Microcirculation Structure and Function

The microcirculation comprises arterioles, capillaries, venules, and terminal lymphatic vessels.

Arterioles

• Small precapillary resistance vessels (10-200 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 α₁ and α₂ postjunctional receptors.
• Represent the major site for regulating systemic vascular resistance.
• Rhythmical contraction and relaxation of arterioles sometimes occurs (i.e., spontaneous vasomotion).
• Primary function of arterioles within an organ is flow regulation, which determines oxygen delivery and the washout of metabolic by-products from the tissue.
• 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 (between adjacent endothelial cells) 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, the number of perfused capillaries can increase during high flow states (e.g., in contracting muscle) in response to arteriolar vasodilation and redistribution of precapillary pressures.

Venules

• Small exchange vessels (10-200 μ) composed of endothelial cells surrounded by basement membrane (smallest postcapillary venules) and smooth muscle (larger venules).
• Fluid and macromolecular exchange occur at small postcapillary 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 out of the tissue.
• 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).

Revised 12/27/2022