Fluid Transport

There is heterogeneity to the mucosal epithelium in several aspects dependent on the location in the alimentary canal. The type, variety, and number of ion transporters, channels, and carrier proteins vary from region to region (e.g., from jejunum to colon). Additionally, the nature of interepithelial cell junctions varies from the proximal to distal intestinal tract, influencing the 'leakiness' of the respective regions. Finally, a clear gradient in cell composition and function between colonic crypt cells and those on the surface exists. Physiologic heterogeneity follows the aforementioned patterns, defining tissue function in these respective areas. For example, the colonic crypts serve more of a secretory function, whereas the villus structures seen most notably in the jejunum exhibit greater absorptive function. This heterogeneity is key in understanding changes in intraluminal osmolality and fluid shifts that occur in the intestine.

Approximately 98% (91 per day) of the daily fluid load handled by the intestine is reabsorbed. Of this, the jejunum absorbs 85%, and the colon absorbs approximately 13% (1.51).

Passive reabsorption of water occurs in the intestines, regulated primarily by electrolyte transport (i.e., following an osmotic gradient). Na-driven or -related transport mechanisms are the primary driving force

Table 2 Electrolyte transport

Transporter

Location

Type

Function(s)

Na and Cl Cl

Na-K-ATPase

Na-H exchangers

NaCl

Protein channel

Cl K

Protein channel Protein channel

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