The microcirculation is composed of arterioles, capillaries, and venules.

 Continual adjustments of vascular diameter of arterioles are required to properly distribute the cardiac output to the various systemic tissues

Arterioles regulate the distribution of blood flow to capillaries (0.5-1.0 μm); small arterioles (metarterioles) can bypass the capillary beds, shunting flow directly into the small venules (10-40 μm).

The independent vasoactivity of different sized arterioles produces blood flow patterns that vary in speed and direction.

Although flow in the arterioles is usually rapid, continuous, and unidirectional.

Capillary flow is highly variable.

Capillaries have a single layer of endothelial cells through which oxygen and nutrients diffuse to adjacent tissues. Venules have an endothelial cell layer surrounded by an adventitia and contractile pericytes and are involved in transvascular exchange of fluid and macromolecules across the vascular wall. The larger venules and veins collect and store blood for return to the heart.

The cellular and molecular mechanisms that control blood flow in the microcirculation are only beginning to be understood.28

Important determinants of capillary exchange through the endothelial cell membrane (diffusion) include (1) the capillary density, which is directly related to the metabolic activity of tissue; (2) lipid solubility of the material to be exchanged; (3) the free diffusion coefficient (small molecules and molecules with very little net electric charge have very high free diffusion coefficients); and (4) the relative concentrations of the material in the blood and the tissue interstitium. Thus, the rate of diffusion for a substance Q moving from the vessel to the interstitial space, dQ/dt is proportional to the capillary wall area (2πrl), the difference in concentration of the substance (ΔC), which represents the driving force for the movement across the vessel wall, and the permeability (P) which is a function of lipid solubility and the free diffusion coefficient: dQ/dt = (2πrl)(P)(ΔC). Permeability for substances varies by capillary bed (eg, whereas capillaries in the brain restrict the diffusion of almost all solutes, liver capillaries have a very high permeability to large solutes such as albumin). Endothelial transport across restrictive beds is accomplished by other processes such as pinocytosis and vesicular transport. Pores occupy less than 1% of the total capillary surface area; there are more present on the venular than arteriolar end of the capillary system, and therefore lipid-insoluble materials (eg, glucose, small ions) exchange slowly. Thus, whereas lipid-soluble materials are considered flow limited, lipid-insoluble materials (except water) are considered to be relatively limited by diffusion.

The transvascular exchange of water occurs primarily through the bulk flow of water through the pores in the capillary walls (QH2O); the amount of bulk flow is a function of the difference in hydrostatic pressure in the vessel (CHP, variable, depending on tissue bed) and interstitium (THP, small but variable), the capillary filtration coefficient (CFC), the plasma colloid osmotic pressure (COP, caused by protein in blood plasma, ~20 mm Hg), and the tissue colloid osmotic pressure (TOP, caused by proteins in the interstitial space, ~4.5 mm Hg). Thus, the net force out of the vessel (filtration) is a hydrostatic force, and the net force into the vessel (reabsorption) is a colloid osmotic force. The effect of these forces on transvascular water flow is described in the Starling equation: QH2O = CFC[(CHP – THP) –σ(COP – TOP)], where σ is the reflection coefficient for the movement of proteins across the capillary wall (the inverse of the permeability of the vessel wall to protein). The capillary filtration coefficient is the product of capillary surface area and permeability and is related to number and size of the pores through which water can pass through the vessel. Because the balance of forces is different across the length of a capillary bed, filtration occurs near the arterial end and reabsorption near the venule end of the capillary.

Of these forces, capillary hydrostatic pressure (CHP) is the principal mechanism responsible for transcapillary exchange of water. CHP increases whenever arterial pressure increases, venous pressure increases, venule resistance to flow increases, or arteriole resistance to flow decreases. Mathematically, CHP = (RV/RA)PA + PV, where RV/RA is the ratio of venule to arteriolar resistance, PA is approximately mean arterial pressure, and PV is approximately central venous pressure. Capillary pressure is far more sensitive to changes in venous pressure than changes in arterial pressure. The ratio of venule to arteriole resistance (RV/RA) is approximately 0.1; thus, arterial pressure must increase 10 mm Hg to cause a 1-mm Hg increase in capillary hydrostatic pressure, but a 1 mm Hg increase in venous pressure will cause a similar increase in capillary hydrostatic pressure. Greater filtration than reabsorption produces tissue lymph flow; the total volume of lymph fluid (important in returning plasma proteins that leaked from the microcirculation and transport of chylomicrons) is approximately 3 to 4 L/d.