Digestive System - Saliva and Gastric Distension Flashcards
What is saliva ?
Composed of water, electrolytes and proteins (mucin, ptyalin, ribonuclease)
Saliva is secreted by submandibular, parotid and sublingual glands
Saliva is hypotonic, alkaline and has a low osmolarity
What are the functions of saliva ?
- Lubrication - mucous cells produce mucous aiding swallowing of food
- Protection - K+ and bicarbonate (HCO3-) added making it alkaline to buffer the acid in food, protecting against dental caries.
- Contains immunoglobulin A and lysozyme which provide the oral cavity with protection against bacteria
- Digestion - serous cells secrete salivary amylase and lingual lipase which begins the digestive process
How is saliva secreted ?
Secretion of saliva controlled from salivatory nuclei in brain stem
Salivary glands are activated mainly by the parasympathetic nervous system using acetylcholine acting via muscarinic receptors
Conditioned reflexes from the salivatory nuclei of the medulla oblongata can stimulate or inhibit salivation due to the thought, smell or sight of food
Partially regulated by appetite area in hypothalamus which receives signals from taste and smell areas in cerebral cortex and amygdala
What are the glands involved in gastric secretions ?
The oxyntic glands which are located in the body and fundus of the stomach are lined with cells that are responsible for the secretion of hydrochloric acid, pepsinogen, intrinsic factor and mucus
Cells in the pyloric glands (located in the antral portion of the stomach) secrete mucus and the hormone gastrin
What are the components of gastric juice ?
- Pepsinogen - secreted by chief cells of gastric glands, converted to pepsin in gastric juice when pH drops below 5
- Pepsin - enzyme which initiates the digestion of dietary proteins
- Hydrochloric acid - secreted by parietal cells, sterilises the food, begins hydrolysis of dietary macromolecules and provides the acidic environment needed for conversion of pepsinogen -> pepsin
- Intrinsic factor - secreted by parietal cells and aids in absorption of vitamin B12
- Mucous - secreted by mucous cells, alkaline protective inner layer of stomach
How is gastric acid secretion stimulated ?
Secretion of gastric acid by parietal cells is under endocrine and nervous control
Acetylcholine released by parasympathetic nervous system (specifically enteric nervous system), stimulates the release of gastric acid in response to vagovagal reflexes
Rate of secretion and formation of gastric acid is directly related to the amount of histamine secreted by the enterochromaffin like cells deep in oxyntic glands
Antral gastrin (G) cells are located in the pyloric end of the stomach and when protein rich foodstuffs reach the antral end of the stomach they stimulate the G cells to release gastrin into the blood
How is pepsinogen stimulated and activated ?
Pepsinogen release by the peptic cells in the oxyntic glands of the stomach occurs in response to two main stimulatory signals
Stimulation of chief cells by acetylcholine released by vagus nerve
Stimulation of the chief cells by the acid in the stomach influencing enteric nervous reflexes
What is intrinsic factor ?
Intrinsic factor is a glycoprotein secreted by the parietal cells of the oxyntic glands
Intrinsic factor combines with vitamin B12 in stomach
Vitamin B12 complex reaches the terminal ileum the intrinsic factor binds with receptors on the brush border membranes of the ileal mucosa, brought into cell by pinocytosis
Vitamin B12 enters the blood and stored in liver
Vitamin B12 required by bone marrow in synthesis and maturation of red blood cells
Describe the cephalic phase of gastric secretion.
- Occurs before food enters the stomach
- Chemo and mechanoreceptors located in the tongue and buccal and nasal cavities are stimulated by tasting, smelling, chewing and swallowing food
- Signals that cause this phase originate in cerebral cortex and appetite centres in medulla oblongata and hypothalamus
- Travel to the stomach via vagus nerve causing the release of gastric secretions in preparation for arrival of food
Describe the gastric phase of gastric secretion.
Once food enters the stomach it excites vagovagal reflexes, local enteric reflexes and the gastrin mechanism
Food mixes with gastric juice and pH of contents rise enhancing gastric secretion
Gastric motility is promoted
Describe the intestinal phase of gastric secretion.
This phase begins when food first leaves the stomach and enters the upper part of the small intestine, particularly the duodenum
Chyme contracts intestinal wall causing release of intestinal gastrin
As more chyme enters small intestine, reverse enterogastric reflex is triggered due to duodenal stretch receptors
This causes release of intestinal hormones - they decrease gastric secretion and motility to slow passage of chyme from stomach to intestine
Ensures gastric contents are delivered to the small intestine at a rate that doesn’t exceed the capacity for digestion and prevents damage to the duodenal mucosa that can result from acidic and hyperosmotic solutions
What are enterogastrones and the different types?
Enterogastrones - duodenal hormones responsible for the inhibition of gastric acid secretion
Gastric inhibitory protein - inhibits acid secretion by parietal cell
Somatostatin - inhibits gastric secretions
Secretin - inhibits acid secretion
Cholecystokinin - inhibits gastric emptying
How is hydrochloric acid secreted?
- Hydrochloric acid (HCl) is secreted in the parietal cell lumen through the hydrogen-potassium pump H+ -K+ ATPase.
- H+ is secreted into the canaliculus in exchange for K+, which is transported into the cell by the H+ - K+ ATPase pump.
- The basolateral H+-K+ ATPase creates low intracellular Na+ reabsorption, allowing most K+ and Na+ in the canaliculus to be reabsorbed into the cell cytoplasm.
- H+ pumps out OH-, forming HCO3- from CO2 and exchanged with Cl- in the extracellular fluid.
- H+ and Cl- combine to form hydrochloric acid (HCl), which is secreted into the gland lumen.
Describe the gastrointestinal hormones .
- Released from the mucosa of the stomach and small intestine by nervous activity, distension and chemical stimulation coincident with the intake of food
- Physiology of the GIT is regulated by 3 divisions of the autonomic nervous system - sympathetic, parasympathetic and enteric nervous system
- Regulated by neural circuits in which a mechanoreceptor/chemoreceptor is stimulated in the mucosa which stimulates other neurones in the myenteric or sub mucosal plexuses that regulate endocrine or secretory cells
- The submucosal plexus is found in the submucosa only in the small and large intestine
- Myenteric plexus is located between the circular and longitudinal muscle layers throughout the GIT from the proximal end of the oesophagus to the rectum
