Introduction to the Functions of the Alimentary Tract Flashcards
Stomach
Stomach is an expanded section of the digestive tube; between oesophagus and small intestine
Angle of his -> meant to prevent regurgitation
Fundus - has basic electric activity
antrum - thick muscular structure that allow grinding to occur
food will make way to duodenum at a pace it can handle -> can sense what kind of food enters and will release enzymes/bile accordingly
Stomach has oblique layer of smooth muscle inside the circular layer, which aids in performance of complex grinding motions.
Storage
Stomach: Food stored here during first stage of digestion; may remain there for ~1hr unmixed (acts as a reservoir) - fundus
Fundus and body of stomach (thinner muscle tone) relaxes, allowing large volume (~1.5L) of food storage
Vagal reflex inhibits smooth muscle tone – mechanoreceptors → fundic relaxation
VIP and NO - important for relaxation
Antral region mixes/grinds food with gastric secretions → Digestion
Colon/rectum: storage of digestive residues and faeces
ANS enables the storage of food in the stomach
Fasting - shrunken stomach
Eating leads to accomdation via VIP and NO - relfex inhibition so mechanoreceptors allow fundus to relax
Emptying - initiated by basic electrical activity as fundua has pacemaker cells hence when theshold is reached, movement occurs -> contraction occurs (due to AcH)
Storage of gastric secretions
Stomach stores 2-3 litres of gastric juice/24hr (Mucus, pepsinogen, intrinsic factor, lipase) which help in digestion and absorption of food
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)
Lipase – converts triglycerides to fatty acids and glycerol
Pepsin (secreted by chief cells): protein digestion
Intrinsic (secreted by parietal cells) – for vitamin B12 absorption
HCl (secreted by parietal cells) – important in defence
pepsinogen and pepsin
pepsinogen = inactive form pepsin = active form
conversion happens due to H+ (hyperacidity) -> once pepsin activated, it can activate all pepsinogen to pepsin
Paracrine secretions
Often called “local hormones” - acts locally so the cells next to the cell that secrete it
Secreted from cells in the mucosa, but the chemical acts locally on adjacent cells via the interstitial fluid (c.f. hormones).
Example: somatostatin inhibits gastrin release in the stomach
Example: somatostatin inhibits gastrin release in the stomach
somastatin secreted by D cells
Gastrin secreted by G cells
very close together therefore not too much acid as gastrin has high H+ secretion and grinding activity
Exocrine secretions
Salivary glands: mucus (lubrication for mastication and speech)
Gastric glands: hydrochloric acid, pepsin, mucus
Pancreas: bicarbonate ions, enzymes (e.g. amylase, lipase, carboxypeptidase) -> mixes with all gall bladder secretion into the duodenum (chyme makes it way there)
Liver: bile acids (xenobiotics, drugs, bilirubin too)
Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection –examples of these are circled in the next slide.
Endocrine secretions
Secretions called ‘hormones’ synthesised by ductless glands enter the blood stream, travel to their target tissue(s) where they bind to specific receptors.
Gastrin: stomach (G-cells in antrum) -> promote acid secretion + thick muscular contraction
Secretin: duodenal mucosa -> has affect on gallbladder to increase bile secretion. cells can sense cells are acidic, secrete secretin which travels in blood -> initiate secretion of bi-carbonate from pancreas
Pancreozymin-cholecystokinin: duodenal mucosa -> Increase contraction of gall bladder to release bile acids (cell recognise fats therefore increase secretion of bile -> emulsification of fats for lipase)
Insulin: pancreas (B-cells)
Exocrine, endocrine and paracrine secretions allow active digestion and control of digestion and motility
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).
Absorption occurs mainly in small intestine.
Fluid absorption occurs in the small intestine and colon.
Colon absorbs 90% of water, reducing volume to 200ml of semi-solid faecal matter
Disorders of fluid secretion and absorption are important (together with motility) in the pathogenesis of diarrhoea.
Motility
The movements of the muscular wall (mostly smooth muscle except extreme ends of the upper oesophagus/rectum):
Movement from one region to another (law of gut)
Storage, e.g. proximal stomach, descending colon
Mechanical degradation, e.g. gastric antrum -> grinding till size if fufilled as the pyloric acts like a sieve therefore only lets small chyme through ( less than 2mm)
Mixing lumen contents, e.g. small intestine
Transport of urea and electrolytes
Excretion
Drugs and some products of normal metabolism may leave the body in:
Saliva
Bile
Faeces
(Vomit)
Indigestible food residues (e.g. tomato skin) leave the body in the faeces
Defence
Like the skin and airways, the gut epithelium is an interface with the “contaminated” outside world.
The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens.
It is also the largest lymphoepithelial organ.
Breaches in the barrier mean “toxins” enter the blood.
Defence mechanism
Gut is unsterile as it is open to external environment
Sight, smell and taste alerts us to harmful food substances
Vomit reflex -> via hand in throat
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
Metabolism
The liver is a major metabolic organ in the abdominal cavity and weighs about 1.3 Kg in an adult.
It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as bile production and bilirubin excretion.
