Phases of Digestion

The return of the bile salts to the liver from the small intestine is the most potent stimulus of bile secretion. In fact, these bile salts may cycle two to five times during each meal. The intestinal hormone secretin, which is released in response to acid in the duodenum, enhances aqueous alkaline secretion by the liver. Secretin has no effect on the secretion of bile salts. During the cephalic phase of digestion, before food even reaches the stomach or intestine, parasympathetic stimulation, by way of the vagus nerve, promotes bile secretion from the liver. 

Figure 6. Details of borosilicate glass dialysis unit for separating phases of digestion...

The cephalic phase of digestion stimulates only a fraction of the maximum possible levels of gastric and pancreatic secretions. This phase does not seem to produce a rise in the levels of gastrin and CCK. Cephalic stimulation of the pancreas, as mediated by the vagus nerve, provokes release of pancreatic enzymes into the small intestine. Cephalic stimulation of the parietal cells, as mediated by the vagus nerve, provokes release of gastric acid into the lumen of the stomach- In humans, the cephalic phase does not seem to result in release of bicarbonate into the lumen of the small intestine. 

The chemical phase of digestion also involves the hormone cholecystokiiun (CCK). Dietary fats and proteins elicit the release of CCK from cells of the intes tines. The versatility of this hormone in digestion is revealed, later in this chapter, via studies involving dogs, rats, and humans. These studies address the influence of CCK on the release of pancreatic enzymes, bile salts, and pancreatic bicarbonate into the lumen of the small intestines-... 

The main features of the rumen phase of digestion include ... 

All chemical and mechanical phases of digestion are directed toward changing the ingested material (food or xenobiotics) into absorptive forms that can pass through mucosal cells into blood and lymphatic vessels. The process of absorption occurs via passive diffusion (para and transcellular) and carrier-mediated process (facilitated or active transport). About 90 % of absorption of nutrients occurs in the small intestine. 

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