Saliva and Gastric Secretions Flashcards
Saliva
- 800-1500mls daily
- pH 6-7
- serous - amylase, starch and glycogen digestion
- mucous cells - mucous, lubricant
- low osmolarity
- hypotonic and alkaline
Salivary Glands
- Parotid
- Submandibular
- Sublingual
- Buccal glands
Constituents of saliva
- water
- electrolytes
- proteins - mucin, ptyalin, ribonuclease, R protein, IgA, IgG, IgM, lipase, lysozyme, EGF
Functions of saliva
- lubrication - swallowing and speech
- protection - alkalinity, immunoglobulins, lysozyme
- digestion - salivary amylase (pytalin), lingual lipase
Secretion of saliva
- Acini secret a solution electrolyte conc. similar to ECF also containing ptylain, mucin
- 1o secretion flows through ducts Na+ reabsorbed and exchanged for K+
- -ve electrochemical gradient causing passive Cl- reabsorption (Cl- conc low)
- HCO3- secreted into duct lumen passively and actively in exchange for Cl-
- Resting conditions conc. of Na+ & Cl- lower than conc. in plasma HCO3- higher
- Maximal salivation secretion by acini can increase x20
- So rapid ductal reconditioning reduced leaving low Na+ and high K+
- ADH & aldosterone modify composition of saliva by decreasing Na+ conc. and increasing K+ conc.
Control of saliva secretion
- secretions controlled by PSNS
- Sour taste and smooth food copious amounts of saliva secretion
- unpleasant taste, rough food - less saliva
- stomach and small intestine reflexes can stimulate secretion
Functions of Gastric Juice
- storage
vagovasal reflex -> receptive relaxation - up to 1.5L mainly in fundus and body - mixing
slow spontaneous rhythmic contractions progressively increase from body
-> antrum
-> mix wiht gastric gland secretions (oxyntic glands HCl)
-> chyme - emptying
intense peristaltic contractions in antrum - controlled by constriction of pyloric sphincter (pyloric glands mucous and gastrin)
Gastric Juice components
Pepsinogen - chief cells, inactive form of pepsin
Pepsin - An enzyme that is involved in the digestion of proteins
Hydrochloric Acid -
- Parietal cells, sterilises food, begins the hydrolysis of dietary macromolecules, produces acid environment for the activation of pepsin.
- Cleaves pepsinogen to pepsin. - HCl isotonic with body fluids
- pH of 0.8
- To concentrate the H+ 1500 calories per litre of gastric juice
- 3 million times that of arterial blood
- HCO3- secreted into venous blood during H+ secretion
Mucus - Provides protection to the stomach lining, lubricates food
Intrinsic Factor - Parietal cells, aids in the absorption of vitamin B12
Gastric Acid Secretion
In the parietal cell
- H2O dissociates in cytoplasm– H+ & OH-
- H+ actively secreted into canaliculi in exchange for K+
- K+ transported into cell on basolateral side leak into canaliculi but are actively recycled back into cell
- Basolateral H+-K+ATPase creates low intracellular Na+ reabsorption
- K+ & Na+ in canaliculus is reabsorbed into cell cytoplasm and H+ takes their place in canaliculus
- OH- accumulates in cytoplasm and forms HCO3-
- HCO3- exchanged with Cl- in ECF then secreted into canaliculus
- Cl- combines with H+ in canaliculus (HCl)
- HCl secreted out through the canaliculus into lumen of gland
- Final secretion H2O, HCl, KCl & NaCl (gastric juice pH 1-2)
Stimulation of gastric acid secretion
Endocrine & nervous control:
- Acetylcholine (PSNS)
- Histamine (ECL cells)
- Gastrin (Gastrin- Histamine release – Acid secretion)
Stimulation and activation of pepsinogen
Released by peptic / chief cells of oxyntic mucosa
Inactive needs processed to active pepsin. Achieved by;
- HCL in stomach
- Pepsin itself
- Pepsin – proteolytic enzyme.
Pepsin released by…
2 main stimulatory signals;
- Ach (vagus nerve)
- In response to acid influence on the enteric NS
Intrinsic Factor (Gastric Juice)
- Glycoprotein
- Secreted by parietal cells
- Needed for vitamin B12 absorption
- Combines with vitamin B12 (intrinsic factor-vitamin B12 complex)
- Binds with receptors on surface of ileal cells
- Absorbed by pinocytosis
- Vitamin B12 enters blood and stored by liver
- Absence or inadequate intrinsic factor leads to pernicious anaemia
Phase 1 of Gastric Secretion
The Cephalic Phase - the sight, smell, taste or thoughts of food
Function - prepare stomach for arrival of food
Duration - short (minutes)
Mechanism - neural, via preganglionic fibres in vagus nerve and synapses in submucosal plexus
Actions -
primary: increased volume of gastric juice by stimulating mucus, enzyme and acid production
secondary: stimulation of gastrin release by G cells
Phase 2 of Gastric Secretion
The Gastric Phase
Functions:
- enhance secretion started in cephalic stage
- homogenize and acidify chyme
- initiate digestion of proteins by pepsin
Duration: long (3-4 hours)
Mechanisms:
- neural: short reflexes triggered by stimulation of stretch receptors as stomach fills and stimulation of chemoreceptors as pH increases
- hormonal: stimulation of gastrin release by G cells by parasympathetic activity and presence of peptides and amino acids in chyme
- local: release of histamine by mast cells as stomach fills
Actions:
- increased acid and pepsinogen production
- increased motility and initiation of mixing waves
Phase 3 of Gastric Secretion
The Intestinal Phase
Function: control rate of chyme entry into duodenum
Duration: long (hours)
Mechanisms:
- neural: short reflexes (enterogastric reflex) triggered by distension of duodenum
- hormonal:
primary: stimulation of cholecystokinin, gastric inhibitory peptide and secretin release by presence of acid, carbohydrates and lipids
secondary: release of gastrin stimulated by presence of undigested proteins and peptides
Actions:
- feedback inhibition of gastric acid and pepsinogen production
- reduction of gastric motility
