Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tracts of animals (including humans) and in the traps of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes are diverse and are found in the saliva secreted by the salivary glands, in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract.
Digestive enzymes are classified based on their target substrates:
- proteases and peptidases split proteins into small peptides and amino acids.
- lipases split fat into three fatty acids and a glycerol molecule.
- carbohydrases split carbohydrates such as starch and sugars into simple sugars such as glucose.
- nucleases split nucleic acids into nucleotides.
- Salivary glands
- Secretory cells in the stomach
- Secretory cells in the pancreas
- Secretory glands in the small intestine
Oral cavity 
Complex food substances that are taken by animals and humans must be broken down into simple, soluble, and diffusible substances before they can be absorbed. In the oral cavity, salivary glands secrete an array of enzymes and substances that aid in digestion and also disinfection. They include the following:
Potassium bicarbonate (KHCO3): The major role of bicarbonate is to neutralize acidity mainly in an attempt to preserve the dentin and tooth enamel and also to neutralize bacterial toxins. Bicarbonate also prevents acid damage to the esophageal lining before food enters the stomach.
- lingual lipase: Lipid digestion initiates in the mouth. Lingual lipase starts the digestion of the lipids/fats.
- amylase: Carbohydrate digestion also initiates in the mouth. Amylase produced by the salivary glands breaks complex carbohydrates to smaller chains, or even simple sugars. It is sometimes referred to as ptyalin.
- Mucin: Mucin functions to make food more pliable and also covers it facilitating its transfer in the esophagus to the stomach, by lubricating the content.
- lysozyme: Considering that food contains more than just the essential nutrients and bacteria or viruses, the lysozome offers a limited and non-specific, yet beneficial antiseptic function in digestion.
- IgA: This is a dimeric form of antibodies produced by the body. Gastrointestinal tract produces only IgA mainly to battle bacterial toxins, and also to tag certain recognizable molecular patterns in viruses and bacteria. IgA-coated pathogens are digested by white cells lining the gastrointestinal tract.
- Haptocorrin (also known as R-factor): Helps with the absorption of Vitamin B12. After Vitamin B12 is released from its original carrier protein in the stomach, it gets bound to Haptocorrin. Haptocorrin protects it from acidic conditions of the stomach but is cleaved in the duodenum by pancreatic proteases. Vitamin B12 can then bind to intrinsic factor (IF) that has been produced by parietal cells. Finally, the IF-Vitamin B12 complex is taken up by in ileum via the cubam receptor.
Of note is the diversity of the salivary glands. There are two types of salivary glands:
- serous glands: These glands produce a secretion rich in water, electrolytes, and enzymes. A great example of a serous oral gland is the parotid gland.
- Mixed glands: These glands have both serous cells and mucous cells, and include sublingual and submandibular glands. Their secretion is mucinous and high in viscosity.
In addition, all salivary secretions are hypotonic with respect to the plasma concentration. This is because the duct cells of salivary cells are impermeable to water, yet there is a continuous abstraction of electrolytes such as sodium (Na) and potassium (K) from the initially secreted juice, causing it to be very dilute (hypotonic) by the time it is released into the mouth.
The enzymes that are secreted in the stomach are called gastric enzymes. The stomach plays a major role in digestion, both in a mechanical sense by mixing and crushing the food, and also in an enzymatic sense, by digesting it. The following are the enzymes produced by the stomach and their respective function:
- Pepsinogen is the main gastric enzyme. It is produced by the stomach cells called "chief cells" in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids. Protein digestion, therefore, first starts in the stomach, unlike carbohydrate and lipids, which start their digestion in the mouth.
- Hydrochloric acid (HCl): This is in essence positively charged hydrogen atoms (H), or in lay-terms stomach acid, and is produced by the cells of the stomach called parietal cells. HCl mainly functions to denature the proteins ingested, to destroy any bacteria or virus that remains in the food, and also to activate pepsinogen into pepsin.
- Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells of the stomach. Vitamin B12 (Vit. B12) is an important vitamin that requires assistance for absorption in terminal ileum. Initially in the saliva, haptocorrin secreted by salivary glands binds Vit. B, creating a Vit B12-Haptocorrin complex. The purpose of this complex is to protect Vitamin B12 from hydrochloric acid produced in the stomach. Once the stomach content exits the stomach into the duodenum, haptocorrin is cleaved with pancreatic enzymes, releasing the intact vitamin B12. Intrinsic factor (IF) produced by the parietal cells then binds Vitamin B12, creating a Vit. B12-IF complex. This complex is then absorbed at the terminal portion of the ileum.
- Mucin: The stomach has a priority to destroy the bacteria and viruses using its highly acidic environment but also has a duty to protect its own lining from its acid. The way that the stomach achieves this is by secreting mucin and bicarbonate via its mucous cells, and also by having a rapid cell turn-over.
- Gastrin: This is an important hormone produced by the "G cells" of the stomach. G cells produce gastrin in response to stomach stretching occurring after food enters it, and also after stomach exposure to protein. Gastrin is an endocrine hormone and therefore enters the bloodstream and eventually returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).
Of note is the division of function between the cells covering the stomach. There are four types of cells in the stomach:
- Parietal cells: Produce hydrochloric acid and intrinsic factor.
- Gastric chief cells: Produce pepsinogen. Chief cells are mainly found in the body of stomach, which is the middle or superior anatomic portion of the stomach.
- Mucous neck and pit cells: Produce mucin and bicarbonate to create a "neutral zone" to protect the stomach lining from the acid or irritants in the stomach chyme.
- G cells: Produce the hormone gastrin in response to distention of the stomach mucosa or protein, and stimulate parietal cells production of their secretion. G cells are located in the antrum of the stomach, which is the most inferior region of the stomach.
Secretion by the previous cells is controlled by the enteric nervous system. Distention in the stomach or innervation by the vagus nerve (via the parasympathetic division of the autonomic nervous system) activates the ENS, in turn leading to the release of acetylcholine. Once present, acetylcholine activates G cells and parietal cells.
Pancreas is both an endocrine and an exocrine gland, in that it functions to produce endocrinic hormones released into the circulatory system (such as insulin, and glucagon), to control glucose metabolism, and also to secrete digestive/exocrinic pancreatic juice, which is secreted eventually via the pancreatic duct into duodenum. Digestive or exocrine function of pancreas is as significant to the maintenance of health as its endocrine function.
Two population of cells in the pancreatic parenchyma make up its digestive enzymes:
- Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to prouduce the hormone secretin and release it to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice. It is interesting to note that secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme.
- Acinar cells: Mainly responsible for production of the inactivate pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin(CCK), which is a hormone/neurotransmitter produced by the duodenal cells called the "I cells." CCK stimulates production of the pancreatic zymogens.
- Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum, into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin.
- Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin.
- Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein
- Several elastases that degrade the protein elastin and some other proteins.
- Pancreatic lipase that degrades triglycerides into fatty acids and glycerol.
- Sterol esterase
- Several nucleases that degrade nucleic acids, like DNAase and RNAase
- Pancreatic amylase that breaks down, besides starch and glycogen, most other carbohydrates. Humans lack the enzyme to digest the carbohydrate cellulose, mainly due to its special hydrogen-bonding structure.
Pancreas's exocrine function owes part of its immaculate function to bio-feedback mechanisms controlling secretion of its juice. The following significant pancreatic bio-feedback mechanisms are essential to the maintenance of pancreatic juice balance/production:
- Secretin, a hormone produced by the duodenal "S cells" in response to the stomach chyme containing high hydrogen atom concentration (high acidicity), is released into the blood stream; upon return to the digestive tract, secretion decreases gastric emptying, increases secretion of the pancreatic ductal cells, as well as stimulating pancreatic acinar cells to release their zymogenic juice.
- Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content. CCK also increases gallbladder contraction, resulting in bile squeezed into the cystic duct, common bile duct and eventually the duodenum. Bile of course helps absorption of the fat by emulsifying it, increasing its absorptive surface. Bile is made by the liver, but is stored in the gallbladder.
- Gastric inhibitory peptide (GIP) is produced by the mucosal duodenal cells in response to chyme containing high amounts of carbohydrate, proteins, and fatty acids. Main function of GIP is to decrease gastric emptying.
- Somatostatin is a hormone produced by the mucosal cells of the duodenum and also the "delta cells" of the pancreas. Somatostatin has a major inhibitory effect, including on pancreatic juice production.
Small intestine 
The small intestine is traditionally divided into three anatomic sections defined from their distance from the pyloric sphincter:
- duodenum: It is the first portion of the small bowel that is itself divided into four distinct anatomic positions called first, second, third, and fourth sections of duodenum. Duodenum is the only portion of the small bowel that is partially retroperitoneal, and peritoneal. Duodenum is a secretary portion of the small intestine.
- jejunum: This is the section of the small intestine that begins immediately from the insertion point of the ligament of Treitz, and is the longest of the three sections. Jejunum is an absorptive surface.
- ileum: This is the terminal portion of the small intestine and as such has a limited but vital role in absorption. Vitamin B12 and bile are absorbed at this portion.
The following enzymes/hormones are produced in the duodenum:
- secretin: This is an endocrine hormone produced by the duodenal "S cells" in response to the acidity of the gastric chyme.
- Cholecystokinin (CCK): This is not by definition a hormone; there is new evidence suggesting that CCK works by a very complex neuronal bi-directional pathway. Regardless of its pathway, its eventual role is to increase secretion of acinar cells and increased production of pancreatic juice. CCK also increases gallbladder contraction, causing release of pre-stored bile into the cystic duct, and eventually into the common bile duct and via the ampulla of Vater into the second anatomic position of the duodenum. CCK also decreases the tone of the sphincter of Oddi, which is the sphincter that regulates flow through the ampula of Vater. CCK also decreases gastric activity and decreases gastric emptying, thereby giving more time to the pancreatic juices to neutralize the acidity of the gastric chyme.
- Gastric inhibitory peptide (GIP): This peptide decreases gastric motility and is produced by duodenal mucosal cells.
- motilin: This substance increases gastro-intestinal motility via specialized receptors called "motilin receptors."
- somatostatin: This hormone is produced by duodenal mucosa and also by the delta cells of the pancreas. Its main function is to inhibit a variety of secretory mechanisms.
Throughout the lining of the small intestine there are numerous "brush border" enzymes whose function is to further cleave the already-broken-down products of digestion into absorbable particles. Some of these enzymes include:
- Lactase: This is a significant brush border enzyme in that a majority of Middleastern and Asian population lack this enzyme and also this enzyme decreases with age, and as such lactose intolerance is often a common abdominal complaint in the Middleastern, Asian, and older population, manifesting with bloating, abdominal pain, and osmotic diarrhea.
- Other disaccharidases
Large intestine (colon) 
The colon is the main reservoir for feces (mainly in rectum) before defecation. It is also where the liquid stool becomes solid, by losing its water, and electrolytes. The colon also actively secretes bicarbonate and potassium, which explains why severe diarrhea can cause metabolic acidosis as well as hypokalemia. The colon also houses symbiotic bacteria that produce vitamin by-products and are essential to the human health and homeostasis.
- Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier, 1st Ed. p. 235
- Bowen, R.  "Exocrine Secretion of the Pancreas"
- Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier, 1st Ed. p. 244
- Morino P, Mascagni F, McDonald A, Hökfelt T.. "Cholecystokinin corticostriatal pathway in the rat: evidence for bilateral origin from medial prefrontal cortical areas." Neuroscience 1994. Apr;59(4):939-52. PUBMED ID:7520138
- Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier. 1st Ed. p. 254