Chapter 5. Water and Electrolyte Absorption and Secretion

• Understand the physiological significance of the regulation of luminal water content and daily fluid balance
• Describe the functional anatomy of the intestinal epithelium that permits it to function as a regulator of fluid movement
  • Surface area
• Understand how transport function is integrated with intestinal motility
• Define pathways via which electrolytes can be transferred across epithelial barriers
  • Passive versus active transport
  • Transcellular versus paracellular transport
• Describe how a limited collection of membrane transport pathways are arranged to assemble transepithelial transport mechanisms
• Identify the major electrolyte transport pathways of the small and large intestines and their intracellular mechanisms of regulation
• Identify how subepithelial elements and other regulatory systems impact on epithelial transport function
• Define major pathogenic alterations in intestinal electrolyte transport and their consequences
  • Heat stable enterotoxin of E. coli as an example of molecular mimicry

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Role and Significance

The intestinal tract is designed primarily to permit assimilation of nutrients. Because chemical reactions are required to digest nutrients into components that can be absorbed across the intestinal epithelium, a fluid environment is needed to support these. Thus, control of the amount of fluid in the intestinal lumen is critical for normal intestinal function. This fluid environment permits contact of digestive enzymes with food particles, and in turn the diffusion of digested nutrients to their eventual site of absorption. The fluidity of the intestinal contents also provides for their transit along the length of the gastrointestinal tract without damage to the lining of the epithelium.

Thus, control of luminal fluidity is central to gastrointestinal function. In fact, large volumes of fluid are handled by the intestine on a daily basis in the course of digesting and absorbing meals. Although some of this fluid is derived from beverages and from the food itself, the majority is supplied by the intestine and the organs that drain into it. The daily fluid load can vary somewhat depending on the types of food and drink ingested, but in normal adults it approximates 9 liters (Figure 5–1). Obviously, in health, this large volume is not lost to the stool, but instead is reclaimed by the intestine to avoid dehydration. The majority of the fluid is reabsorbed in the small intestine in conjunction with nutrients, although the colon is more efficient in conserving fluid and takes up almost 90% of the fluid presented to it. Moreover, both the small and large intestines have a large reserve capacity for absorption, and it is only when this is exceeded that excessive water loss to the stool occurs, seen clinically as diarrhea. Finally, the intestine is not normally the major determinant of whole body fluid and electrolyte homeostasis, a physiological function that is relegated to the kidneys. However, because large volumes of fluid can move into and out of the intestine, especially in disease states, abnormalities of fluid transport in the intestine have the potential to lead to serious derangements in body-fluid balances.