Page 418 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Digestion / 403

the digesta into the duodenum at a con- fasting, the magnitude of the contractions
becomes greater (hunger contractions).
trolled rate. The stomach regularly pro-
VetBooks.ir duces peristaltic contractions, beginning These are apparently a response to an
increase in parasympathetic input during
in the region of the cardia and increasing in
force as they travel over the stomach to the prolonged fasting. These reach maximum
pyloric antrum (see Fig. 20‐8). These mix intensity in humans after about 3 days
and grind the food and force some through without food and weaken progressively
the pyloric sphincter into the duodenum. thereafter. In the horse, hunger contractions
However, much of the food (and especially may begin as early as 5 hours after eating,
larger particles) is held back to allow for when the stomach still contains some food.
more mixing and grinding. The ingesta The intensity of hunger contractions is
forced through the pyloric sphincter, related to the level of blood sugar. As the
termed chyme, is a mushy, semisolid mix- blood sugar level decreases, the intensity of
ture of food, water, and gastric juice. hunger contractions increases.
Similar to the regulation of gastric secre-
tions, the regulation of gastric motility can
be divided into cephalic, gastric, and intes- Physiology of the Small Intestine,
tinal phases. Stimulation during cephalic Exocrine Pancreas, and Liver
regulation occurs via the parasympathetic
nerves, and this increases in response to The small intestine is the primary site of
sight, smell, or taste of food. The hormone chemical digestion and absorption of nutri-
gastrin stimulates overall gastric motility to ents. The exocrine secretions of the pan-
promote mixing (gastric phase). The hor- creas contain most of the enzymes for
mones cholecystokinin and secretin and chemical digestion in the lumen of the small
gastric inhibitory peptides promote a more intestine, but the epithelial cells that line the
forceful contraction of the pyloric sphinc- small intestine (enterocytes) also have in
ter to slow gastric emptying (intestinal their cell membranes enzymes that partici-
phase). The inhibitory effect of the duode- pate in the final steps of chemical digestion.
nal hormones (released in response to The primary digestive function of the liver
chyme entering the duodenum) prevents is to provide bile salts, which facilitate the
the delivery of chyme to the duodenum too enzymatic digestion of lipids. The liver is
fast to be digested normally. not a source of digestive enzymes.
The stomach of a carnivore empties
within a few hours, usually before the next
meal. Other animals require many hours to Small Intestine Secretions and Motility
empty the stomach. Both the horse and pig
require a full day’s fast (24 hours) to empty Intestinal juice is derived from intestinal
a full stomach. The stomach of a nursing glands in the wall of the small intestine.
foal empties slowly, but in an adult pony These include crypts or crypts of
liquid can pass from the stomach to the Lieberkuhn, scattered throughout the
cecum in 2 hours. entire small intestine (Fig. 21‐5), and duo-
In addition to the typical pattern of denal glands, which contribute mucus
stomach contraction when food is present, and are found only in the duodenum. The
waves of peristaltic contractions may occur intestinal juice contains salts and water
over the stomach as a slight ripple. These derived from blood capillaries in the wall
are produced by spontaneous electrical of the intestine. The function of the
depolarizations, which sometimes induce secreted salts is unclear, but the water
action potentials, in the smooth muscle. dilutes the chyme, which is usually hyper-
These begin in the cardia region, and the tonic (higher osmolality than normal
waves of membrane depolarization are plasma). Food in the intestine stimulates
termed gastric slow waves. In prolonged secretion by these intestinal crypt glands.