Absorption

Absorption normally refers to the movement of nutrients, water, and electrolytes through the epithelial cell lining into the blood or lymph. Secretion refers to the movement of water and electrolytes in the opposite direction.

Absorptive Pathway

The pathways taken by nutrients on their journey from the lumen of the intestine to the blood or lymph are shown in Fig. 2. They actually consist of eight different barriers:

1. An unstirred layer of fluid that is crossed by diffusion.

2. The glycocalyx, a filamentous glycoprotein surface coat, whose function is not entirely understood.

3. The apical membrane of the cell, which contains protein carriers for the specific transport of some nutrients.

4. The cytoplasm of the enterocyte.

5. The basolateral cell membrane.

6. The intercellular space.

7. The basement membrane.

8. The wall of the capillary or lymph vessel.

The last three of these are normally crossed by simple diffusion; specific carrier-mediated transport processes exist in the basolateral membrane for most electrolytes and nutrients.

Blood is supplied to each villus by one or more central arterioles, which break up into capillaries that form a network beneath the bases of the absorbing cells. Thus, blood flow ascends centrally and descends peripherally, and absorbed substances enter the descending network. Eventually, this blood drains into the portal vein. Each villus contains a central lymphatic

FIGURE 2 The pathway traversed by absorbed solutes. Beginning in the lumen, barriers consist of an unstirred layer of fluid, the glycocalyx, the apical cell membrane, the cytoplasm of the enterocyte, the basolateral cell membrane, the intercellular space, the basement membrane, and finally, the wall of the capillary or lymphatic vessel. (Modified from Johnson LR, ed., Gastrointestinal physiology, 6th ed. St Louis: Mosby, 2001.)

FIGURE 2 The pathway traversed by absorbed solutes. Beginning in the lumen, barriers consist of an unstirred layer of fluid, the glycocalyx, the apical cell membrane, the cytoplasm of the enterocyte, the basolateral cell membrane, the intercellular space, the basement membrane, and finally, the wall of the capillary or lymphatic vessel. (Modified from Johnson LR, ed., Gastrointestinal physiology, 6th ed. St Louis: Mosby, 2001.)

vessel called a lacteal. The products of fat digestion enter the lacteals and, eventually, the bloodstream via the left thoracic duct.

Transport Processes

The apical membrane of the enterocyte is capable of transporting solutes by a number of different mechanisms. Many solutes cross the apical membrane by the process of simple or passive diffusion. Simple diffusion takes place through pores of the membrane or, in the case of lipid-soluble molecules, through the lipid domain of the membrane. It is governed only by the concentration and electrical gradient of the molecule across the membrane. Many nutrients are taken into the cell by active transport. This requires the combination of the absorbed molecule with a specific protein carrier. The carrier often also requires the binding of Na+. These processes are termed secondary active transport mechanisms because they make use of a Na + gradient derived from the energy of the (Na + ,K + )-ATPase in the basolateral membrane. Active transport results in the accumulation of substances against their electrochemical gradients. Some molecules combine with specific membrane carriers in order to be transported across the membrane, but they cannot be accumulated against a concentration gradient, and no energy is involved. This type of transport also depends on the concentration gradient of the molecule being transported and is called facilitated diffusion. Finally, some large molecules may be transported by the process of pinocytosis, in which the plasma membrane invaginates around the molecule, "swallowing" it. This process usually takes place at the bases of the microvilli. Some movement of water and small electrolytes takes place through the tight junctions directly into the intercellular spaces. This is termed paracellular transport.

Transport processes are not all evenly distributed over the entire length of the small intestine. In general, the absorptive capacity for most nutrients is greater proximally and decreases distally. Active mechanisms for the absorption of vitamin B12 and bile salts, however, are present only in the ileum. The absorptive capacity of the small intestine greatly exceeds the load placed on it. As much as 60-70% of the bowel may be removed without significantly decreasing the absorption of protein and carbohydrate digestion products. However, if the ileum is removed, the ability to absorb vitamin B12 is lost, and steatorrhea occurs because of the inability to actively reabsorb bile salts. The liver cannot synthesize sufficient new bile acids to replace those necessary for fat digestion and absorption.

Maltose ; Limit Dextrin

Maltotriose

Maltose ; Limit Dextrin

Maltotriose

FIGURE 3 Products of exhaustive hydrolysis of starch by «-amylase.
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