The term digestion refers to the breakdown of insoluble food molecules into smaller water-soluble molecules that can be readily absorbed into the watery blood plasma. Digestion can be divided into a number of different phases and depending on the source, they can slightly alter. One of the more common ways to describe the digestive process is to follow the organs through which the injected food moves: mouth, esophagus, stomach, small intestine, and large intestine (National Institute of Diabetes and Digestive and Kidney Diseases [NIDDKD], 2013). Other organs, such as the liver and pancreas also contribute to the digestive process. According to Pandol (2010), the route of food from organ to organ can also be categorized according to three major phases: cephalic phase (gastric secretion at neurogenic stimulation before food enters mouth), gastric phase (gastric juices in stomach help break down food), and intestinal phase (mainly takes place in duodenum and involves hormones and nervous reflexes). As such, the following paper will outline the basic digestive pathways of a cheeseburger, utilizing the above-mentioned organs and phases.
Before the cheeseburger even enters the mouth, the cephalic phase begins at the mere sight, smell, and thought of the food (Pandol, 2010). The senses stimulate the hypothalamus and medulla oblongata, and later the vagus nerve, which eventually produce saliva. Once the cheeseburger enters the mouth, mastication (mechanical digestion) begins as the person begins to chew. The enzyme amylase in the saliva helps to break down the carbohydrates (e.g., the bun) in the cheeseburger into smaller sugars. Also during this time, acetylcholine is released, spurting the secretion of pancreatic enzymes.
Once the cheeseburger is swallowed, it enters the esophagus, where an involuntary process begins, which is under the control of the esophagus and brain (NIDDKD, 2013). In the lower portion of the esophagus, there is sphincter (at the junction of the esophagus and stomach) that relaxes and allows food to pass into the stomach. This process begins the gastric phase of digestion. The upper part of the stomach relaxes in order to receive food. The smooth muscles of the lower portion churn and mix the food with gastric juices that contain digestive enzymes (e.g., HCl, gastric amylase, gastric lipase, renin, and pepsinogen). For example, pepsin helps with protein (e.g., hamburger meat, cheese) digestion and lipase assists with lipid breakdown (e.g., fat in meat and cheese). Salivary amylase also helps with the digestion of starch and glycogen in the stomach (e.g., bun and ketchup).
The partially digested food (also called chyme) then enters the small intestine, which begins the intestinal portion of the process (Pandol, 2010). The upper portion is called the duodenum, where bile and pancreatic juices are secreted to aid in digestion. Bile is secreted by the liver and stored in the gall bladder, which helps further break down fats (so they can later be digested by enzymes). Pancreatic enzymes include: chymotrypsin, carboxypolypeptidase, trypsin, and steapsin. For example, trypsin helps to break down proteins into smaller peptides, while steapsin breaks down triglycerides into fatty acids and glycerol, so that they can later be absorbed into the kidneys and liver. Additionally, the walls of the small intestines absorb any of the nutrients that are present and move them into the bloodstream, where they can be delivered to the rest of the body (NIDDKD, 2013). The waste products (including undigested food and old GI cells) then enter the large intestines, which absorb water and any remaining nutrients. The liquid is then converted into stool, which is stored in the rectum until it is pushed out during a bowel movement.
- National Institute of Diabetes and Digestive and Kidney Diseases. (2013). Your digestive system and how it works.
- Pandol, S. J. (2010). Regulation of whole-organ pancreatic secretion. The Exocrine Pancreas. San Rafael, CA: Morgan & Claypool Life Sciences. Retrieved from: http://www.ncbi.nlm.nih.gov/books/NBK54132/