Defence is a function of the gastrointestinal tract
Gastrointestinal tract
Digesting food
Absorbing nutrients
Eliminating waste
Digestion
1. Physical breakdown (chewing, peristalsis)
2. Chemical breakdown (HCl, enzymes)
Stomach
Expanded section of the digestive tube, between oesophagus and small intestine. Stores food, churns food and begins digestion, produces acid chyme
Vagal reflex
Inhibits smooth muscle tone, allows large volume food storage
Gastric secretions
Mucus, lipase, pepsin, intrinsic factor
Gastric acid secretion
Proton pump
Regulation of gastric acid secretion
Gastrin–ECL–parietal cell axis
Paracrine secretions
Local hormones, secreted from cells in the mucosa, act locally
Exocrine secretions
Secretions from numerous glands with ducts enter the lumen of the gut, involved in digestion, lubrication and protection: mucus (salivary glands), pepsin & HCl (gastric glands), bicarbonate ions & enzymes (pancreas), bile acids (liver)
Endocrine secretions: secretions called "hormones" synthesised by ductless glands that enter the blood stream, travel to their target tissue where they bind to specific receptors - gastrin (stomach), secretin (duodenal mucosa), pancreozymin-cholecystokinin (duodenal mucosa), insulin (pancreas)
Absorption
Nutrients transported across intestinal epithelium into blood or lymph
Fluid absorption occurs in small intestine and colon
Disorders of fluid secretion and absorption are important in pathogenesis of constipation and diarrhoea
Excretion
Indigestible food residues leave body in faeces, drugs and metabolic products may leave in faeces, saliva, bile, vomit
Storage
Proximal stomach, descending colon
Motility
Movements of muscular wall, movement from one region to another, storage, mechanical degradation, mixing lumen contents
The liver is a major metabolic organ involved in carbohydrate, nitrogen and lipoprotein metabolism, as well as bile production and bilirubin excretion
Defence
Gut epithelium is an interface with the outside world, intestine is largest mucosal surface and lymphoepithelial organ, breaches in barrier allow "toxins" to enter blood
Mechanisms of defence
Sight, smell and taste
Vomit reflex
Acid in stomach
Natural bacterial flora
Intestinal immune system (GALT)
The gastrointestinal tract has functions of storage & digestion, motility, storage of waste, defence, exocrine secretion, endocrine secretion, paracrine secretion, and excretion
Mucus
Secreted by goblet and mucus neck cell
Acts as a lubricant by acting as a barrier that protects the stomach and colon
Lipase
Converts triglycerides to fatty acids and glycerol
Pepsin
Secreted by chief cells
Protein digestion
Intrinsic factor
Secreted by parietal cells
salivary amylase: hydrolyses starch into dextrins and maltose
Pepsinogen is...
activated when added to HCl and becomes pepsin. hydrolyses proteins into large peptides
Liver produces...
bile. Helps with the digestion of fats through emulsifying