A&P II C11 The Digestive System

Digestive Enzymes

-Amylase - enzyme in saliva of omnivores Breaks down amylose (sugar component of starch) -Lipase - enzyme that digests lipids May be found in the saliva of some young animals while they are nursing or on a high-milk diet -Sodium bicarbonate and phosphate buffers found in saliva of cattle Neutralize acids normally formed in the rumen -Autonomic nervous system controls most of the glands in the digestive system -Parasympathetic stimulation increases salivation. Anticipation of eating can cause parasympathetic stimulation of the salivary glands. -Sympathetic nervous system stimulation decreases salivation. Fear or parasympathetic nervous system inhibitors like atropine produce dry mouth.

Oral Cavity Functions

-Prehend the food -Initiate mastication (mechanical digestion) Breaks food into smaller particles that increase the surface area available for exposure to the enzymes involved in chemical digestion -Initiate chemical digestion Saliva - added to food as it is chewed; moistens, softens, and shapes food into a form that is more readily swallowed -Prepare food for swallowing

Monogastric Stomach

A monogastric (or simple) stomach is essentially a large dilated tube with the following regions listed in anatomical order from the esophagus to the small intestine: 1. Esophageal region 2. Cardia 3. Fundus 4. Body 5. Antrum 6. Pylorus

How is the young calf's GI tract different from the adult's? What is the role of the reticular groove?

A newborn ruminant's digestive tract functions primarily as a monogastric digestive system. The rumen and reticulum at birth are small (compared with the omasum and abomasum) and are essentially nonfunctional. Little or no fermentative digestion occurs while the animal is primarily receiving a milk diet (nursing). The abomasum (the true stomach) is the largest of the four stomach compartments for the first few weeks of life. The reticular groove, also called the esophageal groove, is a trough in the wall of the reticulum that extends from the esophageal opening to the opening of the omasum. When the calf nurses or suckles, the muscles associated with the groove contract and form a tubular structure that conveys swallowed liquid from the esophageal opening directly to the omasum, essentially bypassing the rumen and reticulum. The suckling act seems to be necessary for this groove to function. With age and development of the rumen and reticulum, this groove formation reflex almost completely disappears.

What role does urea play in the rumen function? Which organ converts ammonia to urea?

Additional nitrogen for rumen microbes comes from the ruminant's production of urea by the liver. Urea makes its way to the rumen via the bloodstream and saliva. Urea is an end product of the liver's activity to convert possibly poisonous amounts of ammonia (NH3) into substances that can be used by the ruminant body. Note that urea is sometimes added to feed as a relatively cheap method of increasing the nitrogen value of poor quality feed.

What are amylase and lipase? Where in the mouth do they come from? What do they do?

Amylase is an enzyme found in the saliva of omnivore species such as rats and pigs, but it is absent in carnivores like dogs and cats. As the name implies, salivary amylase breaks down amylose, a sugar component of starch. Lipase, an enzyme that digests lipids (fats), also may be found in the saliva of some young animals (calves) while they are nursing or on a high-milk diet.

What do CCK and prostaglandins do to the small intestine?

CCK is thought to stimulate intestinal motility. The CCK is secreted by cells in the intestinal mucosa when fats and proteins are present in the lumen of the intestine. Prostaglandins are thought to increase GI motility and secretions.

Large Intestine

Components Cecum - blind sac at ileocecal junction Colon - some microbial digestion Rectum Species variation in structure Primary functions - Recover fluid and electrolytes Store feces until they can be eliminated Carnivores: simple, tubular colon; poorly developed cecum Nonruminant herbivores: very large colon and cecum (hindgut) Fermentation site

Why do some animals get diarrhea when their diet is changed suddenly?

Diarrhea is caused by the changing need for appropriate numbers and types of digestive enzymes to match the changing demands of the diet. Switching from canned food to soft, moist food or dry food, or switching from one brand that uses a high corn protein to another form of protein or carbohydrate, often results in a greater amount of food being incompletely digested. These undigested molecules osmotically retain water within (and may even draw water into) the lumen of the intestine, producing diarrhea.

What are the three segments of the small intestine? Which is usually the longest?

Duodenum, jejunum, and ileum. The ileum is the longest segment.

What are the three segments of the small intestine? Which is usually the longest?

Duodenum, jejunum, ileum. The ileum is the longest segment.

Explain how each of the following plays a role in digestion of fats: emulsification, bile acids, liver, pancreatic lipase, triglycerides, glycerol, fatty acids, monoglycerides, and micelles

Emulsification breaks down fat globules (which have small surface areas for enzymes to work on) to smaller fat pieces so that fat can be adequately digested. Bile acids (secreted into the duodenum from the liver) combine with the fat droplets to keep them from forming back into globules. Bile acids have a hydrophilic (water-loving) end and a hydrophobic (water-fearing or fat-loving) end. The bile acid sticks the hydrophobic end into the fat droplet, leaving the hydrophilic end exposed to the environment of the intestinal tract. In doing so, it makes the resulting fat droplet water soluble. The liver secretes bile acids. Pancreatic lipase (fat-digesting enzymes) penetrate the bile acid coating and digest the fat (triglyceride) molecules to produce glycerol, fatty acids, and monoglycerides. This results in the droplet fragmenting into even smaller pieces called micelles. These bile acid/lipid component micelles allow the lipid components to diffuse readily through the water contents of the intestine and come in contact with the brush border of the intestinal wall, where they are absorbed.

What is the relationship among cellulose, pectin, cellulase, glucose, and VFAs?

Enzymes produced by the digestive glands in monogastric animals cannot digest the cellulose and pectin that make up the cell wall of plant cells. However, rumen bacterial surfaces have cellulase enzymes that can digest cellulose effectively and transform the complex carbohydrate structure of cellulose into much simpler monosaccharides (simple, one-molecule sugars like glucose) and less complex polysaccharides (sugars made of more than one sugar molecule bound together). The glucose sugar produced by this process is not immediately available to the host animal. Instead, the glucose liberated from the plant materials (and other carbohydrate sources, like starch) are absorbed into the microbes and converted biochemically to volatile fatty acids (VFAs).

What is fermentative digestion? How is it different from nonfermentative digestion?

Fermentative digestion is the way ruminants derive energy from food. Rechewed plant material is degraded by bacterial and protozoal enzymes to basic cellular building blocks. In nonfermentative digestion, which occurs in monogastric animals, the enzymes are produced by glands in and along the intestinal tract.

Explain the effect that each of the following has on gastric motility: gastrin, increased acidity in the duodenum, secretin, and cholecystokinin (CCK).

Gastrin, which is produced by the G cells in the antrum of the stomach, not only increases the production of hydrochloric acid in the stomach but also inhibits the muscle activity of the fundus, resulting in relaxation and greater filling of the stomach. When food is first swallowed and begins to distend the body and antrum of the stomach, gastrin is released and signals the fundus to relax so the food can be accommodated. Increased acidity in the duodenum inhibits stomach contraction, resulting in delayed gastric emptying of contents. Secretin, like gastrin, can cause the fundus to relax, but it can also inhibit peristalsis of the body and antrum of the stomach to slow gastric emptying. Cholecystokinin (CCK) decreases contraction of the antrum, body, and (like gastrin) the fundus.

What is the gingiva?

Gingiva is the epithelial tissue that composes the gums around the teeth.

What is hardware disease? With which forestomach is hardware disease usually associated?

Hardware disease occurs when a wire or sharp metallic object that has been swallowed penetrates the cranial wall of the reticulum. This is possible because of the reticulum's location just ventral to the entrance to the esophagus. Because the reticulum is separated from the heart by the diaphragm and only a relatively short distance away, an object piercing the reticulum can also penetrate the diaphragm and pericardium (outer membranous sac surrounding the heart), causing pericarditis (inflammation of the pericardium).

How do diets differ between an herbivore, a carnivore, and an omnivore? To which group would cats, horses, cows, and humans belong?

Herbivores are plant-eating animals, such as horses and cows. Carnivores are meat-eating animals, such as cats. Omnivores eat plant material and meat. Humans are omnivores.

What is ileus? What causes it? Why would antiparasympathetic drugs like atropine cause it?

Ileus is a condition of decreased movement of ingesta (decreased peristalsis) that can be caused by stress from the sympathetic nervous system. Increased stimulation by the parasympathetic nervous system can increase the degree of reflex response that causes peristalsis. Therefore drugs with antiparasympathetic activity inhibit the stretch response and decrease peristalsis.

What are the four types of teeth, rostral to caudal.

Incisors- Grasping teeth Most rostral teeth of upper and lower arcade Canine Tearing teeth Located at the corners of the incisors Longer than other teeth Pointed at the tip Pre molars Cutting teeth Rostral cheek teeth Sharp points and surfaces in carnivores Molars Grinding teeth Caudal cheek teeth Larger, flatter occlusal surfaces Used for grinding Ruminants have no upper incisors or upper canine teeth Dental pad - flat thick connective-tissue structure on the maxilla opposite the lower incisors and canine teeth

Digestive Tract stucture

Mucosa - lining of GI tract; epithelium and loose connective tissue Submucosa - dense connective tissue; may contain glands Muscle layer - outside the submucosa Serosa - outermost layer; thin, tough connective tissue. Mesentery - Sheets of connective tissue Suspend digestive tube in the abdomen from the dorsal body wall Contains blood and lymph vessels and nerves that supply the GI tract

What effect do prostaglandins have on mucus production, gastric blood flow, and the ability of the stomach to heal itself?

PGs stimulate the cells in the gastric glands to produce the bicarbonate ion, which helps make the mucous layer capable of neutralizing the stomach acid to some degree. PGs also enhance blood flow to the stomach, stabilize potentially destructive lysosomes within the gastric cells, and regulate the activity of macrophages and mast cells. All of these functions help the stomach to rapidly repair any damage to the stomach epithelial lining caused by a break in the mucous barrier.

Describe what each of these cells produces: parietal cells, chief cells, mucous cells, and G cells. What do their products do?

Parietal cells produce hydrochloric acid, which helps convert pepsinogen to pepsin. Chief cells produce an enzyme precursor called pepsinogen, which is a precursor for the enzyme pepsin. Under the influence of hydrochloric acid, pepsinogen is converted into the active proteolytic (protein-digesting) enzyme pepsin in the lumen of the stomach. The proteins broken down by pepsin form chains of amino acids (polypeptides and peptides). Mucous cells produce the protective mucus, which is actually a complex of many substances. Mucus provides a gelatinous protective coating for the stomach. G cells secrete the hormone gastrin. The presence of food stimulates the G cells to dump gastrin into the blood. It travels, among other places, to the gastric glands in the proximal part of the stomach, where it stimulates the release of hydrochloric acid from the parietal cells. Gastrin also inhibits the muscle activity of the fundus, resulting in relaxation and greater filling of the stomach.

What is the relationship between pepsinogen and pepsin? What does pepsin do?

Pepsinogen is the precursor of the enzyme, pepsin. Pepsin breaks down proteins into chains of amino acids (polypeptides and peptides).

What is peristalsis? How does it differ from segmentation?

Peristalsis is coordinated contraction and relaxation of muscle layers resulting in movement of a bolus of food or ingesta along the GI tract. Segmentation mixes the GI tract contents and slows their movement through the tract.

What are polysaccharides, disaccharides, and monosaccharides? Give examples of each. What enzymes break down each of these?

Polysaccharides (meaning many sugars) are complex carbohydrates such as starch, glycogen, and various sugars that are normally present in domestic animal foods. Amylase (an enzyme that is produced in the saliva in some but not all species and secreted by the pancreas into the lumen of the duodenum) converts starch into smaller sugar segments called disaccharides (meaning two sugars). These disaccharides, which include sucrose, maltose, isomaltose (also called dextrin), and lactose, are further digested into monosaccharides (meaning one sugar) by the enzymes sucrase, maltase, isomaltase, and lactase, which are found in the cell membranes of the microvilli. The resulting monosaccharides are glucose, galactose, and fructose.

Digestive Tract functions?

Prehension (grasping) of food with the lips or teeth Mastication - mechanical grinding and breaking down of food (chewing) Chemical digestion of food Absorption of nutrients and water Elimination of wastes

What is the relationship among proteases, peptides, and amino acids?

Proteases (enzymes that break down proteins) reduce the long proteins to short peptides (short chains of amino acids) and the amino acid building blocks of the peptides and proteins

What are proteases? What protease is produced by the stomach? What effect do proteases have on polypeptides?

Proteases are protein-splitting enzymes. The stomach produces pepsin. Proteases cleave polypeptides at various sites to form smaller protein molecules such as tripeptides and dipeptides.

Species Variations

Requirements for digestion and absorption of foodstuffs vary depending on the diet of the animal. Herbivores - plant-eating animals (cattle, sheep, goats) Carnivores - meat-eating animals (cats) Omnivores - animals that eat plant material and meat Monogastric animals - simple, single stomachs Ruminants - multiple mixing and fermentation compartments in addition to the stomach

What is rumination vs. eructation? What purposes do they serve?

Rumination is the regurgitation of partially digested plant food (the cud) up the esophagus, chewing it, and reswallowing it. This facilitates the mechanical breakdown of tough plant material through multiple chewing cycles, resulting in a greater surface area on which the rumen microbes and digestive enzymes can act. Eructation is the act of expelling built-up carbon dioxide or methane gas from the rumen, a function of reticuloruminal contractions. Eructation is essential for dispelling excessive gas created by the fermentation process, thereby reducing the risk of too much gas being trapped in the rumen (a condition called bloat ).

What is the role of segmental contractions in the small intestine? Why do animals get diarrhea if segmental contractions are decreased?

Segmental contractions mix the intestinal contents and slow their movement through the length of the intestines. These contractions churn the material in the intestine, ensuring that digestive enzymes secreted into the lumen of the intestine are mixed well with the contents. The churning motion of the segmental contractions also brings digested materials into contact with the surface of the intestinal tract for absorption. Peristaltic movements push ingesta forward, and segmental contractions slow the movement of ingesta; therefore absorption of contents and consistency of the stool depend partly on the balance between peristalsis and segmental contractions. Many cases of diarrhea in dogs and cats are caused by a lack of segmental contractions or overactive peristalsis. The inactive intestinal tract resembles a garden hose through which contents can rapidly slide. The result is a high-volume, small bowel diarrhea.

Digestive Tract Musculature

Skeletal muscle - voluntary control Mouth, pharynx, the cranial part of the esophagus, and the external anal sphincter Allows the processes of chewing, mixing saliva with food, and initiation of swallowing Smooth muscle Wall of the majority of the esophagus, the stomach, the small intestine, the large intestine, and the internal anal sphincter Primarily arranged in circular and longitudinal layers Contraction of circular muscle fibers narrows the tract Contraction of longitudinal fibers shortens the tract

What effect does the parasympathetic nervous system have on the mouth? The sympathetic nervous system?

Stimulation of the parasympathetic part of the autonomic system results in increased salivation. The anticipation of eating can cause parasympathetic stimulation of the salivary glands, resulting in copious saliva production (drooling). Sympathetic nervous system stimulation associated with fear or inhibition of the parasympathetic nervous system by preanesthetic drugs like atropine produce dry mouth because of the decrease in saliva production.

Digestive Tract Epithelium

Stratified squamous epithelium Thick and tough Lines the mouth, pharynx, esophagus, and anus Simple columnar epithelium Runs from junction of esophagus and stomach through the intestines to junction of rectum and anus Nutrients are absorbed through this thin epithelium

Where are the apex, the root, the pulp , the cementum, the dentine and the enamel of a tooth located

The apex is the end of the root, where the blood supply and nerve supply enters. The root of the tooth is not seen and is embedded in the gingiva. The pulp consist of the blood and nerve supply to the tooth and is in the center of the tooth. The dentin surrounds the tooth pulp, is a denser than bone, and helps protect the sensitive pulp. The crown of the tooth is covered with a hard enamel layer, which is the hardest toughest tissue in the body.

What are the cardia, fundus, pyloric antrum, and pylorus of the stomach? What are each of their functions?

The cardia is the area immediately surrounding the opening from the esophagus into the stomach. It helps reduce reflux of stomach contents back up into the esophagus. The orientation of the esophagus as it enters the stomach also provides a natural closure for the cardia as the stomach fills and distends. The fundus is the section of the stomach that forms a distensible blind pouch that expands as more food is swallowed. The body of the stomach is also a distensible section in the middle of the stomach. The fundus and body of the stomach are rich with glands. Typically, the gastric (stomach) glands in this region of the stomach contain (1) parietal cells, which produce the hydrochloric acid; (2) chief cells, which produce an enzyme precursor called pepsinogen, and (3) mucous cells, which produce the protective mucus. The pyloric antrum is the distal part of the stomach that grinds up swallowed food and regulates the hydrochloric acid produced by the fundic and body parietal cells. The glands of the pyloric antrum contain endocrine cells called G cells, which secrete the hormone gastrin. The presence of food stimulates the G cells to dump gastrin into the blood. It travels, among other places, to the gastric glands in the proximal part of the stomach, where it stimulates the release of hydrochloric acid from the parietal cells. Like the gastric glands in the fundus and body, the gastric glands in the pyloric antrum also contain many mucous cells. The pylorus is the muscular sphincter (ring of muscle in a tubular organ) that regulates the movement of chyme (digested stomach contents) from the stomach into the duodenum (first part of the small intestine) and prevents backflow of duodenal contents into the stomach.

What are the four compartments of the ruminant stomach? Which ones are forestomachs and which one is the true stomach?

The four compartments of the ruminant stomach are: one true stomach (the abomasum) and three forestomachs (the reticulum, rumen, and omasum). The abomasum is the true stomach.

What is meant by the lingual surface, palatal surface, labial surface, and buccal surface of teeth?

The inner surface of the lower arcade of teeth that faces toward the tongue is referred to as the lingual surface of the teeth, and the inner facing surface of the upper arcade is referred to as the palatal surface (meaning that it faces the hard palate at the roof of the mouth). The outer-facing surface of the upper and lower arcade at the front (rostral) of the mouth is called the labial surface (labial = pertaining to the lips), and the outer-facing surface of the teeth more caudal in the mouth is called the buccal surface (buccal = pertaining to the cheeks).

What is the difference between the terms mucus, mucous, and mucin? What role does bicarbonate play in the mucous layer?

The mucus produced by the gastric glands is actually a complex of many substances. Mucus provides a gelatinous protective coating for the stomach. Mucins are complex molecules produced by the goblet cells in the gastric glands and are the main constituent of the mucous coating (note that "mucous" is an adjective, "mucus" is a noun). In addition to the mucin, bicarbonate ion is also secreted onto the surface, making the mucous coat more alkaline. By alkalinizing the mucus, the hydrochloric acid contacting it is neutralized to some degree. The mucous coating is essential to protecting the stomach cells from the harsh, acidic environment of the stomach (pH of 2 to 3). A pH of 2 normally would remove paint from many surfaces and cause etching of metal. Mucus is not digested by pepsin, but it is fragmented by the hydrochloric acid in the stomach. This means that for the stomach to be continually protected, all the components of the mucus must be secreted continuously. Failure of the body to do so sets the stomach up for gastritis and gastric ulcers.

Explain how NSAIDs produce side effects in the GI tract.

The net result of the decreased mucous layer and increased acidity caused by NSAIDs can be gastritis (stomach inflammation), gastric erosions (epithelial layer of stomach is eroded away by the acid), or gastric or duodenal ulcers. NSAIDs and glucocorticoids (antiinflammatory steroids including cortisone and prednisone) affect the mucous protective layer and the ability of the stomach to heal itself, potentially causing ulcers that can perforate the stomach and result in a fatal condition called septic peritonitis.

What are the three receptors on the parietal cells that stimulate hydrochloric acid production?

The parietal cell has three receptors on the blood side of the cell (as opposed to the stomach lumen side of the cell) that regulate acid production. These receptors are for gastrin, acetylcholine (the neurotransmitter of the parasympathetic nervous system), and histamine.

How does motility differ in the fundus vs. the body or pyloric antrum?

The proximal part of the stomach (fundus and body) tends to relax with swallowing of food, allowing the stomach to distend and fill with food (act as a reservoir). The body of the stomach also contracts to help mix food within the stomach. In contrast to the fundus, the distal part of the stomach (the pyloric antrum) increases contractions with swallowing of food. Because the pyloric antrum is responsible for most of the grinding activity of the stomach, swallowing of food stimulates vigorous mixing, grinding, and propulsive contractions that move food toward the pylorus.

What are the lower arcade and upper arcade in reference to teeth? What bones in the skull are each of these arcades associated with?

The upper arcade refers to the teeth in the upper part of the mouth (in the maxilla and incisive bones). The lower arcade refers to the teeth in the lower part of the mouth (in the mandible).

What does the ileocecal sphincter do?

This sphincter is an anatomical and functional muscle that regulates movement of materials from the small intestine into the colon or the cecum.

The digestive system

Tube that runs from the mouth to the anus; accessory digestive organs Mouth Salivary Glands Pharynx Esophagus Liver Stomach Duodenum Pancreas Colon Cecum Jejunum Ileum Rectum Anus

What are villi, microvilli, brush border, and crypts? How do they aid digestion and absorption of food?

Villi are millions of tiny cylindrical fingerlike projections of the mucosa in the small intestine that help provide a tremendously large surface area for absorbing nutrients. Each villus contains thousands of very small villi of its own called microvilli. The microvilli are so plentiful that they microscopically resemble short bristles on a brush; hence the layer of microvilli is often called the brush border. The cells of this brush border have many digestive enzymes and carrier molecules embedded in their cell membranes for digestion and absorption of nutrients, minerals (for example, sodium), and vitamins. Crypts are invaginations in the intestinal mucosa surrounding each villus. The crypts constantly produce cells that are pushed from the bottom of the crypt up the villus to replace the oldest cells that are constantly shed at the tip of the villus.

How does the stomach know when to stop producing acid?

When the pH of the stomach contents in the antrum drops below 3, gastrin release is inhibited. With the inhibition of gastrin release, one of the three key stimulants for hydrogen and chloride production is terminated, and hydrochloric acid production declines.

What are the four compartments of the ruminant stomach? which are forestomachs

the abomasum - one true stomach three forestomachs - reticulum, rumen, and omasum