Digestion & Absorption

The mouth is where digestion begins. Chewing begins breaking down the food into smaller pieces and mixes it with saliva. Saliva contains water, electrolytes, mucus and a few enzymes, such as salivary amylase. The saliva moistens the food so it mixes better as the food is broken into smaller pieces and is easier to swallow. This moist food mass is called bolus and swallowed and enters the esophagus.

The esophagus is a hollow muscular tube connecting the mouth and the stomach. The epiglottis is a small flap located in the throat, behind the tongue and in front of the larynx. It is usually in an upright position at rest allowing air to pass into the larynx and lungs. When a person swallows the epiglottis folds back to cover the entrance of the larynx so food and liquid do not enter the windpipe and lungs. After swallowing the epiglottis returns to its original upright position. The esophagus uses peristalsis, involuntary muscle contractions, behind the bolus, to push the bolus of food down the esophagus to the stomach. The Lower Esophageal Sphincter (LES) is located at the bottom of the esophagus. It relaxes and allows food into the stomach. It then closes to prevent backflow of acidic chyme from stomach into the esophagus.

Gastrin is a hormone in the stomach that stimulates digestion through secretion of HCl and increases gastric motility and emptying. Hydrochloric acid (HCl) activates the enzyme pepsin and starts protein digestion. The acidic environment reduces microorganisms ingested from food. Pepsin ia a digestive enzyme that breaks down proteins (including collagen) to smaller chained polypeptides.

Gastric lipase is an enzyme that breaks down a relatively small amount of triglycerides into short chain fatty acids (SCFA). Intrinsic factor is a protein that helps with Vitamin B12 absorption. The ball of food (previously called bolus) is now called chyme. It is a semi-liquid mixture of partially digested food and digestive juices. The pyloric sphincter is located between the stomach and small intestine (SI). It controls the rate of the food entering the SI from the stomach.

Some hormones in the SI include secretin which is produced in the duodenum in response to acidic chyme and slows gastric emptying. Cholecystokinin (CCK) is produced in the duodenum cells in response to chyme with high fat or protein in it.  It stimulates pancreas to release enzymes and the gallbladder to contract and release bile. Gastric Inhibitory Peptide is released in the duodenum and slows gastric emptying.

Trypsin is activated from inactive precursor trypsinogen in the duodenum, breaks down proteins.
Chymotrypsin is activated from inactive precursor chymotrypsinogen in the duodenum, breaks down proteins. Carboxypeptidase is a protease that takes off the terminal amino acid group from a protein.

Both pancreatic lipase and pancreatic amylase are enzymes produced by the pancreas. Pancreatic lipase breaks triglycerides into di- and monoglycerides. Pancreatic amylase breaks down starches into smaller chains. Brush border enzymes in the microvilli further break down chyme to more absorbable particles. One example is lactase. Sodium bicarbonate is also produced by the pancreas. It is a compound that isn’t an enzyme that raises the pH and neutralizes the acidic chyme.

Mucosal folds increase the surface area and assist in movement of chyme. Villi are fingerlike projections that increase surface area. Microvilli (brush border) – increases the surface area further and assists with digestion and absorption. The majority of macro and micronutrient absorption occurs in SI. The ileocecal sphincter is located between the SI and the LI. It prevents backflow of fecal matter into ileum.

The last organ of the GI tract is the large intestine. In addition to the absorption of remaining water and electrolytes, there is bacterial fermentation of fiber and resistant starches. There are no digestive enzymes in the LI. Chemical digestion done by bacteria. There is also Vitamin K and biotin synthesis by gut bacteria. Lastly, the LI is where feces are formed.

The liver is an accessory organ in digestion. A few roles include bile production, processing of nutrients absorbed by the SI, and glucose storage in the form of glycogen.

As already mentioned, the pancreas produces trypsin, chymotrypsin, amylase, and lipase. The gallbladder stores and and concentrates the bile made by the liver.

Examples of improper absorption include Celiac disease, an autoimmune condition when gluten damages villi of SI, causing decreased surface area of the villi which prevents proper absorption of nutrients. Lactose intolerance is when there is insufficient levels of lactase, the enzyme that breaks down lactose. This causes gas, bloating and diarrhea as common symptoms.

In summary, mechanical digestion is the breakdown of food through physical actions such as chewing and muscle contractions.  Chemical digestion is the breakdown of food with acids and enzymes. The GI tract organs are the mouth, esophagus, stomach, SI, and LI.  Accessory organs include the liver, gallbladder, and pancreas. A few examples of improper absorption are Celiac disease and and lactose intolerance.

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