The digestive tract

. Describe the epithelium of a villus and indicate the cell types present.

A single layer of columnar epithelium covers the villi and is composed mainly of enterocytes and goblet cells.

. Explain ways in which the structure of the large intestine is modified for a diet of grass in non-ruminant mammals.

In herbivores which have a simple stomach, the cecum and/or colon is enlarged. Cellulose-digesting bacteria are housed in the cecum/colon. Most digestion and absorption of nutrients occurs within the large intestine.

Name the basic layers of tissue seen in a cross section of the digestive tract, and describe the general structure and function of each layer.

Inner tissue layers:mucosa- lubricant Middle tissue layers: the submucosa and two layers of smooth muscle - supports mucosa Outer tissue layer: serosa- lubricates to prevent friction of muscle movement

Explain how luminal digestion differs from membrane digestion, and indicate where each occurs within the mammalian digestive tract.

Luminal digestion begins in the duodenum. As the liquified food (chyme) leaves the stomach, it enters the duodenum and is mixed with secretions from the liver and pancreas. This luminal digestion continues as the chyme passes into the jejunum and is usually completed before the ileum is reached. Membrane digestion occurs at the surface of the intestinal epithelium. The digestive enzymes are located within the membranes of the microvilli. Some of these enzymes digest peptides to amino acids. Other enzymes digest starch and complex sugars to simple sugars. Thus membrane digestion completes the digestion of proteins and carbohydrates, releasing the amino acids and sugars at the site where they will be absorbed.

Indicate the two main functions of the stomach in most vertebrates.

Mechanical breakdown and liquefaction of food by acid. Initiation of protein digestion by enzymes (pepsin which is active at pH 4).

. List and briefly describe all structures responsible for increasing the absorptive surface of the small intestine.

Plicae circularis are folds found in some species. Villi are finger-like projections found in all species. Microvilli are microscopic extensions of the villus epithelial cells.

Distinguish between a simple and a multi-chambered stomach and give examples of mammals in which each type is found.

Simple stomachs are designed for diets of meat and/or soft plant products, whereas multi-chambered stomachs are able to temporarily store and digest diets containing course plant material. Thus carnivores and most omnivores have simple stomachs, whereas most herbivores have multi-chambered stomachs of varying complexity.

Indicate how the structure of the large intestine of birds differs from that of mammals.

The bird digestive tract has a pair of cecal sacs which are part of the large intestine. The cecal sacs are large in herbivorous birds and contain bacteria that assist in digestion of plant material; they are very small in carivores such as the hawk.

Indicate the location and specific function of the 3 main regions of the human stomach.

The cardiac region secretes mucus to facilitate passage of the food bolus into the stomach. The fundic region and body of the stomach both secrete acid and pepsin to liquify the bolus and begin digestion of protein. They also secrete mucus which protects the stomach lining from abrasion and from the low pH generated by acid secretion. The pyloric region contains mainly mucous glands, but also secretes large amounts of gastrin. This hormone acts on the gastric glands of the stomach to stimulate acid secretion.

Name the regions of the small and large intestine in mammals and indicate the location of each region

The duodenum is the portion which emerges from the stomach. The jejunum is the middle segment of the small intestine and the ileum is the distal region which enters the large intestine.

Compare the inner surface of the mammalian small intestine vs. colon.

The inner surface of the colon is flat and lacks villi.

Compare (in general terms) the anatomy of the digestive tract in herbivorous vs. carnivorous mammals.

The simple stomach is found in carnivores, some omnivores and a few herbivores. Multi-chambered stomachs are found in most herbivores, such as ruminants. Small and large intestine are larger for more absorption of nutrients from their food

Indicate how the esophagus and stomach of birds differs from that of other vertebrates.

The stomach of birds consists of two chambers. The proventriculus secretes enzymes. The gizzard is the muscular part of the stomach were mechanical digestion occurs. The bird esophagus is unique in that it has a crop for food storage. It is a expansion of the esophagus and functions as a storage site for food

. Name each part of the mammalian digestive tract starting in the mouth, and indicate the digestive and/or absorptive functions which occur in each region.

The tubular GI tract consists of the esophagus(carry food, liquids, and saliva from the mouth to the stomach), stomach, The stomach has two main digestive functions. Mechanical breakdown and liquefaction of food by acid. Initiation of protein digestion by enzymes (pepsin which is active at pH 4)., small intestine (most digestion and absorption of nutrients occurs in the small intestine), cecum, colon (The colon absorbs water and packages the feces for elimination. Bacteria are abundant; they synthesize vitamins which are absorbed by the cecum and/or colon.) and rectum (stores feces and absorbs fluids).

Describe the stomach of ruminant mammals, and explain how its structure is related to the digestion of grass.

There are 4 chambers, one of which (the rumen) houses bacteria which are capable of digesting cellulose, the main component of grass and leaves. A second chamber (the reticulum) forms the partially digested grass into a cud which can be regurgitated. Chewing the cud assists in the liquification of the grass which is then returned to the rumen for further digestion.

Describe the major phylogenetic trends regarding the digestive tract as mammals evolved from fish.

increased regional specialization. increased in epithelial surface for better absorption

. Describe the basic structure of an intestinal villus and indicate how its size and shape changes from the duodenum to the ileum to the colon in most mammals.

look up

. Describe the location and function of the following structures of the small intestine: crypts bunner's glands goblet cells microvilli

• Crypts Crypts lie at the base of villi; renewal of the epithelium occurs here. • Brunner's glands The submucosa of the duodenum contains large serous glands called Brunner's glands. These glands produce alkaline secretions (mainly bicarbonate) which neutralize the acid contents of chyme as it emerges from the stomach. • goblet cells Goblet cells are located among the enterocytes covering the villi. The goblet cells secrete mucus to lubricate the digestive tract and to protect the epithelium from digestive enzymes within the lumen. • Microvilli Microvilli are microscopic extensions of the villus epithelial cells. They form the brush boarder. Increase surface area and are involved in secretion and absorption.