small precapillary resistance vessels (10-50 µ) 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 µ) 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 µ) 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).
