Functions of the alimentary tract

• Alimentary tract


  The digestive system
• Stomach


  • Expanded section of the digestive tube, between the oesophagus and small intestine
  • Accommodates food
  • Has oblique layer of smooth muscles inside the circular layer of muscles, which help in the complex grinding motions
• Fundus


  • Bulbus bit of the stomach
  • Thin in muscular structure
  • Can allow for stretching (aka accommodation) to occur
  • Important neuronal factors in this process as the stomach is innervated by the nerves
• Stomach muscles


  • Can relax, which is mediated by nitric oxide and this activates the intestinal peptide and VIP
• Cardiac orifice (aka cardio)


  • Where the oesophagus makes entry into the stomach
• Antrum


  • Found nearer the other end of the stomach
  • Thick in muscular structure, allowing food to be ground which promotes digestion to occur
• Pylorus


  • Found at the end of the stomach after the antrum
  • Allows periodic explosion of material in the stomach
• Digestion


  The breakdown of food into smaller components that can be absorbed and used by the body
• Stomach


  • Allows food to be stored in fundus: food stored here during first stage of digestion may remain there for around an hour unmixed, acting as a reservoir
  • Fundus and body of stomach (thinner muscle tone) relax, allowing large volume (around 1.5 L of food storage
  • Some of that relaxation is innervated by the vagus nerve: vagal reflex inhibits smooth muscle tone. Nitric acid and VIP are quite important in this process of relaxation
  • Antral region mixes/grinds food with gastric secretions, leading to digestion
• Colon/rectum


  • Stores digestive residues and faeces until defecation
• When you fast or haven't eaten in a while

  The stomach contracts
• When you start eating again

  The stomach expands and begins the accommodation process
• Gastric secretions


  Juices secreted into the stomach, which stores 23 litres of gastric juice per day (mucus, pepsinogen, intrinsic, lipase) which helps in digestion and absorption of food
• Secretion of gastric juices

  Dependent upon factors such as the smell of the food, seeing the food, chewing the food and the food being in the stomach itself
• Mucus


  • Secreted by goblet cells and mucus neck cells
  • Acts as a lubricant by acting as a barrier that protects the stomach and colon (prevents trauma to the surface of the alimentary tract)
• Lipase


  • Converts triglycerides to fatty acids and glycerol (digestion of lipids occurs in the stomach)
• Pepsin


  • Secreted by chief cells
  • Involved in protein digestion
  • Not secreted as pepsin, but rather pepsinogen as pepsin is active in its own right
  • Converted to pepsin under acidic conditions provided by the secretion of gastric acid, which occurs due to the food being seen, smelt, chewed and in the stomach
  • Pepsin is very self-sufficient and if its empowered, it can do the job on its own: the next conversion of pepsinogen won't require any action of acid
• Intrinsic


  • Secreted by parietal cells
  • Important for vitamin B12 absorption
• HCl


  • Secreted by parietal cells
  • Important in defence
• Paracrine secretions


  • Often called "local hormones" and are secreted locally
  • Different from endocrine hormones as they travel in interstitial fluid and are usually secreted next to the cells that they're going to act on
• Exocrine secretions


  • Secretions from numerous glands with ducts that enter the lumen of the gut and are involved in digestion, lubrication and protection
• Exocrine secretions


  • Salivary glands (mucus)
  • Gastric glands (hydrochloric acid, pepsin, mucus)
  • Pancreas (bicarbonate ions, enzymes like amylase, lipase, carboxypeptidase)
  • Gallbladder (bile)
  • Liver (bile acids, bile salts)
• Endocrine secretions


  Secretions called 'hormones' synthesised by ductless glands that enter the blood stream and travel to their target tissue(s) where they bind to specific receptors
• Endocrine secretions


  • Gastrin
  • Secretin
  • Pancreozymin-cholecystokinin
  • Insulin
• Absorption


  For food to be of use to the body, the nutrients resulting from digestion must be transported across the intestinal epithelium into the blood (e.g. glucose, amino acids) or lymph (fats)
• Fluid absorption


  • Occurs mainly in the small intestine, with the colon absorbing 90% of water, reducing volume to 200ml of semi-solid faecal matter
• Motility


  The movements of the muscular walls (mostly smooth muscle except extreme ends of the upper oesophagus/rectum) of the stomach and intestines that allow food to move along the alimentary tract
• Segmentation motility


  • Mixes and churns the food, allowing it to be broken down so that enzymes can get in and digestion can occur, and brings the food closer to the surface of the epithelial cells so it can be absorbed
• Functions of motility


  • Storage
  • Mechanical degradation
  • Mixing lumen contents
  • Transport of urea and electrolytes
• Excretion


  Drugs and some products of normal metabolism may leave the body in saliva, bile, faeces, vomit
• Excretion


  • Indigestible food residues (e.g. tomato skin) leave the body in the faeces
  • Migrating motor complex removes indigestible objects, e.g. button, penny etc.
• Gut defence mechanisms


  • Gut is unsterile as it is open to external environment
  • Sight, smell and taste alerts us to harmful food substances
  • Vomit reflex
  • Acid in stomach (HCl) kills most harmful bacteria
  • Natural bacterial flora prevents colonisation of harmful bacteria
  • Aggregation of lymphoid tissue (e.g. Peyer's patches) able to mount a response to food-borne antigens - analyse and respond to pathogenic microbes
• Peyer's patches


  Located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum
• Liver


  • A major metabolic organ in the abdominal cavity, weighing about 1.3 kg in an adult
  • Involved in carbohydrate, nitrogen and lipoprotein metabolism as well as bile production and bilirubin excretion