Digestion and Absorption Are Separate, but Related, Processes
Digestion is the process of breaking down complex nutrients into simple molecules. In contrast, absorption is the process of transporting those simple molecules across the intestinal epithelium (Figure 30-1).
The two processes are the result of different biochemical events occurring within the gut. Both processes are necessary for the assimilation of nutrients into the body; absorption cannot occur if food is not digested, and the process of digestion is fruitless if the digested nutrients cannot be absorbed.Disturbances of nutrient assimilation are common in veterinary medicine and may be caused by a variety of diseases, with some affecting digestion and others affecting absorption. The overt signs of failure of nutrient assimilation are often similar, but the biochemical lesions and specific
FIGURE 30-1 Digestion is the process of reducing macromolecules to their constituent monomers. Absorption is the transport of the resultant monomers across the intestinal epithelium into the bloodstream.
therapies associated with Inaldigestive disease can be quite different from those associated with malabsorptive disease. Therefore, diagnosing the cause of failure of nutrient assimilation is a frequent challenge faced by veterinary clinicians, a challenge that requires a thorough understanding of the physiology of nutrient digestion and absorption. This chapter first reviews the structural characteristics of the small- intestinal epithelium that are of particular importance to the digestive and absorptive processes.