The digestive system breaks down the food we ingest and turns it into energy and waste products.  It is composed of the digestive tract and accessory organs.  The digestive tract is a hollow tube that goes from mouth to anus, with openings at each end.  The accessory organs are the salivary glands, the pancreas, the liver and gallbladder.

Digestion begins in the brain, with the cephalic phase of digestion triggering the release of gastric secretion as a result of the sight, smell and thought of food.  Food enters the mouth where the teeth begin the physical break down food.  Three sets of salivary glands secrete saliva to moisten the food and help with swallowing.  These glands also secrete the enzyme salivary amylase, also known as ptyalin, which begins the chemical breakdown of starch into maltose.  Salivary amylase is in contact with food for a very short time, but deficiency of this enzyme creates a burden further down in the small intestine where pancreatic alpha-amylase has to work harder to break down starches.

The Role That The Mouth and Tongue Play In Digestion

The tongue contains taste buds, and it also pushes food toward the back to the mouth for swallowing.  When we swallow, the bolus enters the esophagus for passage to the stomach through the cardiac sphincter.  The stomach continues the mechanical and chemical breakdown of food.  Gastric juice is secreted by millions of gastric glands located in the mucosal lining of the stomach.  It is composed of mucus, pepsinogen (the inactive form of the enzyme pepsin), and hydrochloric acid (HCl).  HCl activates pepsin from pepsinogen, and it triggers the hormone gastrin to be released into the bloodstream.  This hormone signals the brain to produce more gastric juice.  HCl and pepsin break down protein into peptides.  Pepsin deficiency is generally associated with low levels of HCl, a condition known as hypochloridria.  Symptoms include GERD, burping, bloating, gas, nausea and diarrhea, H. Pylori infections, maldigestion, neurological issues, iron-deficiency anemia, vitamin B12 deficiency, calcium, magnesium and protein deficiency.

The Stomach, pH Levels and the Role They Play In Healthy Digestion

The stomach is all about acid (pH of 1.5 – 3.0).  HCl is excreted into the stomach at a pH of 0.8. The acid bathes the stomach, disinfects it, kills bacteria and parasites, and activates pepsin for protein digestion.  After the stomach churns the bolus and mixes it with gastric juice, the bolus breaks down even more into a paste called chyme, which is released into the upper part of the small intestine (duodenum) through the pyloric sphincter.  When chyme enters the duodenum, the acidic pH of the chyme triggers the goblet cells of the small intestine to secrete mucous. 

The Small Intestines And Digestion

The small intestine has the dual roles of digestive organ and gland. The intestinal walls secrete two hormones into the bloodstream: secretin and cholecystokinin (CCK).  Secretin stimulates the pancreas to release bicarbonate and pancreatic juice, and CCK stimulates the gallbladder to release bile necessary to emulsify and absorb fats.  As part of the pancreatic juice, the pancreas first releases sodium bicarbonate to help raise the pH of the chyme to neutral. Once the chyme pH reaches neutral, the enzyme portion of the pancreatic juice is released to complete the chemical digestion of carbohydrates, proteins, and fats.  Three major groups of pancreatic enzymes are proteases, pancreatic lipase, and pancreatic alpha-amylase.  Other pancreatic enzymes include gelatinase, elastase, deoxyribonuclease, and ribonuclease.  

While digestion of protein begins in the stomach, the bulk of this process takes place in the small intestine and is carried out by pancreatic proteases: mainly trypsin and chymotrypsin.  These enzymes are stored in the pancreas and released in inactive form (trypsinogen and chymotrypsinogen).  When they reach in the lumen of the small intestine, the enzyme enterokinase, present in the intestinal mucosa, converts trypsinogen into its active form, trypsin.   When activated, trypsin activates chymotrypsin.  Both enzymes break down peptides into tripeptides and dipeptides.  The final digestion of tri- and dipeptides into single amino acids is carried out by carboxypeptidase and elastase.  Proteases deficiency causes protein maldigestion, protein deficiency, hypoglycemia, weakness, constipation, hair loss, gingivitis, calcium deficiency, tooth decay, and mood swings.

The Pancreas and Digestion

Pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids.  Bile salts are also necessary for lipase to work as well as for the absorption of fatty acids and monoglycerides.  Pancreatic lipase deficiency causes fat maldigestion, steatorrhea, abdominal pain, weight loss, fat-soluble vitamin deficiencies, and calcium deficiency.

Pancreatic alpha-amylase breaks down starch into maltose, maltotrios and alpha-limit dextrins.  Alpha-amylase deficiency causes carbohydrate maldigestion, malabsorption, gas and diarrhea, and bacterial overgrowth.

By the time chyme leaves the duodenum, it is almost fully digested.  Peristalsis moves the absorbable molecules into the jejunum, where millions of villi and microvilli absorb them into the bloodstream, which carries them to the entire body.  The leftover chyme from the small intestine (indigestible fibers, bile, water, and sloughed off cells) passes into the large intestine through the ileocecal valve.  The large intestine recycles water and the waste material that nourishes the colon cells; it captures any nutrients that are still available, with the help of the bowel flora, and converts the nutrients to Vitamins K/B1/B2/B12 and butyric acid; it forms and expels feces.


des Gachons CP, Breslin PAS. Salivary Amylase: Digestion and Metabolic Syndrome. Curr Diab Rep. 2016 Oct;16(10):102.

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