Absorbed Nutrients Enter the Capillaries by Diffusion from the Lateral Spaces
The collective action of the various intestinal absorptive mechanisms concentrates solutes (nutrients) in the lateral spaces. When the concentrations of individual solutes in the lateral spaces exceed their concentrations in blood, a gradient favoring the diffusion of nutrients from lateral spaces into the capillaries is established.
The movement of solutes by diffusion into the capillaries creates an osmotic force that draws water into the capillaries (waler follows solute). In addition, the oncotic force (the osmotic force exerted by plasma proteins; see Chapters I and 23) also tends to draw water into the capillary lumen. Moreover, hydrostatic pressure in the lateral spaces may force water directly into the capillaries. Lateralspace hydrostatic pressure can be created by the osmotic effect of absorbed solutes. As these solutes attract water from the intestinal lumen, the lateral spaces become distended, developing a small amount of hydrostatic pressure. There are two exits for the relief of this pressure: the light junctions and the capillary endothelium, with the endothelium presenting the route of least resistance to water flow. Thus, water under slight pressure within the Iaieral spaces tends to flow into the capillaries rather than into the intestinal lumen (Figure 30-22).
FIGURE 30-22 ■ Water (H2O) enters the lateral spaces because of osmotic effects created by absorbed solutes, thus creating a hydrostatic pressure head in the lateral spaces. Under pressure, the lateral-space solution can exit through the tight junctions or through the basement membrane of the capillaries. Under normal conditions the route of least resistance is into the capillaries, resulting in little movement of water from the lateral space into the intestinal lumen.