Digestion is defined as the mechanical and chemical breaking down of food into smaller parts, to a form that can be absorbed into the bloodstream.The end of the digestive process is defecation. The food enters the mouth, is chewed by the teeth, and broken down by the saliva from the salivary glands. It travels down the esophagus and into the stomach where acid breaks down most of the food. The remainder goes through the small intestine, then through the large intestine or bowel, and comes out as waste through the anus. It’s a form of catabolism. In other words, it’s the process of the conversion of complex substances into simpler ones for easy absorption and assimilation by the body. This is the purpose of the digestive tract.
People can eat nutritious food but still be tired. They can develop chronic diseases and age prematurely. Why does this happen? It might be due to poor assimilation and poor absorption of food and nutrients. This is why the digestive system operating properly is so important. Digestive enzymes, which are responsible for the metabolism of food, include Carbohydrases, (such as sucrase, cellulase, amylase, and maltase), lipase, proteases (such as rennin), lactase, and pepsin. Hydrochloric acid is also important for proper digestion.
The Digestive Process
The digestive process begins in our mouth. Chewing breaks down and grinds our food into smaller pieces so they may be swallowed. Three different types of amylase are secreted in our mouth. The amylase is mixed with food when we chew to digest the carbohydrates we eat. It is so important to chew food well, because the longer we chew the more amylase we introduce to the food, which is in our mouth.
After we swallow our food, it travels down our esophagus to our stomach, which lies in the upper left region of our abdomen, just below our diaphragm. Our stomach is a pouch-like structure and serves as a reservoir for our food during the early stage of digestion. There are two sections to our stomach. The upper section is called the cardiac section and is where the majority of carbohydrate digestion occurs from the amylase, which was in our saliva.
The cardiac section is called the enzyme section, because this is where active enzymes in the food we have eaten will partially digest our food. The food will stay there for about 30 to 45 minutes then it will enter the pyloric section, which is the lower section of our stomach. The pyloric section secretes hydrochloric acid and pepsin to digest proteins. Our stomach produces between 1 and 2 liters of gastric juice daily. The food stays there for an average of 2 hours.
The amount of protein eaten and the efficiency of our digestive system are what will mandate the length of time the food stays there. Different proteases break down different proteins. Rennin facilitates the digestion of milk protein. Muscular movements of our lower stomach along with the hydrochloric acid and the pepsin will produce a watery mixture called chyme. The chyme then leaves our stomach and after the chyme enters our small intestine our pancreas secretes lipase and our gall bladder secretes bile to assist in digesting fat. Lactase found in the brush border membrane of our small intestine breaks down any lactose that we may have eaten. The secretions from our pancreas also neutralize acid from our stomach. At this point the law of adaptive secretion takes place and our body manufactures only the amount of enzymes needed to digest any undigested carbohydrates, proteins or fats. If our food is well digested at this point our body will not have to produce large amounts of enzymes.
Next the food will enter our large intestine. Our large intestine is responsible for absorbing water and electrolytes (any ion of sodium or calcium that in biological fluid regulates or affects most metabolic processes). The majority of the process involves reabsorbing gastric juices. Our large intestine also is home to numerous bacteria that live off the food, which reaches our large intestine without being digested. They are beneficial bacteria and make some of the vitamins we need and keep a check on harmful bacteria that can cause disease. Once these liquids and vitamins are extracted the waste is eliminated through our anus.
The enzymes released from the pancreatic acinar cells comprise the major enzymes, which are involved in the digestive process. Many are secreted as inactive precursors. The acinar cells contain zymogen granules, which are the sites of storage of the enzyme or enzyme precursor proteins. The enzyme precursors include trypsin, chymotrypsin, proteolytic enzymes, carboxypeptidase, elastase, and phospholipase A. The enzymes secreted as active enzymes include lipase, alpha-amylase, ribonuclease, and deoxyribonuclease. The reason for enzymes being released as inactive precursors is so that activated enzymes don’t digest the pancreatic tissue.
In 1974 George Palade discovered the mechanism of secretion in the acinar cell and was awarded the Nobel prize for his work. The mechanism is as follows:
- The enzymes or precursors are synthesized on the rough endoplasmic reticulum of the cells.
- The molecules then are released into the cisternae of the endoplasmic reticulum.
- Buds, which contain the enzymes or enzyme precursors break off of the cisternal membranes.
- The buds coalesce in the region of the Golji complex and form condensing vacuoles.
- The vacuoles migrate towards the luminal membrane.
- At the luminal membrane the zymogen granules fuse with the cell membrane and the vesicles break open and release their contents. This process is called exocytosis.
The different enzymes are packaged together in the zymogen granules and are probably released together. It is exocytosis that is contolled by hormones and neurotransmitters.
The Purpose of the Digestive Tract
The purpose of the digestive tract is to break down our food into absorbable nutrients. Our food consists of carbohydrates, proteins and fats, which need to be broken down into sugars, amino acids and fatty acids. If our digestive system can’t break down our food adequately our body could absorb macromolecules, which could include bacteria. This could lead to infection, irritation to the intestine, intestinal toxemia or other diseases. In other words, without adequate digestion good health is not possible.
The specific functions of our digestive system are:
- Receive food
- Break down food into absorbable nutrients
- Move food through the digestive system
- Secrete hydrochloric acid, mucus, digestive enzymes, bile and pepsin
- Absorb nutrients
- Elimination of toxic waste products and unabsorbed food