CHAPTER 14 SUMMARY
Introduction
•The four basic digestive processes are motility, secretion, digestion, and absorption.
•Depending on the species of animal, the three classes of energy-rich nutrients are digested into
absorbable units as follows: (1) Dietary carbohydrates in the form of the polysaccharides starch,
glycogen, chitin and cellulose are digested into their absorbable units of monosaccharides,
especially glucose. (2) Dietary proteins are digested into their absorbable units of amino acids
and a few small poypeptides. (3) Dietary fats in the form of triglycerides are digested into their
absorbable units of monoglycerides and free fatty acids.
•The digestive tract consists of a continuous tube that runs from the mouth to the anus, with
local modifications that reflect regional specializations for carrying out digestive functions. The
lumen of the digestive tract is continuous with the external environment, so its contents are
technically outside the body. This arrangement permits digestion of food without self-digestion
occurring in the process.
Mouth, Pharynx, Crop and Esophagus
•Food enters the digestive system through the mouth, where it is chewed and mixed with saliva
to facilitate swallowing. The salivary enzyme, amylase, begins the digestion of polysaccharides, a
process that continues in the stomach after the food has been swallowed until amylase is
eventually inactivated by the acidic gastric juice. More important than its minor digestive
function, saliva is essential for articulate speech and plays an important role in dental health.
Salivary secretion is controlled by a salivary center in the medulla, mediated by autonomic
innervation of the salivary glands.
•The swallowing center in the medulla coordinates a complex group of activities that result in
closure of the respiratory passages and propulsion of the food through the pharynx and
esophagus into the crop or the stomach. The esophageal secretion, mucus, is protective in
nature. No nutrient absorption occurs in the mouth, pharynx, crop, or esophagus.
Stomach
•The stomach, a saclike structure located between the esophagus and small intestine, stores
ingested food for variable periods of time until the small intestine is ready to process it further for
final absorption.
•The four aspects encompassing gastric motility are gastric filling, storage, mixing, and emptying.
(1) Gastric filling is facilitated by vagally mediated receptive relaxation of the stomach
musculature. (2) Gastric storage takes place in the body of the stomach, where peristaltic
contractions of the thin muscular walls are too weak to mix the contents. (3) Gastric mixing takes
place in the thick-muscled antrum as a result of vigorous peristaltic contractions. (4) Gastric
emptying is influenced by factors in both the stomach and the duodenum.
•The volume and fluidity of chyme in the stomach tend to promote emptying of the stomach
contents. The duodenal factors, which are the dominant factors controlling gastric emptying, tend
to delay gastric emptying until the duodenum is ready to receive and process more chyme. The
specific factors in the duodenum that delay gastric emptying by inhibiting stomach peristaltic
activity are fat, acid, hypertonicity, and distension.
•Carbohydrate digestion continues in the body of the stomach under the influence of the
swallowed salivary amylase. Protein digestion is initiated in the antrum of the stomach, where
vigorous peristaltic contractions mix the food with gastric secretions, converting it to a thick liquid
mixture known as chyme.
•Gastric secretions into the stomach lumen include (1) HCl, which activates pepsinogen,
denatures protein, and kills bacteria; (2) pepsinogen, which, once activated, initiates protein
digestion; (3) mucus, which provides a protective coating to supplement the gastric mucosal
barrier, enabling the stomach to contain the harsh luminal contents without self-digestion; and
(4) intrinsic factor, which plays a vital role in vitamin B 12 absorption, a constituent essential for
normal red blood cell production.
•The stomach also secretes the hormone gastrin into the blood, which plays a dominant role in
regulating gastric secretion. Histamine, a potent gastric stimulant that is not normally secreted, is
released into the stomach lumen with devastating effects during ulcer formation.
•Both gastric motility and gastric secretion are under complex control mechanisms, involving not
only gastrin but also vagal and intrinsic nerve responses and enterogastrone hormones (secretin,
cholecystokinin, and gastric inhibitory peptide) secreted from the small intestine mucosa.
Regulation of the stomach is aimed at balancing the rate of gastric activity with the ability of the
small intestine to handle the arrival of acidic, fat-laden contents from the stomach.
•No nutrients are absorbed from the acidic stomach.
Pancreatic and Biliary Secretions
•Pancreatic exocrine secretions and bile from the liver both enter the duodenal lumen.
•Pancreatic secretions include:
1. potent digestive enzymes from the acinar cells, which digest all three categories of
foodstuff; and
2. an aqueous NaHCO3 solution from the duct cells, which neutralizes the acidic contents
emptied into the duodenum from the stomach.
This neutralization is important to protect the duodenum from acid injury and to allow the
pancreatic enzymes, which are inactivated by acid, to perform their important digestive functions.
Pancreatic secretion is primarily under hormonal control, which matches the composition of the
pancreatic juice with the needs in the duodenal lumen.
•The vertebrate liver (comparable to the fat body of insects), performs many varied biochemical
functions. Its contribution to vertebrate digestion is the secretion of bile, which contains bile
salts. Bile salts aid fat digestion through their detergent action and facilitate fat absorption
through formation of water-soluble micelles that can carry the products of fat digestion to their
absorptive site. Between feeding, in some species bile is stored and concentrated in the gall
bladder, which is hormonally stimulated to contract and empty the bile into the duodenum during
digestion of a meal. After participating in fat digestion and absorption, bile salts are reabsorbed
and returned via the hepatic portal system to the liver, where they are not only are resecreted
but act as a potent choleretic to stimulate the secretion of even more bile.
•Bile also contains bilirubin, a derivative of degraded hemoglobin, which is the major excretory
product in the feces.
Small Intestine
•The midgut of invertebrates or vertebrate small intestine is the main site for digestion and
absorption in an animal. Segmentation, its primary motility, thoroughly mixes the food with
pancreatic, biliary, and small intestinal juices to facilitate digestion: it also exposes the products
of digestion to the absorptive surfaces.
•The juice secreted by the vertebrate small intestine does not contain any digestive enzymes.
The enzymes synthesized by the small intestine act at the brush border membrane of the
epithelial cells. These enzymes complete the digestion of carbohydrates and protein before these
nutrients enter the cells and can be made available to the animal. The energy-dependent process
of Na+ absorption provides the driving force for Cl-, water, glucose, and amino acid absorption.
•Fat digestion is accomplished entirely in the lumen of the small intestine by pancreatic lipase.
Because fats are not soluble in water, the products of fat digestion must undergo a series of
transformations that enable them to be passively absorbed, eventually entering the lymph.
•The small intestine absorbs almost everything presented to it, from ingested food to digestive
secretions to sloughed epithelia cells. Only a small amount of fluid and nondigestible food residue
passes on to the large intestine.
•The small intestine lining is remarkably adapted to its digestive and absorptive function. It is
thrown into folds that bear a rich array of fingerlike projections, the villi, which are furnished with
a multitude of even smaller hairlike protrusions, the microvilli. Altogether, these surface
modifications tremendously increase the area available to house the membrane-boune enzymes
and to accomplish both active and passive absorption. This impressive lining is replaced
approximately every three days to ensure an optimally healthy and functional presence of
epithelial cells in spite of harsh luminal conditions.
Large Intestine and Ceca
•The colon and ceca serve primarily to concentrate and store undigested food residues and
biliary waste products until they can be eliminated from the body as feces.
•Postgastric fermentation occurs in all animals but is particularly well developed in some species.
The site of the fermentation chamber is the combined colon and cecum. By positioning the
fermentation chamber after the stomach, the host animal has the first chance at the available
carbohydrates and protein in the food but loses the opportunity to exploit the high quality
microbially synthesized protein. Further, the digestive capabilities of the stomach and small
intestine are not able to contribute to the digestion of microbial products. However, VFAs and
some vitamins synthesized by the microbes in the cecum and colon are absorbed through the
mucosa of the large intestine.
•With few exceptions, nutrient digestion and absorption is completed in the small intestine.
Haustal contractions slowly shuffle the colonic contents back and forth to accomplish absorption
of most of the remaining fluid and electrolytes, which converts the colonic contents into feces.
Ruminants
•The rumen is the first of four compartments of the stomach of ruminant animals. Digestion of
fibrous vegetation by herbivores requires the metabolic activities of populations of anaerobic
microbes that colonize chambers of the ruminant stomach. These enlarged compartments delay
the transit time of food particles to allow microbial digestion of fiber into digestible units prior to
the food material reaching the acidic stomach. These fermentations occur at nearly neutral pH
and convert plant materials into volatile fatty acids that can be directly absorbed through the
lining of the rumen. Protein derived from the microbes is eventually digested in the ruminant
abomasums and used as a source of amino acids and energy.