Page 421 - Anatomy and Physiology of Farm Animals, 8th Edition

406 / Anatomy and Physiology of Farm Animals

oligosaccharides (a carbohydrate com-
posed of a small number of monosaccha-
VetBooks.ir rides, usually two to four). The enzymes

maltase and sucrase, components of
enterocyte cell membranes, further digest Bile duct 1
the oligosaccharides to monosaccharides.
Lactase, to digest lactose (milk sugar), is A Liver and
gallbladder
present in enterocytes of young mammals
but not in all adults. Lipase hydrolyzes tri- Gastrin 2 3
glycerides into fatty acids and glycerol. This Pancreas
action is most effective after the fats have
been emulsified by bile (discussed later). Stomach
Seeing or smelling food stimulates vagal 4
stimulation, and food in the stomach Secretin Stomach
prompts release of gastrin. Control of pan- Duodenum
creatic exocrine secretion depends on stim- 5
ulation by vagal autonomic nerves that Pancreas
innervate the pancreas and on three intesti-
nal hormones, cholecystokinin, secretin,
and gastrin. A summary of the action of B
these three hormones is found in Figure 21‐8. Bile duct 6
The greatest amount of pancreatic exo-
crine secretion occurs when the acid Liver and
chyme and food components in the duode- gallbladder
num stimulate the release of cholecysto- C 7
kinin and secretin from cells in the Cholecystokinin
duodenal mucosa (intestinal phase of Stomach
control). These two duodenal hormones Duodenum
also feed back to the stomach to decrease 8
secretions and slow down the activity and Pancreas
emptying of the stomach until the duode-
nal chyme has been degraded by the
enzymes and adjusted in pH by the pancre-
atic bicarbonate. Bile duct 9
Liver and
gallbladder
Liver Digestive Function and
Secretion of Bile Sphincter of Oddi Duodenum

Liver cells (hepatocytes) are responsible Figure 21-8. Summary of the steps for hormonal
for bile formation. Bile is a greenish‐yellow control of digestion by gastrin (A), secretin (B), and
salt solution consisting primarily of bile cholecystokinin (C). Source: adapted from Reece,
salts, cholesterol, phospholipids (leci- 2009. Reproduced with permission of John Wiley &
thins), and bile pigments (bilirubin). Sons, Inc.
Hepatocytes synthesize the bile salts
(primarily sodium salts of glycocholic and In an aqueous solution, such as the duo-
taurocholic acids) from cholesterol. These denal chyme, lipids tend to clump together
salts assist in digestion and absorption of and form large droplets (recall the appear-
lipids (triglycerides), and the production ance of an oil and vinegar salad dressing
and secretion of these salts is the most after shaking the bottle). Such large lipid
important digestive function of the liver. droplets present a small surface area for