The small intestine is the organ of the gastrointestinal tract where the final steps in the digestion and absorption of most of the nutrients and food components take place. It is the receptacle for pancreatic juice (the secretions of pancreas) and bile (produced by liver), the two key players in the hydrolysis of proteins, lipids, oligosaccharides, and polysaccharides, as well as conjugated food components. But it also has its own hydrolases (especially lactase, sucrase, and D-glucoamylase) synthesized by the enterocytes, and it is particularly active in absorbing amino acids, lipids, fatty acids, monosaccharides, vitamins, and most minerals. In a day, the small intestine receives ±2 l of dietary fluid plus ±7 l of gastrointestinal secretions; out of this ±9 l of fluid, only ±2 l will pass into the colon.
The exocrine pancreas is composed of alveolar glands that are similar to salivary glands. The glands are composed of acinar cells that discharge granules containing digestive enzymes (the zymogen granules) by exocytosis into the small pancreatic ducts that coalesce into the duct of Wirsung. The exocrine pancreas produces two secretions: one is electrolytic, and the other is enzymatic. The electrolytic secretion is alkaline (pH = 7.5-9) because it is rich in HCO3D (±113 meq/l). It buffers the acidic chyme coming out of the stomach and creates the optimum pH for the activity of the enzymes secreted by the pancreas. The enzymatic secretion of the pancreas contains a large number of different hydrolases (proteases, lipases, and amylases) (Table 2.2). The very powerful protein-splitting enzymes are secreted as zymogens, the inactive proenzymes that become activated in the duodenal lumen through an interconnected cascade of enzymatic reactions (Figure 2.3).
Trypsin also activates a prophospholipase A2 into phospholipase A2 that hydrolyses lecithin to produce lysolecithin. The other hydrolases (lipases, amylases, and nucleases) are secreted in their active forms. The exocrine pancreatic secretions are primarily under the control of two hormones: (1) secretin, which stimulates the electrolytic secretion and (2) cholecystokinin (CCK), which acts on the acinar cells to cause release of the zymogen granules. The secretion of CCK is stimulated by the presence of peptides, amino acids, fatty acids, or Ca in the duodenum.
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