Cover Story Overview on Digestive System Chief cel ls or peptic cel ls: secrete pepsin, a proteolytic enzyme, which is largely responsible for partial digestion of proteins. G cel ls: secrete the hormone gastrin.
Mechanism of acid secretion The daily volume of gastric secretion is around 2000 ml. The HCl is secreted by the parietal cells. The K+ activated ATPase and the enzyme carbonic anhydrase are necessary for the production of HCl. This needs energy which is provided by the hydrolysis of ATP. The hydrogen ions are then secreted into gastric lumen. Side by side with the HCl secretion into the stomach, bicarbonate is also produced, which is absorbed into blood stream. This would account for the al kaline tide of plasma and urine, immediately after meals. Gastrin, the gastro-intestinal hormone secreted by G cells, stimulates the secretion of HCl. The secretion of gastrin is cut off by acidic pH by a feed back regulatory control. The most potent stimulus for acid secretion is histamine, which acts through specific H2 receptors on the gastric mucosa.
Cephalic phase The feelings of seeing, smelling and anticipating food are processed in the brain. So the brain informs the stomach that it should prepare for the arrival of a meal. Brain communicates through parasympathetic vagus nerve to the enteric nervous system. This results in stimulation of G cells
causing secretion of the hormone gastrin. This in turn stimulates parietal cells to secrete small amounts of acid.
Gastric phase When food enters the stomach, distension of stomach wall is sensed by enteric neurons, which further stimulates both G cells and parietal cells. More amounts of acid and gastrin are secreted. Vigorous smooth muscle contractions are also resulted.
Intestinal phase As food is emptied from stomach into the small intestine, the small intestine sends inhibitory signals to the stomach to slow down the secretion and motility. Nervous and endocrine signals are used. Moreover, enteric hormones such as cholecystokinin and secretin are released from the small intestine, which contribute to suppression of gastric activity.
Abnormal acid secretion Hyperacidity (increased acid secretion) results in duodenal ulcer. Increased quantity of acid wil l eat away the stomach muscles, leading to ulcers. There are many rare causes for hyper acidity, such as multiple endocrine neoplasia and Zollinger Ellison syndrome. Hypoacidity is seen in Gastritis, Gastric carcinoma, Pernicious anemia and Chronic iron deficiency anemia.
Anatomy of small intestine In most animals, the length of the small intestine is roughly 3.5 times the body length. In man, it is about 6 meters in length. The small intestine is the longest section of the digestive tube and consists of three segments: Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic duct and bile duct. Jejunum: considered to be roughly 40% of the small gut in man. Ileum: the portion after jejunum is called Ileum. It empties into the large intestine. Ileum consists of about 60% of the length of total intestine in man. All of the absorption takes place on the surface of small intestinal epithelial cells. If the small intestine is viewed as a sim ple pipe, its inner surface area would be one half of a square metre. But in reality, the absorptive surface area of the small intestine is roughly 250
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TECHAGAPPE
APRIL-JUNE 2016