lab 9 (PART 1) Digestive Function

digestive system

The digestive system processes food so that it can be absorbed and utilized by the body’s cells. The digestive organs are responsible for food indigestion, digestion, absorption, and elimination. These functions occur along a tubular tract approximately 9m long (mouth to anus) that is open to the exterior at each end.

Groups of the digestive system

The digestive system can be broken down into 2 main groups of organs the Gastrointestinal (GI) tract (aka alimentary canal organs) and the accessory digestive organs and structures.

1. The GI tract organs: directly associated with the digestion of food. These organs extend from the mouth to the anus and include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine

2. Accessory organs and structures: assist in digestion either by mechanically or chemically manipulating ingested food. Includes lips, cheeks, palate, tongue, teeth/gums, salivary glands, gallbladder, liver, and pancreas

Mouth function

ingestion of food

Mechanical digestion by chewing

Initiates propulsion by swallowing

Pharnyx and esophagus function

propulsion of food during swallowing

Stomach function

Holding area for ingested food

Further mechanical breakdown of food

Chemical digestion (mainly protein digestion)

Secretion of intrinsic factor, required for intestinal absorption of vitamin B12

Some absorption (alcohol, aspirin and some other drugs)

Prepares food for entry into small intestine

Small intestine function

Main site of absorption of food

Chemical digestion completed

Mechanical digestion and propulsion

Large intestine function

Chemical digestion: some food residues digested by enteric bacteria.

Absorption of water, electrolytes, and bacteria

Propulsion of feces toward rectum

Storage of feces until defecation

Defecation: the elimination of feces from the body

Human Food Processor

The food ingested must undergo a number of changes before it can be absorbed by the digestive tract and used by the cells in our body. Substances like water, vitamins, and minerals can be readily absorbed, whereas more complex molecules like carbohydrates, fats and proteins must be physically and chemically broken down first by enzymes. The processing of foodstuffs occurs throughout most of the length of our digestive system. A regional survey of enzymes and substances involved in the digestive process is given on the next page.

Mouth digestive enzymes

Contains a variety of salivary glands that produce saliva. One of the digestive enzymes found in saliva is amylase, which starts the process of breaking down starchy foods. Lingual lipase, a fat-digesting enzyme, is secreted by serous cells at the back of the tongue and becomes active in the stomach.

Pharynx and Esophagus digestive enzymes

no digestive enzymes in this region

Stomach digestive enzymes

Enzymes that work int he stomach require a very low pH to work properly. Gastric pits contain a variety of cell types that contribute to highly acidic gastric juice. it is the job of parietal cells to secrete Hydrochloric Acid (HCl) making the stomach contents so acidic. The pH drop initiates protein digestion by denaturing these molecules. While HCl creates the necessary acidic environment, it has the potential to damage the surrounding tissue, so the stomach (and small intestine) must have mechanisms to protect these tissues from the low pH that is normally present in the stomach after a meal.

Stomach digestive enzymes part 2

Mucus neck cells and the surface epithelium of the stomach secrete bicarbonate-rich mucus to protect the underlying tissues. Chief cells produce the main gastric enzyme pepsinogen, which is the presence of HCl, is converted to its active form, pepsin that digests protein. HCl also enables the cell walls of plant-based foods to be broken down in the stomach. Intrinsic factor is another important substance secreted by the parietal cells of the stomach. Gastric lipase continues to digest fats in the stomach.

Small Intestine

The duodenum (first region of the small intestine) recieves chyme from the stomach. The pH of chyme is very low and has the potential to damage the cells of the small intestine, thus interfering with the ability to absorb nutrients efficiently. Neutralization of the pH for the incoming chyme is essential to maintain physiological functioning in this region. It is achieved by the release of a bicarbonate-rich juice from the pancreas into the duodenum. The rise in pH is also promoted by the production of slightly alkaline intestinal juice by glands and mucus cells in the duodenum. Intestinal juice also aids in the absorption of nutrients by acting as a carrier fluid.

Liver

supports digestive activities by producing bile and storing it in the gall bladder until needed. Bile emulsifies fats by breaking it down into smaller particles that are easier to digest. It is secreted into the duodenum whenever fatty chyme enters this region. Cholecystokinin (CCK), a duodenal hormone stimulated by fatty chyme, causes gallbladder to contract. CCK also relaxes the hepatopancreatic sphincter permitting bile and pancreatic juice to enter the duodenum.

Pancreatic Juice

Pancreatic juice, in addition to contributing an alkaline environment in the small intestine, is rich in enzymes. Many of these enzymes are secreted into the duodenum where they are converted from an inactive to their active form. Epithelial cells of the duodenum release enteropeptidase, which activvates pancreatic trypsinogen into trypsin which then activates more trypsinogen. Trypsin also converts the pancreatic proteases, pro-carboxypeptidase and chymotrypsinogen, into their active forms carboxypeptidase and chymotrypsin respectively. The pancreas also secretes active enzymes such as amylase, sucrase, lactase, maltase, lipases and nucleases into the small intestine. In the duodenum, intrinsic factor (secreted by the stomach ) binds vitamin B12. From here, the complex travels to the ileum, the last region of the small intestine, to be absorbed into the bloodstream.

Large intestine

There are no digestive enzymes in the large intestine. Contents passing through this region contain relatively few nutrients. Bacteria that normally inhabit this region play an important role by synthesizing B complex vitamins and vitamin K that are then absorbed into the blood. These bacteria are also capable of metabolizing some undigested polysaccharides