Page 417 - Anatomy and Physiology of Farm Animals, 8th Edition

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Plasma in Interstitial fluid Parietal cell of fundic Lumen of
capillary gland region stomach
VetBooks.ir HCO – 3 HCO + H + H +
–
3

H CO 3
2
Carbonic anhydrase
CO 2 CO + H O
2
2
Cl – Cl –
Figure 21-4. Mechanism of hydrochloric acid secretion by parietal cells of the gastric mucosa.
Source: Reece, 2009. Reproduced with permission of John Wiley & Sons, Inc.

the patient resulting in a metabolic alka- a component of gastric juice) is activated by
losis that must be corrected along with the low pH. By its proteolytic activity, pepsin
the dehydration. can activate more pepsinogen. The low pH
The parasympathetic neurotransmitter also promotes the activity of pepsin, because
acetylcholine also stimulates parietal cells the most favorable pH range for its proteo-
to secrete HCl, but all three regulators lytic activity is 1.3 to 5. Chief or peptic cells
(gastrin, histamine, and acetylcholine) (Fig. 21‐3) secrete pepsinogen to begin pro-
must be present for the most efficient tein digestion in the stomach, but protein
hydrochloric acid secretion. The hista- digestion is completed in the small intestine
mine receptors on parietal cells (H by other digestive enzymes. Chief cells also
2
receptors) are different from those on produce a proteolytic enzyme called rennin
cells involved in allergic reactions (H that functions to coagulate milk and reduce
1
receptors). The specific H receptor its rate of passage through the gastrointesti-
2
antagonists provide a means to reduce nal tract. This is particularly important in
acid secretion with few side effects. The neonates and is found in the abomasum of
antihistamines used for allergies do not young ruminants.
bind to H receptors and thus do not dis- A layer of mucus covers the epithelial
2
turb digestion. The hormones cholecys- lining of the stomach and protects the epi-
tokinin, gastric inhibitory peptide, and thelium from the low pH of the gastric fluids.
secretin inhibit hydrochloric acid secre- This mucus is produced by cells in the gastric
tion. These hormones are released from glands (Fig. 21‐3) and is secreted from mucus
the duodenal epithelium in response to the glands onto the surface of the epithelium.
presence of food in the duodenum. The Mucus secretion is stimulated by prosta-
release of these hormones that act to glandins, which are also produced locally
inhibit gastric function is part of the intes- in the wall of the stomach. Nonsteroidal
tinal phase of gastric regulation. Parietal anti‐inflammatory drugs (such as aspirin
cells also produce intrinsic factor, a carrier and phenylbutazone) inhibit the synthesis
protein for vitamin B that binds to the of prostaglandins, and are associated with
12
vitamin, and the resultant complex passes gastric ulcers. It is presumed that a lack of
through the tract to the ileum, which mucus secretion contributes to the devel-
absorbs the B . opment of the ulcers.
12
The pH of gastric juice in mammals can
be 2 or less. The low pH is protective in that
most foreign microbes ingested with food Gastric Motility
cannot survive such an acidic environment.
The low pH inhibits HCl secretion to pre- Gastric movements mix the ingesta with
vent it from becoming too acidic. Pepsinogen the gastric juice, continue mechanical
(an inactive form of the enzyme pepsin and digestion (to liquefy the digesta), and pass