Page 416 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Digestion / 401

calves usually is attributed to milk entering Gastric lumen
the rumen, where it is not properly digested.
VetBooks.ir The use of buckets with nipples tends to

prevent appreciable amounts of milk from
entering the rumen. After weaning, fluid
drunk from open containers largely passes Surface
into the rumen and reticulum. The groove epithelial cells
has no known function in adult animals. Mucous neck
cell
Omasum
Parietal cell
The omasum is nearly always found packed Chief cell
tightly with rather dry roughage in animals
examined after death. The appearance of
the omasal leaves, studded with short Endocrine cell
horny papillae, suggests a burr type of
grinder capable of excess fluid absorption.
Experimental vagal stimulation elicits Figure 21-3. Gastric pit (gastric gland) in the
strong contractions of the omasal wall, but lining of the stomach. Note the different cell types
movement of the leaves is limited. found in the epithelium lining the gland.

Gastric Physiology cells in the gastric epithelium (Fig. 21‐3).

Gastrin and histamine stimulate parietal
Gastric Glands and Secretions cells in gastric glands to secrete hydro-
chloric acid (HCl) (Fig. 21‐4). Carbonic
The term gastric juice refers to the combi- anhydrase facilitates the formation of
nation of substances secreted into the H CO from CO that diffuses into the
3
2
2
stomach lumen by gastric glands, also parietal cells from the interstitial fluid.
termed gastric pits because of their pit‐like H CO dissociates into H and HCO
+
−
2
3
3
extension into the wall of the stomach within the parietal cell. The parietal cell
(Fig. 21‐3), and epithelial cells of the stom- actively secrets H and Cl into the lumen
+
−
ach mucosa. Gastric juice contains water, of the stomach and this causes a gradient
hydrochloric acid, mucus, intrinsic factor, for diffusion of Cl from the plasma into
−
pepsinogen (an inactive form of pepsin, a the parietal cell. The loss of Cl from
−
proteolytic enzyme), and the enzyme plasma is followed by diffusion of HCO
−
3
rennin. The regulation of gastric juice into plasma so that electrical neutrality is
secretion has three phases: cephalic, gas- maintained. The net result after the inges-
tric, and intestinal. tion of food is therefore an increase in
Stimulation of gastric secretions during plasma bicarbonate concentration associ-
the cephalic phase is in response to the ated with the secretion of HCl into the
sight, smell, or taste of food. These induce lumen of the stomach. Prolonged vomit-
a neural response that increases parasym- ing (emesis) can result in significant
pathetic (vagal nerve) stimulation to the dehydration as well as abnormalities in
stomach, and this stimulates gastric secre- acid‐base balance. Secretion of HCl into
tions. The gastric phase begins when food the gastric lumen continues during eme-
enters the stomach. sis, resulting in the continued loss of
The presence of food, especially gastric HCl accompanied by persistent
proteins, stimulates the secretion of the diffusion of HCO into the plasma. This
3‐
hormones gastrin and histamine from effectively increases the plasma pH of