Chapter summary

Introduction

The digestive system consists of the digestive tract, also called the gastrointestinal or alimentary tract, and its accessory organs. The accessory organs include the teeth, tongue, salivary glands, liver, pan­creas, and gall bladder.

Functions of the digestive tract

Functions of the digestive system include ingestion, propulsion, mechanical processing, digestion, secre­tion, absorption, excretion, and immunity.

Peritoneal cavity

1. The peritoneal cavity is formed from a serous membrane, called the peritoneum. The perito­neal membrane has a serosa, or visceral layer, that covers the organs in the peritoneal cavity, and a parietal peritoneum that lines the inner surface of the body wall.

2. The mesentery consists of two layers of serous membranes fused back to back, and suspends portions of the digestive tract from the body wall. The mesenteries (1) provide a route for blood vessels, lymphatic vessels, and nerves to travel to the digestive system; (2) hold organs in place; and (3) store lipid.

3. Some organs are located between the posterior parietal peritoneum and the posterior abdomi­nal wall, and are thus outside this cavity and are said to be retroperitoneal. These organs include the kidneys, adrenal glands, ureters, duodenum, ascending colon, descending colon, and pancreas.

Blood supply of the digestive organs

The splanchnic circulation serves the digestive organs and hepatic portal system.

Histology of the digestive tract

1. The digestive tract includes four major layers. Listed from the lumen outward, they include (1) the mucosa, (2) the submucosa, (3) the muscula­ris externa, and (4) the serosa.

2. The mucosa layer consists of three sublayers including a layer of epithelial tissue in direct contact with the contents of the digestive tract, the lamina propria, and the muscularis mucosae.

3. The submucosa consists of dense irregular con­nective tissue, and contains large blood vessels, lymphatic vessels, and in some regions, exocrine glands secreting buffers and enzymes into the lumen.

4. The muscularis externa generally consists of two layers of smooth muscle: an inner layer of circu­lar smooth muscle fibers and an outer layer of longitudinal smooth muscle fibers. These layers control peristalsis and segmental contractions.

5. The serosa. Most portions of the digestive tract lie within the peritoneal cavity. The outermost portions or superficial layer of the digestive tract is lined with the adventitia. There is no serosa around the oral cavity, pharynx, esophagus, or rectum.

Enteric nervous system

1. The digestive tract has its own nervous system called the enteric nervous system. It is composed mostly of two large plexuses: the submucosal plexus and myenteric plexus.

2. The submucosal plexus, or Meissner's plexus, is located within the submucosal layer.

3. The myenteric plexus, or plexus of Auerbach, is located between the two layers of smooth muscle fibers in the muscularis externa.

Functional anatomy of the digestive system

The digestive system shows great variation among species depending on whether the animal is a carni­vore (meat-eating), herbivore (plant-eating), or omnivore (meat- and plant-eating). In nonruminants, the stomach and small intestine are relatively small, whereas the cecum is well developed. Ruminants have a complex stomach that accommodates micro­bial digestion. Carnivores have a short small intes­tine, a poorly developed cecum, and an average colon. Omnivores have an intermediate size colon since this is a major site of microbial digestion.

The mouth

1. It is also called the oral cavity, or buccal cavity, and is where food first enters the digestive tract.

The mouth is lined with stratified squamous epi­thelium, which protects against friction.

2. The palate is the roof of the oral cavity and it consists of a rostral bony part called the hard palate and a caudal musculomembranous portion called the soft palate.

Tongue

1. The tongue is composed of interlacing bundles of skeletal muscle fibers and is involved in grip­ping, repositioning food, mixing food with saliva, and forming the compact mass of food called a bolus.

2. The superior surface of the tongue has many papillae that are named for their shape.

Salivary glands and saliva

1. Salivary glands are extramural glands associated with the oral cavity. The secretions of the salivary glands can be serous, mucous, or mixed.

2. The major salivary glands include the parotid salivary gland, the mandibular (submandibular, submaxillary) salivary gland and the sublingual salivary gland.

3. Saliva solubilizes food; provides alkaline buffer­ing and fluid; removes wastes, lubrication, and binding; initiates starch digestion; provides oral hygiene; and helps with evaporative cooling.

Teeth

1. Domestic animals have two types of teeth— low-crowned (brachydont) and high-crowned (hypsodont).

2. All domestic species have two sets of teeth, deciduous and permanent. Deciduous teeth are smaller and fewer in number.

3. Low-crowned teeth are simple teeth as found in man, carnivores, pigs, ruminant incisors, and horse deciduous incisors. They consist of a crown, neck, and root.

4. Teeth are composed of three layers: cementum, enamel, and dentin.

5. Teeth are divided into groups according to their location and function. Incisors are located in the rostral portion of the mouth. The canine is the large tooth between the incisors and cheek teeth. Cheek teeth are those teeth caudal to the canine and incisors in the maxillary. They include the premolars located in the rostral cheek area and molars located caudal to the premolars.

Pharynx

1. The pharynx is the common passageway for food and air. It includes the oropharynx, naso­pharynx, and Iaryngopharynx.

2. In birds, there is no sharp distinction between the pharynx and mouth.

Swallowing

1. Swallowing, or deglutition, involves three stages: (1) voluntary stage—bolus moved into the oro­pharynx, (2) pharyngeal stage—bolus moves involuntarily through the pharynx to the esopha­gus, and (3) esophageal stage—bolus moves involuntarily through the esophagus to the stomach.

2. In many species of birds, the upper portion of the esophagus is expanded to form the crop.

Stomach

1. The stomach has four functions: (1) storage of ingested food, (2) mechanical breakdown of ingested food, (3) disruption of chemical bonds of food through the action of acids and enzymes, and (4) production of intrinsic factor required for vitamin B₁₂ absorption from the small intestine.

2. While monogastric animals have a single, simple stomach, ruminants have a complex stomach consisting of four chambers.

3. The true stomach is that area which produces hydrochloric acid. In ruminants, this is the abomasums, while in birds, it is the proventriculus.

4. In addition to the circular and longitudinal smooth muscle layers, the stomach has an addi­tional inner oblique or transverse layer.

5. The stomach is typically divided into four regions: the cardia, fundus, body, and pyloric region.

6. The stomach contains gastric glands, which contain mucous neck cells; parietal cells that secrete hydrochloric acid (HCl) and intrinsic factor; chief cells, which secrete pepsinogen; and enteroendocrine cells that produce a variety of hormones including gastrin, hista­mine, endorphins, serotonin, Cholecsytokinin, and somatostatin.

7. Protein digestion is initiated in the stomach via the action of pepsin.

8. Parietal cells increase secretion in response to ACh and gastrin.

Gastric motility

1. With the arrival of food, the stomach can stretch to accommodate this increased volume (reflexive relaxation.)

2.

Vomiting and egestion

1. The presence of irritants or toxins in the stomach can stimulate vomiting, or emesis.

2. Egestion is a process unique to birds in which nondigestible materials such as bone, fur, or feathers are orally eliminated from the digestive tract.

Regulation of gastric secretions and emptying

1. Gastric secretions are controlled by neural and hormonal mechanisms. Stimulation of the vagus nerve (i.e., parasympathetic nervous system) increases secretory activity of the stomach, while sympathetic stimulation inhibits stomach secretion.

2. Gastric secretions are controlled at three levels including the central nervous system, stomach, and small intestine, termed the cephalic phase, gastric phase, and intestinal phase, respectively.

3. During the intestinal phase, stimulation of che­moreceptors and stretch receptors triggers the enterogastric reflex, which inhibits gastrin pro­duction and gastric motility, and stimulates contraction of the pyloric sphincter, thus slow­ing gastric emptying into the duodenum. In addition, the enterogastrone reflex, a hormonal reflex, causes the release of CCK and GIP, which inhibit gastric secretions as well as gastric motility.

Anatomy of the stomach of ruminants

1. Ruminants have a specially modified stomach that consists of three, nonsecretory forestomachs and a secretory "true" stomach. The forestom­achs include the reticulum, rumen, and omasum, while the true stomach is the abomasum.

2. The lining of the reticulum has a "honeycomb" arrangement of ridges. The rumen and reticulum act as a functional unit, the reticulorumen.

3. The rumen, sometimes called the "pouch," occu­pies almost the entire left side of the abdominal cavity.

4. The omasum is kidney-shaped and is sometimes called the "book stomach" since its interior looks like pages of a book.

5. The abomasum consists of two glandular regions equivalent to the fundus and pyloric region of the monogastric stomach.

6. The ruminant stomach (1) allows animals to use feedstuffs too fibrous for monogastrics, (2) breaks down cellulose, (3) allows the use of nonprotein nitrogen sources (urea, uric acid) which are converted by the rumenal microbes to high-value organic nitrogen compounds, and (4) provides B complex vitamins due to the action of microbes as long as cobalt is present in the diet.

Motility of the ruminant stomach

1. The mixing, or A, sequence spreads across the reticulorumen in a "Z" pattern and provides extensive mixing of the rumen contents. The B, or eructation, sequence moves gas from the rumen toward the oral cavity, thus allowing the formation of a gas bubble which is eventually forcibly ejected into the esophagus by contrac­tion of the ventral rumen.

2. Feed is then returned to the oral cavity through a process called rumination.

Rumenal microbial fermentation

1. Fermentation involves the anaerobic action of bacteria and protozoa. Products of the bacteria and protozoa carbohydrate digestion include short-chain VFAs, carbon dioxide, and methane. The major VFAs are acetic, propionic, and butyric acids.

2. Rumen microorganisms hydrolyze dietary pro­teins to peptides and amino acids. In addition, these microorganisms can make amino acids from nonprotein nitrogen sources such as uric acid, urea, and ammonia.

3. Triglycerides are hydrolyzed by rumenal bacte­ria to glycerol and fatty acids, and glycerol is generally metabolized to propionic acid while the fatty acids pass to the duodenum where they are absorbed.

Anatomy of the stomach of birds

Birds have a two chambered stomach including the proventriculus (pars glandularis) and gizzard. The proventriculus is the glandular or true stomach.

Small intestine

The small intestine is the area where most digestion and 90% of absorption occur.

Anatomy of the small intestine

1. The small intestine is divided into three sections: (1) duodenum, (2) jejunum, and (3) ileum.

2. The pancreatic and bile ducts empty into the descending duodenum at the hepatopancreatic ampulla.

3. The longest part of the small intestine, the jejunum, is the site of the bulk of chemical diges­tion and absorption.

4. While in mammals the ileum is distinguishable from the jejunum, in birds, it is generally sepa­rated from the jejunum at the yolk stalk (diver­ticulum Vitellinum), formally called MeckeTs diverticulum.

Histology of the small intestine

1. The interior of the small intestine contains trans­verse folds called plicae, or plicae circulares.

2. The mucosa has fingerlike projections called intestinal villi. The villi are covered with simple columnar epitheliums that have microvilli that make up the brush border.

3. In the center of each villi is a lymphatic capillary called a lacteal, or central lacteal.

4. At the base of each villi are entrances to intestinal glands, or crypts of Lieberkuhn. Located at the base of each gland are stem cells that produce new epithelial cells. Also located within the crypts are Paneth cells, which secrete lysozyme and are part of the immune system.

Intestinal juices and brush-border enzymes

Intestinal juices are secreted from the mucosal lining of the small intestine. Also embedded in the micro­villi of the absorptive epithelial cells lining the small intestine are enzymes called brush-border enzymes.

Mechanical digestion and motility in the small intestine

1. Small intestine motility consists of two types of movements: segmentation and peristalsis. Segmental contractions are a nonpropagating type of movement resulting in churning and mixing of the luminal contents.

2. Peristaltic contractions propel chyme aborad along the length of the digestive tract. Peristalsis in the small intestine is controlled by the MMC.

Chemical digestion in the small intestine

1. Carbohydrate digestion. Starch and glycogen are acted upon by salivary and pancreatic amylases to form maltose, maltotriose, and «-dextrins. Brush-border enzymes involved in carbohydrate digestion include dextrinase and glucoamylase, maltase, sucrose, and lactase to produce glucoses, fructose, and galactose.

2. Protein digestion. Chemical digestion of protein begins in the stomach by the action of pepsin. Once in the small intestine, trypsin, carboxypep­tidase, and chymotrypsin secreted by the pan­creas break down proteins into peptides. The Brush-border enzymes aminopeptidase and dipeptidase further cleave the proteins.

3. Lipid digestion. Triglycerides and phospholipids are digested by lipases. Bile salts assist in emul­sifying dietary lipids in the small intestine lumen. Pancreatic lipase cleaves off two fatty acids from triglycerides producing two free fatty acids and monoglyceride.

4. Nucleic acid digestion. Pancreatic nucleases digest these molecules to their nucleotide mono­mers, which are then acted upon by brush­border nucleosidases and phosphatases that release free bases, pentose sugars, and phosphate ions.

Absorption in the small intestine

1. About 90% of absorption occurs within the small intestine with the rest occurring in the stomach and large intestine. Absorption occurs via diffu­sion, facilitated diffusion, osmosis, and active transport.

2. Absorption of monosaccharides. Fructose is absorbed by facilitated diffusion, and therefore can only move down its concentration gradient. Glucose and galactose are absorbed via second­ary active transport.

3. Absorption of amino acids, dipeptides and tri­peptides. Amino acids, di- and tripeptides, are actively absorbed in the small intestine. Some amino acids enter the epithelial cells by a second­ary active transport system similar to that described for glucose, while some amino acids

utilize a sodium-independent cotransporter in which the amino acids enter along with H⁺ instead of Na⁺.

4. Absorption of lipids. Since lipids are fat soluble, monoglycerides and free fatty acids can cross the epithelial membrane by simple diffusion. Short-chain fatty acids, those having fewer than 12 carbons, pass into the hepatic portal system similarly to amino acids and mono­saccharides. The remaining triglycerides and monoglycerides are resynthesized into triglycer­ides. These triglycerides are then coated with lipoproteins into droplets called chylomicrons. In mammals, these chylomicrons then enter the central lacteal.

Accessory organs

Pancreas

1. The pancreas has both endocrine and exocrine functions. Its exocrine function is to release enzymes involved in the digestion of all nutri­ents including carbohydrates, lipids, proteins, and nucleic acids.

2. Pancreatic juice contains mostly water, but it also contains salts, sodium bicarbonate, and enzymes. Pancreatic enzymes include pancreatic amylase, several protein-digesting enzymes including trypsin, chymotrypsin, carboxypeptidase, and elastase, pancreatic lipase, ribonuclease, and deoxyribonuclease.

3. Regulation of pancreatic secretions. During the cephalic and gastric phases of gastric secretion, parasympathetic signals carried via the vagus nerve increase secretion of pancreatic enzymes. Partially digested lipids and proteins within the duodenal lumen stimulate the secretion of CCK, which stimulates the secretion of pancreatic enzymes. Decreased pH in the duodenal lumen stimulates the release of secretin that stimulates release of bicarbonate ions from the pancreas.

Liver and gall bladder

1. Bile formed in the bile canaliculi moves into the bile ducts that fuse to form the common hepatic duct.

2. Each lobe of the liver has liver lobules, the func­tional unit of the liver. The cells of the liver, hepa­tocytes, are arranged in plates that radiate longitudinally outward from the central vein.

3. At each corner of the hexagonal lobule is a portal triad. Instead of capillaries, between the hepatic plates are cavities called sinusoids. The sinusoids allow even large plasma proteins to pass out of the bloodstream and into the spaces surrounding the hepatocytes.

4. Bile is secreted by hepatocytes and enters the bile canaliculi, which are narrow intercellular canals between the hepatocytes.

5. Bile composition and function. Bile consists of water, bile salts, bile acids, cholesterol, the phos­pholipid lecithin, bile pigments, and ions. The bile salts include sodium and potassium salts of bile acids, mostly cholic acids and chenodeoxy- cholic acid.

6. Functions of the liver. When blood glucose levels are high, the liver converts glucose to glycogen (glycogenesis), and when blood glucose levels drop, the liver can breakdown glycogen to glucose (glycogenolysis). Hepatocytes can store triglycerides as well as use fatty acids to synthe­size ATP, and synthesize lipoproteins. Choles­terol can be synthesized in the liver. Hepatocytes remove deamination amino acids so they can be used for ATP synthesis. Hepatocytes also synthe­size carbohydrates and fats from certain amino acids. Hepatocytes can synthesize various plasma proteins. The liver is an important site of detoxification. The liver detoxifies and can alter and excrete steroid hormones. The liver is the primary storage site of fat-soluble vitamins, as well as vitamin B₁₂. Glycogen and certain miner­als are also stored in the liver. Kupffer cells destroy aged blood cells and microbes that may have entered via the hepatic portal blood. The liver combines with the skin and kidneys to syn­thesize the active form of vitamin D.

Large intestine

The primary functions of the large intestine are elec­trolyte and water absorption, microbial digestion and vitamin production, formation of feces, and expulsion of feces.

Anatomy of large intestine

1. The large intestine is divided into the cecum, colon, rectum, and anal canal.

2. The cecum is a blind diverticulum that extends off of the colon at the ileocecal valve. In those animals that are hindgut fermenters (i.e., horse), the cecum is a major site of digestion.

3. The colon consists of three segments called the ascending, transverse, and descending colon.

Mechanical digestion in the large intestine

1. The ileocecal valve controls the movement of chyme into the large intestine.

2. Haustra churning is characterized by the slow filling of a haustrum until it is distended, at which time its walls contract, squeezing the con­tents into the next haustrum.

3. Peristalsis also occurs along the large intestine. Finally, mass peristalis occurs when a strong peristaltic wave begins near the middle of the transverse colon and quickly forces the contents to the rectum.

Chemical digestion in the large intestine

1. While no chemical digestion occurs in the large intestine, considerable fermentation takes place, especially in those animals that are hindgut fermenters.

Gut microflora

1. Gut microflora is more complex than that of its host, and it is now known that there are about 150 times more microbial genes than human genes in the human body.

2. The gut microflora may regulate an animal's genes and immune system. It is now believed that commensal microbiota may "program" aspects of T-cell differentiation, thereby influenc­ing the host genome and the function of the adaptive immune system.

Review questions and answers are available ¾ online.

References

Budras, K.-D., W.O. Sack, and S. Rock. 2003. Anatomy of the Horse: An Illustrated Text. Schliitersche GmbH & Co., Hannover, Germany.

Constantinescu, G.M. 2001. Guide to Regional Ruminant Anatomy Based on the Dissection of the Goat. Iowa State Press, Ames, Iowa.

Constantinescu, G.M. 2002. Clinical Anatomy for Small Animal Practitioners. Iowa State Press, Ames, Iowa.

Constantinescu, G.M. and LA. Constantinescu. 2004. Clini­cal Dissection Guide for Large Animals. Iowa State Press, Ames, Iowa.

Gheorghe, M., G.M. Constantinescu, and LA. Constanti- nescu. 2004. Clinical Dissection Guide for Large Animals: Horse and Large Ruminants, 2nd edition. Iowa State Press, Ames, Iowa.

Lee, Y.K. and S.K. Mazmanian. 2010. Has the microbiota played a critical role in the evolution of the adaptive immune system? Science 330: 1768-1773.

Martini, F.H. 2004. Frmdamentals of Anatomy and Physiol­ogy, 6th edition. Benjamin Cummings, San Francisco, California.

Pasquini, C., T. Spurgeon, and S. Pasquini. 1995. Anatomy of Domestic Animals, 8th edition. SUDZ Publishing, Pilot Point, Texas.