Agriculture Reference
In-Depth Information
FUNCTION OF OMASUM
The role of omasum in digestive physiology of ruminants
is briefl y summarized in this section, but further informa-
tion is presented in Chapter 6. Size of the omasum is
considerably larger for cattle than for goats. It is an addi-
tional site for fermentation and absorption, but its primary
function appears to be regulation of the fl ow of digesta
from the reticulo-rumen to the abomasum. Ingesta from
the reticulum fl ows to the omasum through the reticulo-
omasal orifi ce that is open during the secondary phase of
the primary cycle contractions of the reticulum. Prolonged
and powerful contractions of the omasal body tend to
empty the materials trapped between leaves into the
abomasum. Particle size of digesta in the omasum (about
1 mm) is similar to that found in the reticular area adjacent
to the orifi ce, and its physiochemical conditions resemble
those of the cranial and ventral regions of the reticulo-
rumen. The omasum has a large surface area (leaves) rela-
tive to its volume, a factor that gives this organ a large
capacity for absorption of VFA, electrolytes, and water. In
this organ, chloride (instead of bicarbonate) plays a major
role in VFA absorption.
hydrolysis of specifi c glycosidic bonds in mucopolysac-
charides that constitute some bacterial cell walls. Ruminant
animals secrete large amounts of specially adapted lyso-
zymes into the abomasal lumen. Active at low pH, lyso-
zymes resist pepsin digestion. Acid, pepsin, and lysozymes
chemically and enzymatically digest microbial protein and
other digesta, preparing the chyme to enter the duodenum
for further digestion.
INTESTINAL DIGESTION
Acidic chyme (a mixture of partially digested feeds and
digestive juices) enters the duodenum for further enzy-
matic digestion in and absorption of monomers from the
SI. Since the SI is a primary site for both digestion and
absorption, luminal fl ow is regulated so as to provide
mixing of luminal contents with digestive juices, time for
luminal digestion of nutrients, and exposure of digested
nutrients to the intestinal wall for absorption.
Presence of acid in the duodenum provides the stimulus
for the intestinal wall to release the hormone secretin in
the portal blood that in turn stimulates the pancreas and
gallbladder to release bicarbonate-rich fl uid into the duo-
denum to partially neutralize acidic chyme. Cholecystokinin
(CCK) from the SI wall is released in the presence of fats
or proteins in the duodenum to stimulate the pancreas to
release enzyme-rich digestive juices (and some bicarbon-
ate) and the gallbladder to release bicarbonate as well as
bile acids and salts. The pancreas secretes all of the
enzymes necessary to digest lipids, proteins, and carbohy-
drates; however, in ruminants most (50-90% of starch) of
the readily available carbohydrates and lipids (small
amounts of triglycerides and galactolipids in the diet) are
cleaved to free fatty acids in the rumen so supply of these
nutrients is limited. This may explain why pancreatic juice
of ruminants is not rich in lipolytic and amylolytic enzymes.
Pancreatic proteolytic enzymes are secreted in proen-
zyme form, being exopeptidases such as carboxypeptidase
A and B or endopeptidases such as trypsinogen, chymo-
trypsinogen, and elastase. Trypsinogen is activated by
intestinal enterokinase to form trypsin that in turn activates
remaining trypsin and other proenzymes. The end-products
of pancreatic proteolytic digestion are amino acids and
oligopeptides (up to 6-10 amino acid chain). Amino acids
and some oligopeptides are actively transported into the
epithelial cells where more than 90% of the oligopeptides
are hydrolyzed to amino acids and actively transported into
the blood. The remaining 10% of dipeptides and tripep-
tides may diffuse directly into the bloodstream.
Sucrose is not digested in the small intestine, but most
will be fermented before reaching the small intestine.
FUNCTION OF ABOMASUM
The abomasum or “true stomach” receives more or less
continuous (but at variable rates) input of ingesta contain-
ing partially fermented materials, fl uids, or particle clumps
(of variable composition) from the omasum and, following
acidifi cation, passes digesta in a reasonably constant fl ow
to the duodenum. The abomasum plays two important
roles in ruminant digestion: (1) transfer of partially digested
feed and (2) chemical and enzymatic breakdown of ingesta.
The cardiac and fundus regions of the abomasum are
responsible for nonacid secretion while the antrum/pyloric
region secretes acid.
Distension of the pyloric region, a rise in abomasal pH,
and presence of VFA and lactic acid all serve to stimulate
gastric secretions and contractions. The presence of acidic
conditions and fat in the duodenum inhibit gastric motility
and gastric emptying. The G cells in the pyloric gland area
release gastrin hormone into the blood that stimulates pari-
etal cells to release hydrochloric acid. Feedback control of
acid release comes from a low pH (approaching pH = 2)
of gastric contents that stimulates release of somatostatin.
A low duodenal pH also can inhibit acid release, likely
through inhibiting gastric emptying.
Pepsinogen, an inactive proteolytic enzyme, is released
by chief cells of the gastric mucosa. Pepsinogen is autolyti-
cally activated and converted to pepsin in the presence of
hydrochloric acid. In addition, lysozymes catalyze the
