Digestion starts with the cephalic phase, when the senses of smell, sight, sound, or the thought of food activates neural centers in the brain, which stimulate the facial and glossopharyngeal nerves that in turn stimulate the salivary glands to secret saliva. Also the vagus nerves are activated, which in turn stimulate the gastric glands to secrete gastric juice. This stage prepares the mouth and stomach for eating the food.
The gastric phase of digestion starts in the mouth with ingestion of the food. Here, food is being masticated first with the help of tongue, teeth, mandible, muscles of the mandible, and temporomandibular joints. To help mastication, the salivary glands secret saliva, which helps dissolve the food and start digestion. Saliva lubricates the food, preparing it for its journey down into the esophagus. It also produces an enzyme that digests starch. Mechanical digestion involves grinding of the food by the teeth, mixing the food with the saliva, and maneuvering it by the tongue to be swallowed. During this process, the food is transformed into a bolus which is being forced back of the mouth, by the tongue, into the oropharynx. The next step is the swallowing of the bolus/food, passing it from the mouth into the pharynx. This is a voluntary action, hence the voluntary stage. Immediately after the swallowing, digestion becomes an involuntary action: this is the pharyngeal stage of swallowing. The tongue rises against the palate, nasopharynx closes off by the upward movement of uvula and soft palate, and breathing is temporarily interrupted; larynx rises and epiglottis seals it off, vocal chords come together, and the bolus of food passes through the laryngopharynx into the esophagus when the upper esophageal sphincter relaxes to allow the bolus into the esophagus. The bolus has now reached the esophageal stage where peristalsis occurs and the bolus is moved downward the esophagus, toward the stomach. The esophagus has mucus glands in its wall which also help to further lubricate the food on its journey down to the stomach, and muscular waves which travels from the top to the bottom of the esophagus to get the food down to the stomach. At the end of the esophagus, the lower esophageal sphincter relaxes and allows the bolus to pass into the stomach.
The food lands into the stomach four-five seconds after it has been swallowed. Right next to where the esophagus comes into the stomach is the fundus, where the gas formed inside the stomach builds up and is eliminated through a burp. Now the stomach wall is stretched, no more rugae because this was a big meal. pH of the contents increases because some of the acid in the stomach got buffered by the proteins in the food, which triggers nerve impulses to stimulate the flow of gastric juices made of pepsinogen - pepsin and intrinsic factor, which initiates mixing waves, which macerate the food, mixing it with secretion of the gastric glands. Pepsine breaks up the protein into smaller units. This generates the production of the pasty chyme. The mixing waves generates gastric emptying – passing a little bit of the chyme through the pyloric sphincter, which is partially closed, into the duodenum. During the stomachal chemical digestion, pepsin breaks the peptide bonds between amino acids in proteins, which in turn are fragmented into peptides. When reaching the small intestine, the chyme contains partially digested carbohydrates and proteins. In two to four hours after eating, the stomach has finished emptying the chyme into the small intestine, and the intestinal phase of digestion begins.
The small intestine is the real engine-room of the bowel, as this is where the food is finally broken down into its elementary constituents, and absorbed into the bloodstream. Here the chyme will remain for three to five hours. The mechanical digestion in the small intestine includes segmentation and peristalsis. The chemical digestion is a joint effort of pancreatic juice, bile, and the intestinal juice. The pancreas secrets pancreatic juice to inactivated the pepsin from the stomach and generate an optimal environment for the enzymes in the small intestine, continuing the digestion of proteins begun in the stomach. The pancreatic juice includes pancreatic amylase for digesting starches; trypsin, chymotrypsin and carboxypeptidase to digest proteins; pancreatic lipase to digest triglycerides, and ribonuclease/deoxyribonuclease to digest nucleic acids. Enzymes of the small intestine break down disaccharides into monosaccharides: the sucrase enzyme breaks down sucrose, the maltase enzyme breaks down maltose, and the enzyme lactase digests lactose. The bile salts produced by the liver, stored in the gallbladder, are also secreted to emulsify triglycerides. At the end of this breaking down process, the molecules – the final products of digestion, are ready to be absorbed through the mucosa into the blood and lymphatic streams. Ninety percent of this absorption happens in the small intestine, through simple diffusion, osmosis and active transport. Nutrients broken down during digestion will travel through the bloodstream into the liver, the main metabolic factory of the body, where they will be processed according to the body’s needs. In the liver, glycogen is broken down to glucose, and some amino acids, lactic acids plus other sugars are also converted into glucose. The liver breaks down fatty acids to produce ATP, synthesizes lipoproteins to transport triglycerides and cholesterol, synthesizes cholesterol and uses it to produce bile salts. The liver stores glycogen, vitamins A, D, E and K, iron, copper, and participates in the synthesis of vitamin D.
The rest of ten percent of absorption occurs during the stomachal stage of digestion and in the large intestine. The material left over after absorption occurs in the small intestine, passes through the ileocecal sphincter into the large intestine. This is a slow passage, as the ileocecal sphincter is slightly contracted. Here the chyme will remain for three to ten hours. In the large intestine, completion of absorption occurs, some vitamins are synthesized, and feces are formed and eliminated. Peristalsis occurs slower. Bacteria ferment the remaining carbohydrates, releasing gases that contribute to the flatus. Other components that are broken down here by bacteria are the remaining proteins, and some vitamins. Bilirubin is being decomposed into pigments, one of them being stercobilin which gives the brown color to feces – the solidified chyme. This material is pushed into the rectum through mass peristaltic movements. Defecation reflex is initiated and the rectum is being emptied through the anus. This is the end of the journey of digestion, from the tongue to the anus.
Resources:
Coleman, D. (Director). (2005). Anatomy for beginners: Lesson 3, Digestion. (Reality TV Show). United Kingdom
Tortora, G.J., Derrickson B. (2010). Introduction to the Human Body: The Essentials of Anatomy and Physiology (8th Ed.). Hoboken , NJ
