Ingestion and Swallowing: Swallowing is a complex event that is coordinated by the swallowing center in the lower portion of the brainstem. During this process, food passes from the mouth to the pharynx and into the esophagus.
Oral phase: initially, the food bolus is moved to the back of the tongue. This triggers swallowing, by stimulating touch receptors in the pharynx. Then, the anterior of the tongue lifts to the hard palate and forces the bolus to the pharynx.
Pharyngeal phase: during the pharyngeal phase, the larynx is pulled forward and upward under the tongue by muscular contraction. As the larynx rises, the epiglottis moves backwards and downwards to seal off the glottis (the entrance to the respiratory system).
Esophageal phase: during the esophageal phase, the food bolus is pushed through the esophagus by involuntary muscle contractions called peristalsis.
Stomach: The stomach is a J-shaped organ, directly under the diaphragm. The uperior portion is a continuation of the esophagus. The inferior portion (pylorus) empties the stomach contents into the first segment of the small intestine.
Gastric juices: The main component of gastric juices released during digestion is gastric acid. Gastric acid is hydrochloric acid produced by the parietal cells, and it makes the lumen of the stomach very acidic with a pH of 2-3. This increased acidity contributes both to the conversion of pepsinogen to pepsin and to the breakdown of foods.
Liver and Gallbladder
Bile production and storage: The liver is involved in many aspects of nutrient metabolism, and the regulation of the products of digestion in the blood: (A) Carbohydrate metabolism, (B) Protein and lipid metabolism and (C) Vitamin storage.
Bile is an alkaline fluid produced by hepatocytes in the liver, and it helps to emulsify fats during digestion and absorption in the small intestine. Bile contains taurocholic and deoxycholic salts; these salts combine with fat globules and break them down into small droplets for absorption in the small intestine.
Structure: The pancreas is an elongated organ, adjacent to the stomach and in close association with the first segment of the small intestine, the duodenum.
Digestive enzymes: The pancreas produces a number of enzymes used in the process of digestion: (a) trypsinogen and chymotrypsinogen, (b) pancreatic lipase and (c) amylase.
Small Intestine: The small intestine is 8-22 ft. in length in an adult; it is divided into three main segments: duodenum, jejunum and the ileum. The small intestine is the site were most of the nutrients from digested food takes place.
Large Intestine: The large intestine is shorter in length (4-5 ft) than the small intestine, but it is so named because of its increased diameter. The large intestine is divided into four major areas: the ascending, transverse, descending and sigmoid colon.