As suggested in the previous anatomical discussions there are a great many different digestive processes at play in the digestive system. In this section we provide an overview of these processes and provide additional details where appropriate.
Children under 13 need know this material only at a base level. Those 13 and older should understand the processes in some significant detail but need not recall details such as which enzyme is responsible for specific digestive processes. Such details are provided as additional foundational information for those who may be interested in exploring digestive processes in more detail elsewhere.
Ingestion involves taking food and liquids in via the mouth. This stimulates the secretion of saliva by the various salivary glands which serve to begin the digestive process and soften the ingested materials into a moist clump called a bolus. Enzymes in the saliva immediately begin breaking down carbohydrates into more simple sugars that will later be again modified and stored by the body. The bolus is then swallowed and directed downward toward the esophagus.
The body secretes numerous chemicals and compounds that facilitate the digestive process. As discussed, Saliva is secreted into the mouth to initiate digestion and to soften food. Nearly all of the digestive system organs secrete mucous liquid that helps protect the organ and allows material to move through the organ easily.
In the stomach various enzymes, mucous, and hydrochloric acids are secreted by the stomach walls to further break down the ingested materials into chyme. The primary secretions are hydrochloric acid and protease enzymes. Protease enzymes require an acidic environment into order to begin breaking down proteins into simpler molecules. This is the primary reason for the acidic content of the stomach. The mucous secretions are bicarbonate-rich and line the inside of the stomach to protect it from the acidic mixture in the stomach.
As chyme is moved into the small intestines the gallbladder (and/or liver) and the pancreas secrete various chemicals and enzymes that help the small intestines further digest and absorb nutrients in the chyme. A key secretion of the pancreas is the bicarbonate rich alkaline solution that serves to neutralize the hydrochloric acid in the chyme. This allows further break down of protein products by trypsin enzymes in the small intestine.
The pancreas also secretes lipase which digests fat, and amylase which digests carbohydrates. These enzymes break down larger molecules into forms that can be more easily absorbed in the small intestine. Amylase continues the work of breaking down carbohydrates that began with saliva in the mouth.
Bile salts from the gallbladder keep fats from coagulating into larger masses. This allows lipase to work on smaller clumps of fatty material so that these fats can be more readily broken down into fatty acids and glycerol. These smaller molecules are then transported by the circulatory system to all cells in the body which use them for energy.
Mixing and Movement
The esophagus, stomach, small intestine and large intestine all use involuntary wave-like muscle contractions called peristalsis to move ingested materials in one direction. The movements propel the digestive content through each organ and from one organ to the next. These contractions also cause these materials to be thoroughly mixed, which helps enzymes, bacteria, and acids interact with all of the material in the digestive system.
Digestion is a multi-part and often multi-step process that converts larger and more complex molecules into smaller molecules that can be distributed throughout the body by the circulatory system. Different enzymes are used to break down specific types of larger molecules.
Fat digestion is one of the most complex processes in the human digestive system. Most of the fats in the human diet are in the form of triglycerides. These are complex molecules that are too large to be absorbed through the walls of the small intestine. These fat molecules are not water soluble and tend to bond together to form large clumps. Even though the clumps are large, the combined surface area of one large clump is less than the surface area of the same quantity of material in many smaller clumps. This means it is more efficient to digest fatty materials if they exist as many smaller globules or droplets rather than as a few larger ones.
This is where bile salts benefit the digestion of fats. These salts prevent fats from forming into larger masses and helps break up larger masses into smaller ones. This process is called emulsification. This allows the lipase enzyme to operate on a larger surface area to more efficiently break down these fats. Bile salts are released into the small intestine by the gallbladder when fats are detected in the digestive system.
The enzyme lipase then breaks down a single triglyceride molecule into two fatty acid molecules and one monoglyceride molecule. These smaller molecules then combine with the bile salts to form micelles which are some 200 times smaller than an emulsified droplet. Because of the smaller size the micelles can fit between the villi on the small intestine walls. Here the fatty acid and monoglyceride molecules are released from the bile salts and are absorbed through the walls of the small intestine into the enterocyte. Within the enterocyte two fatty acid molecules and one monoglyceride molecule are reassembled into a triglyceride. Yes, a triglyceride was digested into smaller molecules that could be absorbed through the intestinal wall only to be immediately reassembled back into a triglyceride.
The newly reformed triglyceride molecule is then coated with proteins and vitamins to form chylomicrons. But these are now relatively large particles that will not transport through the capillaries and into the bloodstream. So they must find their way into the circulatory system through an entirely different process.
The chylomicrons move through lacteals (lymphatic capillaries) into veins of the lymphatic system. The lymphatic veins then empty into larger blood veins in the thorax.
Only about 50% of ingested fat is processed through the small intestine. The remainder is eliminated as waste.
Carbohydrates are composed of complex sugars called saccharides. Saccharides are links of multiple sugar molecules called polysaccharides.
As you chew, saliva containing salivary amylase works to both soften the food and to begin breaking down polysaccharides into less complex sugars called disaccharides. This breakdown continues while a bolus forms in the mouth and as it travels down the esophagus. The process stops, however, once the bolus becomes part of the high acid chyme in the stomach.
As chyme enters the small intestine the hydrochloric acid in the chyme is neutralized by pancreatic juices. A key component of the many elements in these pancreatic juices is pancreatic amylase. With a less acidic environment this enzyme can once again begin to convert polysaccharides into disaccharides. Disaccharides are less complex sugars consisting of a chain of only two sugars. But this is simply another intermediate step for the digestive system.
The disaccharides must be broken down further before they can be absorbed by the small intestines. This is accomplished by the enzymes dextranase, glucoamylase, sucrase, lactase, and maltase. These enzymes are produced in the small intestine and are attached to the villi in the brush border of the small intestine. They do no float freely in the chyme. These enzymes convert various disaccharides into glucose which can then be transported through the lining of the small intestine. These enzymes perform the following processes.
|Sucrase||Sucrose||Glucose and Fructose|
Not everyone produces the lactase enzyme. A deficiency in lactase may lead to lactose intolerance, or an inability to properly digest milk products.
Glucose and fructose are then absorbed through the villi and enter the blood stream via capillaries in each villi. They are transported through ever larger veins until they enter the hepatic portal vein which transports them to the liver.
Proteins are long chemical chains of amino acids. Digestion breaks these long chemical chains down into the fundamental amino acids so they can be absorbed and utilized by the body.
A protease refers to any enzyme that breaks down protein molecules into amino acids. The three most prevalent protease in the human body are pepsin, trypsin and chymotrypsin. Proteins are large and complex molecules that must be broken down in multiple stages.
Proteins first begin digestion in the stomach. The stomach releases pepsinogen when proteins are present in the chyme. Pepsinogen is essentially inert. When pepsinogen is exposed to an acidic environment it is converted into the active pepsin enzyme. Pepsin begins to break down proteins into smaller molecular structures called polypeptides. About 20% of protein digestion occurs in the stomach.
As chyme leaves the stomach the pepsin enzymes become inactive due to the lower acidity levels in the small intestine. At this point trypsinogen and chymotrypsinogen, inactive forms of the enzymes trypsin and chymotrypsin, are released with other pancreatic juices into the small intestine. These inactive forms are produced so that these highly effective enzymes do not attempt to digest the pancreas or other tissues outside of the small intestine.
The trypsinogen is converted to trypsin when it comes in contact with the enterokinase enzyme secreted by the microvilli in the intestinal wall. The trypsin then converts chymotrypsinogen into chymotrypsin. These two active enzymes work in concert to further break down polypeptides into smaller protein molecules called peptides, and to then break the peptides down into amino acids that can be absorbed by the villi of the small intestine.
Amino acids pass into the villi and then pass through capillaries and eventually into the hepatic portal vein that carries the amino acids to the liver.
Bacteria in the large intestine work to further break down and ferment any residual materials in the ingested content. Blood cells destroyed by the liver entered the small intestine with the bile salts and any undigested portions of these blood cells mix with the undigested materials which turns the mass a dark brown color. Various enzymes continue to break down any undigested materials as the material moves through the large intestine. Vitamins and gasses are released during these processes. Any water soluble vitamins, minerals, or other substances are retrieved when water is absorbed by the large intestine. As the material finally flows, via peristalsis, to the rectum it is held until such time as it is voluntarily eliminated from the body.