The gastrointestinal organization in humans and other advanced members of Kingdom Animalia is furnished with well-developed structures and unique features that are capable of performing a variety of tasks, for example, selection of food, grinding, mastication, lubrication, oral digestion, enzymatic secretions, acidic release, absorption and packaging the residue for waste disposal through excretory system. Here follow some of the common tasks assigned to food breakdown mechanism, particularly, in human body with reference to the associated organs:

Food Selection

The first and foremost function of digestive system is to make selection of healthy and digestible food with the help of three senses, namely, smell, taste and touch. Before you insert a morsel into mouth, you need to smell it, and if you sense some unpleasant odor, it should be rejected because it might be rotten or polluted. The next step in sequence is to taste it with taste-buds present on the upper layer of your tongue, and similarly if the taste doesn’t appeal to your mood, the morsel might be rejected. The sense of touch aids in the detection and elimination of any hard or indigestible object that, if engulfed, might injure or scratch your alimentary canal at any point and pose difficulties in the normal mechanism of food breakdown.

Mastication/Grinding

The major parts of digestive system located in mouth cavity include tongue, teeth and cheeks. After the desired quality and quantity of food has been selected, it needs to be chewed and converted into smaller pieces, which is done with the help of different types of teeth, like canines, premolars and molars. These teeth are facing their counterparts on the opposite jaws and are brought firmly together to crush the food between them.

Wetting and Lubrication

The watery secretions in mouth cavity help to moisten and lubricate the food to be digested, so that it may become soft and swallowed easily. In this way, there remains no hard object or large particle that can cause complications in the overall digestive system function. As a result, a soft, wet, oval mass of food, called bolus, is ready to pass through esophagus by peristaltic movements to stomach for the second phase of digestion.

Oral Digestion/Saliva Secretions

The first phase of mechanical and biological breakdown of dietary items (i.e. starch and lipids or fats) starts in the oral cavity with the help of saliva that is 99.5% composed of water and the remaining 0.5% is contributed by enzymatic secretions, glycoproteins, antibacterial compounds, mucus and electrolytes. Complex starch compounds are broken down into simpler sugars with the help of ptyalin or salivary amylase that is released, by glandular organs, into the mouth cavity. In this way, the first function of digestive system gets completed.

Another enzyme, called ‘salivary lipase’ is of vital importance for newborn infants because at such an early age, the pancreatic lipase is in the process of development.

Protection against Tooth Decay/Infection

The salivary lipase, along with fat and lipid breakdown, plays a significant role in combating the digestive system diseases. It cleans the teeth by breaking down any adhered or trapped particles among them. Moreover, the same biological catalyst prevents the build-up of harmful bacteria on teeth.

Killing Germs

It is one of the digestive system facts that various germs, like bacteria and other disease causing agents are killed either by grinding/salivation in oral cavity or in the strongly acidic medium of stomach where the optimum level of pH is 2. This low level of pH dissolves the living germs (bacteria) as quickly and efficiently as it does with undigested bolus contents, thus eliminating the risk of any mild, acute or chronic disease associated with them.

Neutralization of Toxins

Natural detoxification mechanisms are found in various organs associated with the systems, like, digestive system, immune system, respiratory system, excretory system, and so on. There is very possibility that the water we drink and the food we eat would be containing toxins or other harmful substances that can easily inflict injuries to the sensitive cells and tissues of the body. So their timely neutralization or elimination becomes quite necessary to avoid malfunctioning in any part of the body.

Transportation of Liquid/Solid/Semi-Solid Food

The alimentary canal of gastrointestinal system resembles a hollow tubular structure surrounded by multi-dimensional muscle organizations. Circular and longitudinal arrangement of muscles perform coordinated contractions and relaxations, thus creating a wave-like propelling movement, called peristalsis, for the easy and one-way transportation of gastrointestinal contents, thus accomplishing one of the important digestive system function.

Role of Gastric Secretions

The food that enters the stomach, through esophageal peristalsis, is in the form of masticated, lubricated, soft and wet mass of bolus. The next step in digestive system function involves the conversion of the chewed and semi-digested food mass into chyme through churning and mixing, and by the action of acidic & enzymatic secretions that are poured into stomach as soon as a series of boluses get entry into it through cardia or gastroesophageal junction. Gastric juice secreting structures in the stomach wall, viz. cardiac, fundic, and pyloric glands, secrete gastric acid, enzymes (pepsinogen), hormones (somatostatin, serotonin, endorphins, cholecystokinin, histamine and gastrin), intrinsic factor and mucus.

The protease, or protein digesting enzyme is secreted in an inactive form, called pepsinogen which has to be activated in a strongly acidic medium where it is converted into pepsin. The mucus forms an inner lining of stomach which renders protection to stomach wall and other internal organs of the body against acid-burns and injuries caused by the contact with gastric acid.

Role of Pancreatic Secretions

The pancreatic juice secreted by this glandular organ contains digestive enzymes that carry out the further breakdown of partly-digested food or chyme, coming from stomach and entering into small bowel through pyloric sphincter. All the major organic components of chyme, namely, proteins, lipids and carbohydrates, are acted upon by the biological catalysts present in pancreatic fluid. Another role of these secretions is to facilitate the absorption of finely digested dietary compounds passing through small intestine.

Role of Bile Juice

Bile secretions are synthesized by liver, a vital gland, and then transferred to gallbladder (an accessory organ for concentration and storage), which then releases them into small intestine at the time of dire need. Being a surfactant, bile assists a great deal in the emulsification of fatty ingredients of food, so that their surface area may be increased for pancreatic lipase and other enzymes to act upon them. In composition, bile consists of about 85% of water, and the remaining part is contributed by other components, namely, inorganic salts, bile salts, fats, pigments and mucus.

Apart from being an emulsifier, other major functions of the hepatic juice include absorption of finally digested food and other fat-soluble substances, like vitamins (A, K, E & D). Hepatic fluid is very necessary for the digestion of fats because fat-digesting enzymes can work only in an alkaline environment which is achieved due to the alkaline nature of bile juice, otherwise, the chyme coming from stomach has acidic pH. Moreover, some of the bile salts also act as germ-killers and eliminate any bacteria or other microbes entering through food into the duodenum of small intestine.

Absorption of Nutrients

Another function of digestive system is that it plays very significant role in the absorption of finely digested food particles, salts, minerals, vitamins or any other essential nutrients into the blood stream. After the chyme is completely broken down into the simplest absorbable particles, the process of its intake starts, and about ninety-five percent (95%) of absorption takes place in the small intestine. However, water, minerals and vitamins that are synthesized by bacteria in the large bowel (vitamins K & B7) are absorbed across the wall of colon into the circulatory system.

Excretion of Indigestible Materials

Any organic or inorganic substances present in the large bowel, that are indigestible, are excreted out of the body through the posterior end of excretory system, called anus.