Introduction to the functions and control of the alimentary tract Flashcards
Describe the function of exocrine secretions in reference to the alimentary tract and give an example
Paracrine secretions - Local hormones, secreted from cells in the mucosa, the chemical acts locally on adjacent cells via interstitial fluid
Salivary glands: Mucus, lipase
Gastric glands: HCl, pepsin, mucus
Pancreas: bicarbonate ions, enzymes
Liver: bile salts, bile acids
Secretions from numerous glands with ducts enter the lumen of the gut, and are involved in digestion, lubrication and protection.
Describe the function of motility in reference to the alimentary tract and give an example
Gastrin promotes motility i.e. pushing material towards the pyloric sphincter so food enters duodenum at a pace that can be handled.
Storage e.g. proximal stomach, descending colon.
Movements of muscular wall allows: Movement from one region to another, mass evacuation, mechnical degradation e.g. gastric antrum, mixing lumen contents e.g. small intestine, transport of nutrients, water and of urea and electrolytes
Describe the function of digestion in reference to the alimentary tract and give an example
Storage
Stomach: Food is stored here during the first stage of digestion. Fundus and body of stomach relaxes, allowing large volume of food storage. The vagal reflex inhibits smooth muscle tone -> Mechanoreceptors, lead to fundic relaxation (VIP and NO). The antral region grinds food with gastric secretions, leading to digestion.
In the colon/rectum, storage of indigestive residues and faecal matter.
The stomach can store 2-3 litres of gastric juice for 24 hours, helping in digestion and absorption of food. These include mucus (lubricant, barrier that protects stomach and colon from acid), lipase (converts triglycerides), pepsin (protein digestion), HCl (defence) and intrinsic (vitamin b12 absorption)
Describe the function of absorption in reference to the alimentary tract and give an example
Nutrients from digestion must be transported across intestinal epithelium into the blood or lymph via lacteals for absorption. This mainly occurs in the small intestine, and absorption of fluid occurs in the small intestine and colon.
Colon absorbs 90% of water. Disorders of fluid secretion and absorption are important in the pathogenesis of diarrhoea.
Describe the function of defence in reference to the alimentary tract and give an example
Gut epithelium is exposed to the contaminated outside world. Intestine is the largest mucosal surface in the body and is exposed to a lot of antigens.
Defence mechanisms
Sight, smell and taste alert us to harmful food substances
Vomit reflex
HCl kills most harmful bacteria
Mucus secretions
Natural bacterial flora prevent colonisation of harmful bacteria
Aggregation of lymphoid tissue (Peyer’s patches) mount a response to food-borne antigens - analyse and respond to pathogenic microbes - E.g. food poisoning response, some feel fine while others don’t depending on the strength of their lymphoid tissue.
Peyer’s patches: located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum
Describe the function of endocrine secretions in reference to the alimentary tract and give an example
Hormones synthesised by ductless glands, enter the bloodstream and travel to target tissues, bind to receptors to elicit effects.
Gastrin: Stomach (G-cells in antrum)
Secretin: Duodenal mucosa
Pancreozymin-cholecystokinin: Duodenal mucosa
Insulin: Pancreas
CCK secreted by duodenal mucosa, promotes contraction of the gallbladder, while bile salts emulsify fats.
All secretions allow for active digestion and control of digestion, gastric motility and energy homeostasis
Describe the function of excretion in reference to the alimentary tract and give an example
Drugs and some products of metabolism leave the body in saliva, bile, bilirubin, faeces and vomit.
Indigestible food residues leave the body in the faeces.
Describe the autonomic innervation of the gut in terms of the extrinsic parasympathetic (e.g. vagal) and sympathetic (e.g. splanchnic) supply and the enteric nervous system
The vago-vagal reflex: The afferent and efferent axons are in the vagus nerve trunk. Response to gut stimuli via NTS and DMVN
NTS - Nucleus tractus solitarius - Site of termination of vagal afferents
DMVN - Dorsal motor vagal nucleus - Site of origin of vagal efferents (motor) supplying the gut
The reflex is active during receptive relaxation of the stomach in response to swallowing, going from stomach to brain then stomach. Promotes motility and acid secretion.
Describe the role of the autonomic nervous system in enabling the stomach to store food during digestion and to empty partially digested food into the duodenum
Food in the stomach is broken down to move to the duodenum. Ripples of contraction move food towards the antrum, which grinds the food. The food moves through the pyloric sphincter. Any food too large is pushed back to the antrum for further grinding.
Rate of emptying is dependent upon material’s ability to be absorbed -> Carbohydrates, protein, then fats.
Fatty acids in the duodenum cause a decrease in gastric emptying by increasing the contractility of pyloric sphincter. They take longer to be emulsified.
Describe the role of the enteric nervous system in mediating peristalsis in the small intestine
The gut has an intrinsic enteric nervous system. In the gut wall exists plexuses, namely the submucosal Meissner plexus, which regulates digestive glands, and the myenteric Auerbach plexus, which is involved with gut motility
Peristalsis is wave of coordinated contractions, moving contents of the gut towards the anus. Distension initiates contraction. Vagal inhibitory (VIP, NO) and excitatory fibres (ACh, SP) control movement.
Describe the roles of nerves, hormones and paracrine factors in the regulation of gastric hydrochloric acid secretion
Gastrin is a hormone secreted by G cells (gastrin secretion cells). Travels through blood. Controlled by parasympathetic system. Gastrin controls contraction of the antrum and secretes gastric juice e.g. HCl.
Gastrin can act directly on parietal cells for gastric juice secretion. Its effects are also mediated by enterochromaffin cells, which secrete histamine, binds to H2 receptors on the parietal cells causing the secretion of HCl, causing gastric juice secretion. Acetylcholine can also bind to parietal cells via muscarinic receptors (M2) as well as ECL cells, causing histamine secretion. Acid secretion is therefore mediated by both gastrin and acetylcholine.
Paracrine control involves somatostatin secretion from the D cell, influenced by hyperacidity (H+). This stimulates D cells. Too much results in an ulcer. Somatostatin travels through interstitial fluid, inhibiting G cells, and preventing acid secretion.
