Page 422 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Digestion / 407

the action of the pancreatic lipases. Bile common bile duct and block passage of
acids act as emulsifiers to reduce droplet
VetBooks.ir size and make the lipids more accessible to proteolytic enzymes from the exocrine
pancreas from reaching the duodenum.
the lipases. Lipases can function without
bile salts, but lipid digestion is inefficient Pancreatitis is inflammation of the
pancreas and can result in severe life‐
without them. Micelle is the term for the threatening illness if those proteolytic
small droplets formed in the intestinal enzymes begin to self‐digest the pan-
chyme that contain lipids, bile salts, and creas. While pancreatitis has many
products of lipid digestion. etiologies, gallstone obstruction of the
In all farm animals except the horse, bile common bile duct or major duodenal
is stored in the gallbladder. Since the horse papilla can be a medical emergency in
has no gallbladder, the bile passes directly humans and animals.
from the liver to the duodenum by way of the
bile duct and its tributaries at a fairly con-
tinuous rate. The gallbladder stores bile for Nutrient Absorption in the Small
intermittent discharge into the duodenum Intestine
and concentrates the bile by reabsorbing
water from the stored bile. Cholecystokinin The small intestine is the major site of
stimulates gallbladder contraction and the nutrient absorption. Most of the products
release of stored bile. Since food entering the of carbohydrate, protein, and lipid diges-
duodenum stimulates the release of chole- tion are absorbed as the digesta pass
cystokinin, this coordinates the release of through the small intestine. The small
bile with the presence of food. intestine is also the primary site of absorp-
Most of the bile salts released from the tion for vitamins, minerals, and water.
liver remain mixed with the digesta as it The epithelium lining the small intes-
passes into the terminal part of the small tine has structural features that increase
intestine (ileum). Here, enterocytes reabsorb the surface area for nutrient absorption.
bile salts, which enter the blood. The reab- The mucosa is covered with villi (finger-
sorbed bile salts are transported to the liver like projections) that extend into the
via the hepatic portal vein, and here hepato- lumen, and the individual enterocytes have
cytes take up the bile salts from the portal microvilli on their cell membrane facing
blood. These bile salts can then be secreted the lumen (Fig. 21‐5).
by the hepatocytes into bile for reuse. An Individual amino acids and monosac-
increase in bile salts in portal blood, such as charides (simple sugars) are the simplest
during the digestion of a meal, is the primary products of protein and carbohydrate
stimulus for bile salt secretion by hepato- digestion, respectively. The cellular mech-
cytes. The recycling of bile salts between the anisms of absorption of amino acids and
digestive tract and the liver is enterohepatic monosaccharides (primarily glucose) are
circulation. similar in that the transport across the
The liver is capable of synthesizing cho- cell membrane on the luminal surface
lesterol, and the liver makes much of the involves sodium‐linked cotransporters
cholesterol in bile. The liver can also elimi- (see Chapter 2). The cotransporters all
nate excessive dietary cholesterol via the bind sodium, but different cotransporters
bile. Cholesterol is insoluble in water, but are used by glucose and amino acids. At
the bile salts and lecithin normally change least five cotransporters have been found
it to a soluble form so that it can exist in to transport various amino acids. These
the bile. However, sometimes cholesterol are all characterized as secondarily active
precipitates from the bile in the gallbladder transport, for they depend on the gradient
or bile ducts, forming gallstones. Stones for sodium between the intracellular fluid
in the gallbladder can have serious con- of the enterocytes and the fluid in the
sequences if they become lodged in the lumen of the small intestine (see Chapter 2).