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Biology - GI Tract

Think of food as consisting of three kinds of substances; carbohydrate, protein, and fat. Carbohydrate is sometimes referred to as starch. Digestion is the process of taking food in and using enzymes to breakup the starch, the protein, and the fat into small molecules that can be absorbed. You should know the five stations of the human digestive tract; mouth, esophagus, stomach, small intestine, and large intestine. 

The mouth secretes an enzyme called amylase. Amylase is in the saliva and it begins the breakdown of starch. From the mouth, food moves to the esophagus. The esophagus then wriggles and pushes the food into the stomach. The rhythmic wriggling motion of the esophagus is called peristalsis. Peristalsis therefore moves food through the esophagus into the stomach. The rhythmic movement continues throughout the digestive tract. 

The esophagus doesn’t secrete any enzymes. No digestion occurs in the esophagus. When food reaches the stomach, the process of digestion continues. The stomach is acidic. The parietal cells of the stomach secrete hydrochloric acid into the stomach and the stomach is, therefore, acidic. Stomach acid secretion is stimulated by the vagus nerve. One of the reasons that the stomach is acidic is this; the chief cells of the stomach secrete an enzyme called pepsin. 

Pepsin helps digest protein and it works best in an acidic environment. The important enzymes that function in the small intestine are produced in the pancreas and they reach the small intestine by traveling through the pancreatic duct. They are called pancreatic enzymes. These enzymes function in the digestion of starch, protein, and fat. There’s another duct that makes the delivery to the small intestine called the common bile duct. It delivers bile. Bile is made in the liver and then sent to the gallbladder to be stored. 

When food enters the small intestine, the gallbladder sends bile through the common bile duct into the small intestine. Bile is not a digestive enzyme. It’s an emulsifier. It breaks fat particles into smaller fat particles. It does not, however, break fat particles into smaller molecules. Bile is important because if large particles of fat were not broken into smaller particles of fat, lipase, which is the enzyme that digests fat, would not be able to do as good a job. The liver is an important organ. It not only makes bile, but it converts glucose to glycogen, converts amino acid to keto acids and urea, detoxify some drugs and poisons in the body and phagocytosis worn-out red blood cells. 

All you really have to know about the large intestine is that it reabsorbs a lot of water. Once food has passed through the stomach, it enters the small intestine through the pyloric sphincter. In the intestine, food is exposed to enzymes that digest starch, protein, and fat. In their inactive state, these enzymes are called zymogens. Zymogens must be cleaved to be activated. The pancreatic enzyme that digests starch is called pancreatic amylase. The pancreatic enzyme that digests fat is called pancreatic lipase. 

There are two pancreatic enzymes that digest protein, one is called trypsin and the other is called chymotrypsin. Once food is broken down, it is absorbed in the small intestine, enters the blood, and is processed by the liver. Waste products are passed on to the large intestine. When the pancreas secretes its pancreatic enzymes into the small intestine, it secretes a lot of water along with them. That water becomes a part of the mush that enters the large intestine. 

The large intestine then reabsorbs a lot of that water and takes it into the bloodstream. What is left to food after water is absorbed in the large intestine is feces. It moves out of the large intestine into the rectum and from there, it is excreted.