BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.4. Nervous and hormonal regulation of digestive gland functions

8.4.3. Pancreatic juice and bile

When acidic chyme passes from The Stomach into the duodenum, secretin and CCK are released (see above). Secretin is secreted in response to the arrival of acid, whereas CCK secretion is stimulated by the presence of partially digested fats and Proteins. Both Hormones regulate The production of pancreatic juice and Bile. In terms of its action, secretin acts as an anti-acid hormone. It stimulates The formation of bicarbonate ions in the Pancreas and Liver, increasing the alkalinity of pancreatic juice and bile. The resulting alkaline environment neutralizes the acid coming from the stomach. CCK stimulates the synthesis of digestive Enzymes by the pancreas and triggers the contraction of the Gallbladder to release bile into the duodenum (see Fig. 8.11, which shows the positions of the pancreas, liver, and gallbladder). Bile is produced in the liver, but is stored and concentrated in the gallbladder. The pH of bile ranges from 7.6 to 8.6.

The secretion of pancreatic juice and bile is also stimulated by nervous Reflexes. During the cephalic and gastric phases of gastric Digestion (Section 8.4.2), the Vagus nerve stimulates the liver to secrete bile and the pancreas to secrete enzymes.

Table 8.5 lists the hormones secreted by various PARTS OF THE digestive tract and their target Organs.

Class="center">Table 8.5. Hormonal control of digestive secretions and their target organs