Lecture 118 of 373: Absorption of Vitamins and Minerals; Neural Regulation of Gastrointestinal Process (31 mins) | CUET (Common University Entrance Test) PG Zoology (SCQP28) | Complete Video Course 373 Lectures [222 hrs : 42 mins]

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Absorption of vitamins and minerals

Fat soluble vitamins -A, D, E, and K follow the pathway for fat absorption. They are solubilized in micelles. Any interference from with the secretion of bile or the action of bile salts in the intestine decreases the absorption of the fat-soluble vitamins (malabsorption), leading to deficiency of fat-soluble vitamins. For example, non- tropical spruce where loss of intestinal surface area takes place due to a sensitivity to the wheat protein gluten, leading to vitamin D malabsorption. This results in a decrease in calcium absorption in the GI tract.

Water soluble vitamins (vitamin B and C) are absorbed by the diffusion or mediated transport. Vitamin B-12 absorption has a special case. As it is a very large, charged molecule it needs to be first bound to a protein known as intrinsic factor, secreted by the acid secreting cells in the stomach. Intrinsic factor bound vitamin B-12 then binds to a specific sites on the epithelial cells in the lower portion of the ileum, where it is absorbed by endocytosis. Vitamin B-12 is required for erythrocyte formation and deficiencies result in pernicious anemia.

Among minerals, sodium absorption is a primary active process, using Na⁺ /K⁺ - ATPase pumps. Chloride and bicarbonate ions are absorbed with the sodium ions and contribute another large fraction of the absorbed solute. Iron absorption depends upon the body՚s iron content. Iron ions are actively transported into intestinal epithelial cells, where most of them are incorporated into ferritin, the protein-iron complex, that functions as an intracellular iron store. The absorbed iron that does not bind to ferritin is released on the blood stream where it circulates throughout the body bound to plasma protein transferrin.

Neural regulation of the gastrointestinal processes

The gastrointestinal tract (GI tract) has its own local nervous system known as enteric nervous system, in the form of two nerve networks, the myenteric plexus and the submucosal plexus. These neurons either synapse with other neurons in the plexus or end near smooth muscles, glands and epithelial cells. Myenteric plexus stimulates smooth muscle activity while submucosal plexus influences secretory activity. They are independent of CNS or central nervous system. These work on the principle of short reflexes.

CNS works on the long reflex principle. Nerve fibres from both sympathetic and parasympathetic branches of the autonomic nervous system enter the intestinal tract and synapse with neurons in both plexuses. Efferent and afferent branches of nerve fibres are working to get the stimulus and response feedback. Thus, CNS can influence the motility and secretory activity of the gastrointestinal tract.

Hormonal regulation of the gastrointestinal processes

The hormones are secreted from the endocrine cells scattered throughout the epithelium of the stomach and small intestine. Out of the several hormones identified, four best understood are – gastrin (from stomach), secretin, cholecystokinin (CCK) and glucose -dependent insulinotropic peptide (GIP) (all three secreted from small intestine). Each hormone participates in feedback control system and affects more than one type of target cell. The presence of fats and amino acids in the small intestine (SI) triggers CCK secretion from cells in SI into the blood. Circulating CCK stimulates secretion by the pancreas of the digestive enzymes. CCK also causes gall bladder to contract, delivering to the intestine the bile salts required for micelle formation. As amino acids or fats are absorbed, stimuli for the CCK are removed too. The following flow chart can explain the functioning of CCK

Both hormonal and neural control of gastro intestinal system has cephalic phase (initiated when receptors in the head are stimulated by sight, smell , taste and chewing of food by vagus nerves), gastric phase (stimuli in the stomach initiate reflexes for acid secretion, distention of stomach wall) and intestinal phase (stimuli in the intestine).