Digestion (D2)

• The major exocrine glands that secrete digestive enzymes into alimentary canal include: salivary glands in mouth, gastric glands in stomach & the exocrine glands of pancreas. Endocrine glands secrete hormones directly into bloodstream. Exocrine glands secrete either to surface of the body/ into lumen of the gut. 
• Cholera is a disease caused by infection by the bacterium Vibrio cholera. The bacterium releases a toxin that binds to a receptor on intestinal cells. The toxin is then brought into the cell by endocytosis. Once inside the cell, the toxin triggers a cascade response that → efflux of Cl- ions into the intestine. Water follows by osmosis → watery diarrhoea. Water is drawn from the blood into the cells to replace the fluid loss from the intestinal cells. Quite quickly severe dehydration can → death if the patient does not receive rehydration.
• Adaptations of villus epithelial cells include microvilli & mitochondria. Microvilli are tiny, finger-like foldings of the cell surface facing the lumen of the gut & they greatly ↑ SA in contact with material to be absorbed. Mitochondria are organelles present in large no.s due to the many active transport processes. Pinocytotic vesicles are the sites of pinocytosis, where fluid is taken up/ released in tiny vesicles across the plasma membrane of a cell. Tight junctions bind tgt the indi epithelial cells, so the only way into the tissues of the body is through the epithelium.
• The major exocrine glands do not secrete fluid continuosly, but is regulated by nervous & hormonal mechanisms, ensuring that digestive juice is secreted when & where it is req. Also, vol & composition of the digestive juice is appropriate to the food substance. Stomach acid is corrosive so the body produces a natural mucus barrier which protects lining of the stomach from being attacked by the acid. In some people this barrier may have broken down allowing acid to dmg stomach, → bleeding aka an ulcer, hence, it is important to control. 
• Sight/ smell of food causes brain to send nerve impulses via vagus nerve from the medulla. Gland cells in stomach wall are stimulated to secrete components of gastric juice. Stretch receptors detect stomach distension, sending impulses to the brain. Brain responds by sending impulses via vagus nerve to endocrine cells in the wall of the duodenum & the part of the stomach nearest to the duodenum, stimulating them to secrete hormone gastrin.
• Gastrin stimulates secretion of acid & pepsinogen by 2 types of exocrine gland cell in the stomach wall. Secretin & somatostatin, inhibit gastrin secretion if pH in stomach ↓ too low (control). 
• Acid secreted by stomach disrupts the extracellular matrix that holds cells tgt in tissues → denaturing of proteins, exposing the polypeptide chains. The acid conditions within the stomach convert inactive pepsinogen to pepsin so that the enzyme pepsin can hydrolyse the bonds within the food polypeptides. Impaired gastric acid secretion predisposes to infection w a variety of pathogens, ∴, stomach acid prevents infections by neutralising pathogens in food.
• For some ppl, the circular muscle at the top of the stomach cannot prevent gastric fluid from escaping the stomach & thus acid escapes & irritates the oesophagus. This aka acid reflux and it can cause symptoms such as heartburn.
• The parietal cells of the gastric glands pump out H+ ions. This proton pump is powered by ATP & transports H+ ions out & K+ ions in & is aka H+/K+ ATPase pump. Proton-pump inhibitor (PPI) drugs are used to treat gastroesophageal diseases w symptoms of low stomach pH. PPIs bind irreversibly to a single pump. The effect on the overall acid prod system is not permanent cus pumps are ≈ recycled & replaced w new pumps. However, use of these drugs may also encourage growth of the gut microflora which may ↑ patient’s susceptibility to bacterial colonizations. 
• Until recently, emotional stress & excessive gastric juice secretion were believed to be a major contributory factor in the development of stomach ulcers, but, Helicobacter pylori, is a more significant cause. Hp doesn’t invade cells, but survives by attaching to receptors on plasma membrane of stomach mucosa cells, underneath the mucus lining. An inflammation rxn occurs at the site of infection. Cells of the stomach lining become exposed to the protease & HCL of the gastric juice which may → stomach ulcer.
• Dietary fibre is material such as cellulose, lignin & pectin that cannot be readily digested. A healthy balanced diet contains fibre as it ↑ bulk of material passing through intestines & helps prevent constipation as it draws water into the faeces inside the intestine. Conversely, when there is ↑ % of digestible matter content, the food stays in the intestine for a longer time. (If too long = constipation) This is due to more digestible material which ↑ water absorption into large intestine. The outcome is hardened stool & ↑ residence time in the intestine.