lecture 2- stomach Flashcards
Stomach functions
Store food (release into S.I), secrete HCI & enzymes (begins protein digestion), create chyme (from mixing food with secretions)
Gastric filling- Receptive relaxation
of the stomach as it receives food- folds get smaller and flatten out (allows stomach to increase in size)
Gastric mixing
ICC’s create electrical rhythm, slow wave potentials sweep from fundus to pyloric sphincter via gap junctions. If SWP reaches threshold in smooth muscle cells, action potentials are triggered (contraction/peristalsis= mixing)
Gastric emptying
Direction of peristaltic wave pushes food forward gradually from body into antrum, towards pyloric sphincter
Duodenum affects gastric motility via neural responses-
intrinsic nerve plexuses and autonomic nerves (short and long reflexes)
Duodenum affects gastric motility via hormonal responses-
Hormones secretin and cholecystokinin (CCK) inhibit antral contractions
External factors influencing motility
Increased sympathetic activity can alter motility (e.g. fear and pain decrease motility)
Gastric secretions
secretory cells are located in mucosal infoldings (may be exocrine, paracrine or endocrine secretions)
Gastric juices=
collective exocrine secretions
2 areas of gastric mucosa
pyloric gland area (antrum) and oxyntic mucosa (funds and body)
Hydrochloric acid-
aids digestion, activates pepsinogen enzyme, breaks down connective tissue and muscle fibres
Pepsinogen
major digestive component- synthesised and packaged by ER and Golgi complex in chief cells
Pyloric glad area- G cell
secretes gastrin, stimulates acid secretion by ACh and acts on ECL and parietal cells, release peptide hormone, chief cells secrete pepsinogen
Oxyntic area- ECL cell
secretes histamine (amplifies gastrin signal and ACh, stimulates acid secretion by acting on parietal cells
Pyloric glad area- D cell
secretes somatostatin, inhibits gastrin and acid secretion, acts as a “turning off” signal
3 phases of Gastric secretion
Cephalic, Gastric, Intestinal Phase
Cephalic phase
increased secretion of HCI and pepsinogen in response to thinking about eating food (vagal stimulation of the intrinsic plexuses sends signal to medulla oblongata
Gastric phase
Food reaches the stomach and proteins increase gastric secretions via both vagal and direct stimulation of the intrinsic nerve plexuses (enteric nervous system)
Intestinal phase
Encompasses inhibitory factors originating from the S.I. This stops the flow of gastric juices and gastric motility as chyme is processed in the S.I
How can the gastric mucosa withstand strong acid and proteolytic enzymes without damage?
Epithelial cells impermeable to HCI, tight junctions prevent acid diffusing between cells, mucus stops acid from physically penetrating, Bicarbonate secreted into mucus neutralises acid and inactivate pepsin
Stomach body digestion
the semi-solid mass of food is not well mixed with gastric secretions (due to thinness of muscle), salivary amylase digestions of carbs can continue in centre of food mass.
Stomach Antrum digestion
Digestion by gastric juice as food mass mixed with HCI and pepsin (protein digestion)
HCI
hydrochloric acid (protein)
Absorption
no food or water absorbed, small amount of lipid soluble compounds can diffuse across cell membranes
why is alcohol absorption slowed when consumed with a meal?
Fat entering slows activating in the stomach, alcohol sits much longer in stomach due to food. S.I is much more efficient at absorption
Myenteric plexus
between circular and longitudinal layers of muscularis externa, controls motility;
Submucosal plexus
submucosa, controls blood flow and secretions
Briefly describe the 3 functions of hydrochloric acid as a component of gastric secretion
hydrochloric acid acts by activating pepsinogen to pepsin, denaturing proteins, breaking down connective tissue, killing microorganisms. It is not an enzyme, but helps with protein digestion.
