Gastric Phase

The arrival of food into the stomach stimulates the gastric phase of regulation. Gastric secretion is stimulated in response to two factors: (1) distension of the stomach, which is determined by the amount of chyme, and (2) the chemical nature of the chyme.

While intact proteins in the chyme have little stimulatory effect, the partial digestion of proteins into shorter polypeptides and amino acids, particularly phenylalanine and tryptophan, stimulates the chief cells to secrete pepsinogen and the G cells to secrete gas-trin. Gastrin, in turn, stimulates the secretion of pepsinogen from chief cells, but its effect on the parietal cells is primarily indirect. Gastrin stimulates the secretion of histamine from ECL cells, and the histamine then stimulates secretion of HCl from parietal cells, as previously described (fig. 18.30). A positive feedback mechanism thus develops. As more HCl and pepsinogen are secreted, more short polypeptides and amino acids are released from the ingested protein. This stimulates additional secretion of gastrin and, therefore, additional secretion of HCl and pepsinogen. It should be noted that glucose in the chyme has no effect on gastric secretion, and the presence of fat actually inhibits acid secretion.

Fish Pepsinogen Secretion

■ Figure 18.30 The regulation of gastric acid secretion. The presence of amino acids in the stomach lumen from partially digested proteins stimulates gastrin secretion. Gastrin secretion from G cells is also stimulated by vagus nerve activity. The secreted gastrin then acts as a hormone to stimulate histamine release from the ECL cells. The histamine, in turn, acts as a paracrine regulator to stimulate the parietal cells to secrete HCl. (© = stimulation; Q = inhibition.)

Secretion of HCl during the gastric phase is also regulated by a negative feedback mechanism. As the pH of gastric juice drops, so does the secretion of gastrin—at a pH of 2.5, gastrin secretion is reduced, and at a pH of 1.0 gastrin secretion ceases. The secretion of HCl thus declines accordingly. This effect may be mediated by the hormone somatostatin, secreted by the D cells of the gastric mucosa. As the pH of gastric juice falls, the D cells are stimulated to secrete somatostatin, which then acts as a paracrine regulator to inhibit the secretion of gastrin from the G cells.

The presence of proteins and polypeptides in the stomach helps to buffer the acid and thus to prevent a rapid fall in gastric pH. More acid can thus be secreted when proteins are present than when they are absent. The arrival of protein into the stomach thus stimulates acid secretion in two ways—by the positive feedback mechanism previously discussed and by inhibition of the negative feedback control of acid secretion. Through these mechanisms, the amount of acid secreted is closely matched to the amount of protein ingested. As the stomach is emptied the protein buffers exit, the pH thus falls, and the secretion of gas-trin and HCl is accordingly inhibited.

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Responses

  • Lawrence
    What is the pathway of pepsinogen secretion?
    8 years ago
  • amanda
    Which phase stimulates gastric secrection?
    8 years ago
  • ISAIAS
    What stimulates release of pepsinogen?
    8 years ago
  • Milena
    What stimulates secretion of hcl into stomach?
    8 years ago
  • iole
    How do amino acids stimulate hcl secretion?
    8 years ago
  • jasper
    What stimulates gastrin secretion?
    8 years ago
  • marcel marquis
    How gastrin stimulates gastic secretion?
    8 years ago
  • Katja
    What stimulates the secretion of pepsinogen?
    8 years ago
  • Peter
    What stimulates hcl in parietal cells?
    8 years ago
  • MIRABELLA
    Which gastric phase is gastrin secreted?
    8 years ago
  • dominik gottlieb
    What stimulates acid secretion in the stomach?
    8 years ago
  • welde
    What cell affect stomach ph?
    8 years ago
  • Jaakko
    Which human stomach cells secrete pepsinogen?
    8 years ago
  • Edward Reilly
    What is phase of gastic secretion tht is stimulated by the presence of food in the stomach?
    7 years ago
  • ursula
    When phase is pepsinogen released?
    7 years ago
  • Aidan Kerr
    What is thhe gastroc phase?
    7 years ago
  • demet
    How is gastric secretion stimulated?
    6 years ago
  • domenica
    What phase of gastric secretion that is stimulated by the presence of food in the stomach?
    6 years ago
  • kerstin
    What does the stomach do in response to stimulation by the brainstem during the gastric phase?
    6 years ago
  • JAMES
    What food in the stomach stimulates the gastric phase?
    5 years ago
  • primula
    What does positive feedback in gastric phase involve?
    2 years ago
  • Lucia
    What type of feed back mechanism is HCL secretion in the stomach?
    2 years ago
  • Ernest
    Which phase of gastric secretion is stimulated by the distention of the stomach?
    1 year ago
  • gary
    What proteins stimulate gastrin?
    9 months ago
  • natalia j
    Are gastric secretion negative or positive feedback?
    5 months ago
  • lena
    What do proteins do in the intestinal phase?
    5 months ago

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