I. ANATOMY AND PHYSIOLOGY

The human digestivesystem is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste. Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food asit makes itsway through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.THE DIGESTIVE PROCESS

The start of the process - the mouth:The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smallermolecules).On the way tothe stomach: the esophagusAfter being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to thestomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from thethroat into the stomach. This muscle movement gives us theability to eat ordrink even when we're upside-down.In the stomachThe stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed withstomach acids is called chyme.In the small intestineAfter being in the stomach, food enters theduodenum, the firstpart of the smallintestine. It then enters thejejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder),pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.In the large intestineAfter passing through the small intestine, food passes intothe large intestine. In thelarge intestine, some of thewaterand electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillusacidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine iscalled the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon.The end of the processSolid waste is then stored in the rectum until it is excreted via the anus.Layers of the Stomach Wall

The epithelial cells that line the stomach form themucosal epitheliumlayer.When the stomach is empty the mucosa lies in large folds calledrugaeand which look like wrinkles. The rugae flatten as the stomach fills. In order to identify the different types of cells that line the stomach and the functions of each of these types of cells it is necessary to describe the inner-layers of the stomach (i.e. the layers of the mucosa).The surface of the mucosa is a layer ofnonciliated simple columnar epithelial cells calledsurface mucous cells. There are also manycolumnsof secretory cells calledgastric glandsthat line narrow channels calledgastric pits. That is, epithelial cells not only line the insidesurfaceof the stomach, but also formmany narrow passagescalledgastric pitsthat lead from the lumen of the stomach outwards towards the inner stomach wall, orsubmucosa.

The Mucosa(of the Stomach)

The cells identified in the diagram above are: Surface Mucous Cells(or "Mucous Surface Cells") - which secrete mucus. Neck Mucous Cells(or "Mucous Neck Cells") - which secrete mucus whose acidity is more neutral than that secreted by the cells at the surface of the stomach lining. Chief Cells Parietal Cells, and G CellsGastric Glands lining the Gastric Pits:There are three special types of exocrine glands (neck mucous cells,chief cellsandparietal cells) that secrete chemicals into the stomach. There is also one type of hormone-producing cell in the lining of the gastric pits, which is called aG cell.Secretions from cells located in the StomachType of CellSecretionNotes

Surface Mucous Cells(or "Mucous Surface Cells")- secrete mucus

Neck Mucous Cells(or "Mucous Neck Cells")- secrete mucuswhose acidity is more neutral than that secreted by the cells at the surface of the stomach lining.Themucussecreted by themuscous neck cellshas a more neutral pH than that secreted by the cells at the surface of the stomach lining.

Chief Cells- secretepepsinogenPepsinogenis an inactive gastricenzymewhich is converted topepsin, aprotein-digesting enzyme.

Parietal Cells- secretehydrochloric acid, and anintrinsic factor(involved in the absorption of vitamin B12).Hydrochloric acidassists in the conversion of pepsinogen to pepsin (mentioned above).Insufficientintrinsic factorcan lead to pernicious anemia because vitamin B12is necessary for the production of red blood cells (callederythrocytes).

Gastric Juice:The secretions of the surface mucous cells, neck mucous cells, chief cells, and parietal cells are known collectively asgastric juice.(Hencegastric juiceincludesmucous,pepsinogen,hydrochloric acidandintrinsic factor.)

G Cells- produce and secrete the hormonegastrin.Gastrin: stimulates secretion of gastric juice, increases the contractions of the gastro-intestinal (GI) tract, and relaxes the pyloric sphincter.