gastric secretion

what are the components of the gastric juice? (4)

- HCl

- pepsinogen

- intrinsic factor

- mucous

which cells release which components in the body?

parietal - release HCl and intrinsic factor

chief - release pepsinogen

which cells release which components in the pyloric antrum?

neck and surface mucous-secreting cells - secrete mucous

what is the first step in HCl secretion from parietal cells? (2)

- CO₂ and H₂O from aerobic respiration combines to form carbonic acid, catalysed by carbonic anhydrase

- this then dissociates to form H+ and HCO₃⁻

what happens to the H+? (2)

- pumped out of the apical membrane into the stomach lumen by H+/K+ ATPase

- hence K+ are pumped into the parietal cells

what happens to the HCO₃⁻? (3)

- transported out of the basolateral membrane by Cl-/HCO₃⁻ exchangers

- it is absorbed into the veins and is responsible for the 'alkaline tide' that is found in gastric venous blood

- because it is an exchanger, Cl- is drawn into the parietal cells

what happens to the Cl-?

Cl- diffuses out via Cl- channels in the apical membrane

what are the 3 substances that stimulate HCl secretion?

- Histamine

- AcetylCholine

- Gastrin

where is histamine released from?

ECL (enterochromaffin-like cells)

where are ECL cells?

in the gastric glands - they are a type of neuroendrocine cell

how does histamine reach its target cells?

paracrine - to nearby parietal cells

what does histamine bind to?

H2 receptors on parietal cells

how is acetylcholine released? and what is the term for this?

- from vagus nerves that innervate the gastric mucosa

- neurocrine

what does acetylcholine bind to?

muscarinic (M3) receptors

what cells secrete gastrin?

G cells

what is the second and indirect stimulation by acetylcholine?

it stimulates ECL cells to release more histamine

how does gastrin reach its target cells?

endocrine - through the blood

what does gastrin bind to?

CCKB receptors on parietal cells

what is the second and indirect stimulation by gastrin?

stimulates ECL to release more histamine

what are the 3 phases of gastric secretion stimulation?

- cephalic phase

- gastric phase

- intestinal phase

what are the stimuli for the cephalic phase?

- smelling, tasting, chewing

- anticipation of food

which mechanisms do the cephalic stimuli promote?

- direct vagal stimulation of parietal cells (ACh)

- indirect vagal stimulation of parietal cells (ACh stimulates ECL cells to release histamine)

what are the stimuli for the gastric phase? (2)

- distension of stomach

- presence of amino acids and small peptides

what does distension of the stomach stimulate?

direct and indirect vagal stimulation of parietal cells

what does distension of the pyloric antrum in particular stimulate?

triggers local reflexes that stimulate G cells

what do amino acids and peptides do?

act of G cells to trigger gastrin release

what ingested drugs can stimulate gastric HCl secretion? (2)

- alcohol

- caffeine

what are the stimuli of the intestinal phase?

products of protein digestion

when is HCl secretion inhibited? (2)

- when HCl is no longer needed as chyme enters small intestine

- technically when pH decreases

what cell is responsible for HCl secretion inhibition?

D cells

what do D cells release?

somatostatin

what does somatostatin do?

- directly and indirectly inhibit HCl secretion

which cells release pepsinogen?

chief and mucous cells

what stimulates pepsinogen secretion? (2)

- vagus nerves release ACh

- H+ triggers chief cells to secrete pepsinogen

(when pH is low enough for pepsinogen -> pepsin)

what is an intrinsic factor?

a mucoprotein

what is intrinsic factor vital for?

vitamin B12 absorption in the ileum