DIGESTION*This chapter discusses the functions of processes of the digestive tract, the absorption and breakdown of food and particles. *THE DIGESTIVE TRACT: The process of digestion begins in the mouth. Mastication (chewing) mixes with saliva and it's antimicrobial agents called salivary amylase, an emzyme that can catalyze the partial digestion of starch. Deglutition (swallowing) begins as a volunatary action where the larynx is raised so that the epiglotis covers the entrance to the respiratory tract. Deglutition involves three phases-oral, pharyngeal and esophageal. Perstaltic contractions in the esophagus deliver food to the stomach. Once in the stomach, the ingested material is churned with hydrochloric acid and pepsin.


Defecation: After all needed electrolytes and water have been absorbed, the remaining waste material passes to the rectum. This leads to an increase in rectal pressure and ultimately the urge to defecate. During this process, the longitudinal rectal muscles contract to increase the rectal pressure, and the internal/external anal sphincters relax. This process is also aided by contractions of the abdominal and pelvic muscles.

Phases of Gastic Function:** 
Cephalic Phase
1. Sight, smell and taste of food cause stimulation in the brain.
2. Vagus nerve stimules acid secretion.
a. Indirect stimulation of parietal cells (major effect).
b. Stimulation of gastrin secretion. (minor effect)
Gastric Phase
1. Distention of stomach stimulates vagus nerves causing the stimulation to secrete acid.
2. Amino acids and peptides also stimulate acid secretion.
a. Direct stimulation of parietal cells (minor effect).
b. Stimulation of gastrin secretion causing acid secretion (major effect).
3. Gastrin secretion inhibited when pH of juice falls lower than 2.5
Intestinal Phase
1. Neural inhibition of gastric emptying and acid secretion.
a. Arrival of chyme cuasing distention and osmotic pressure increase.
b. These stimuli activate a reural reflex inhibiting gastric activity.
2. Responding to the fat in chyme, duodenum secretes a hormone that inhibits gastric acid function.

APPLICATION:Recently, I had the amazing opportunity to assist in Pierre Hospital's surgery unit. I was able to see three seperate Laporascopy Cholecystectomies (otherwise known as gallbladder removals). I was able to see and assess the different gallbladders, which all varied in different levels of gallbladder disease and all contained stones. It was a great visual experience to get to view the stomach first hand and see what the structures look like to the human eye. Not only was it the gallbladder, but also the different ducts, stomach and duodenum! Additionally, I can apply digestion to my career because the absorption of electrolytes and different nutrients is vital in the healing process associated with any patient and also their nutritional status. ***Critical Thinking Questions:Chose two organs of the digestive system. Explain how the structure of that organ contributes to the function of that organ. STOMACH: The J-shaped structure of the stomach is the where the food is churned by the muscular contractions of the organs walls and were the mucous is formed. The secretion of mucous stimulates pepsin, another digestive agent.

SMALL INTESTINE: The small intestine, measuring about three feet long is divided into three sections beginning with the duodenum, jejunem and that last three-fifths being the ileum. Most of the chemical digestion and absorption takes place here. The pyloric sphincter lets the food into the intestine from the stomach. The small intestine is lined with microvilla, helping to increase the surface area to absorb food. Peristalsis allows the food to be pushed along.

The three molecules, carbohydrates, proteins and lipids are important molecules in food that need to be physically and chemically digested by the digestive system. For each nutrient:*Describe where the nutrient is digested in the digestive system.*Describe the enzymes that help to hydrolyze the molecules. *Describe how the nutrient is absorbed.*Described how the body uses the molecule.

* Carbohydrates begin to be digested in the mouth by the amylase, an enyzyme in the saliva. It then continues to be digested again in the duodenum by the pancreatic amylase.
*Maltose, maltroise and oligosaccharides are hydrolized by brush border enzymes located on the microvilli in the small intestine. They are moved by secondary active transport.
*The body uses carbohydrates for energy.
*Proteins start being digested in the stomach by pepsin, with most of the digestion occuring in the duodenum and jejunum.
*Pancreatic juices (trypsin, chymotrypsin and elestase) cleave peptid bonds.
*Polypeptide chains are digested into free amino acids, dipeptides, and tripeptides. Free amino acids are absorbed by cotransport. The dipeptides and tripeptides enter epithelial cells by the action of a single membrane carrier that has recently been characterized.
*Proteins are needed to build and repair the body's tissues.
* In neonates, the salivary glands and stomach produce lipases. In adults very little lipid digestion occurs until the lipid globules in chyme arrive in teh duodenum. In a process called emulsification, bile salt micelles are secreted into teh duodenum and act to break up the fat droplets into tiny emulsification droplets of triglycerides. Emulsification gives droplets a greater surface area in which the pancreatic lipase can act. *Through hydrolysis, lipase removes two of the three fatty acids from each triglyceride molecule and lliberates free fatty acids and monoglycerides.
*Lipids are mainly used for energy storage. The good lipids are called HDL and work as a scrub brush on the walls of blood vessels. The bad lipids are called LDL and can stick to the walls of blood vessels and cause problems (atherosclerosis).