December 11, 1999
The process that the body undergoes when eating a Hamburger, fries and a drink is quite complex. First upon walking into Joe’s famous “Eat Here And Get Gas” corner store, restaurant and gas station, the wonderful smell of burning grease, and the sight of the dripping fries starts your body’s Cephalic gastric secretions. Your salivary glands are parasympathetically stimulated by Acetylcholine to start producing the exocrine secretion saliva, which is composed of water, amylase, and mucin.
Seven and a half minutes later when your food gets to the booth, You begin the artery clogging consumption at the headgut. This structure is the anterior opening of the alimentary canal more commonly referred to as the mouth. Here, the food (if you can call it that) is chewed by teeth (mastication) and moved around by the tongue. The chemicals in the food, the act of chewing and the tastes all further stimulate the production of saliva, which is used to lubricate the bolus of chewed food, which aids in swallowing. Along with mucin, the saliva contains digestive enzymes such as amylase, which start the chemical digestion.
The bolus then travels to the foregut where it passes through the esophagus leading from the mouth to the stomach. Myogenic smooth muscle movement called peristaltic movement aids the movement of the “food”. The bolus then leaves the esophagus through the Esophageal (cardiac) sphincter and enters the stomach where chemical, mechanical and enzymatic digestion takes place. Humans have a monogastric stomach consisting of a single muscular compartment. The internal layer of the stomach is lined with thousands of gastric pits, which are filled with gastric juices from the gastric glands. In the gastric glands, the chief cells (zygomatic) secrete pepsinogen, the parietal cells secrete HCl (hydrochloric acid), and the goblet cells secrete mucus. The release of these digestive juices is mediated by both endocrine (gastrin, CholeCystoKinin, and secretin) and exocrine pathways (water, HCl, pepsinogen). Once the food is broken down to the consistency of soggy oatmeal, it passes through the Pyloric Sphincter and enters the Duodenum.
This is the beginning of the Midgut, and the duodenum is where much of the mucus and fluids are secreted. This occurs a few hours after eating. The midgut is the major site for the chemical digestion and absorption of carbohydrates, proteins and fatty acids. The acidic contents from the stomach have to be buffered when they reach the small intestine in order not to digest the alimentary canal walls. This is accomplished by secretions from the pancreas. The pancreas secretes bicarbonate and water to lessen the acidity, but it also secretes enzymes (chymotrypsin, trypson, pancreatic amylase, lipases, proteases, carboxypeptidases and nucleases) to further break down the food. These are regulated by both the endocrine and exocrine systems and released in the duodenum through the pancreatic duct. The pancreas itself acts as an important exocrine organ. The liver also has a secretion which ends up in the duodenum. The liver produces bile salts, which are carried to the gallbladder for storage as bile fluid. The bile helps to buffer the acidic contents of the stomach as well as to emulsify fats and make their digestion easier. The production and release of bile is regulated by CCK from the parasympathetic nervous system and the endocrine system. CCK is secreted in response to the presence of fatty acids in the duodenum.
Further down the small intestine you have the jejunum, which also secretes fluid, further digests food and starts the absorption of nutrients. The most posterior part of the small intestine is the ileum, which is the primary site for nutrient absorption. The food (intestinal chyme) is passed through the small intestine by peristalsis. It can also be moved back and forth by segmentation. The small intestine has thousands of villi and millions of microvilli, which increase the surface area and aid in the absorption of the food. There is a glycocalyx that traps water and mucus in order to slow the chyme and allow for more absorption.
Once the bulk of nutrients have been absorbed in the ileum, the contents are moved into the hindgut (large intestine) via peristalsis. The main functions of the hindgut are to reabsorb water and to store the material remaining after the bulk of nutrients were removed. The “leftovers” are consolidated into feces and then they are expelled from the body via the anus. This process is stimulated when the large intestine gets “full”, and it completes the Eat At Joe’s cycle.