Nutrition not only involves the macro nutrients and micro nutrients, it also involves their digestion that how they get into our body or becomes part of human muscle mass. Micro nutrients that involve minerals and vitamins they are not digested by the digestive system but are absorbed as it is. Macro nutrients include carbohydrates, proteins and fats. After being digested, they are absorbed into the human body through different routes. There are different sites for the digestion of macro nutrients. For example, main site for protein digestion is stomach and for that of lipids it is the small intestine. In this article, the discussion will be about the process of digestion in the whole GIT system that how the digestion and absorption takes place.
Components of digestive system:
The components of digestive system involve:
Digestion in oral cavity:
Oral cavity performs many functions, Such as:
Selection of food
Grinding and lubrication of food
Digestion of starch
In the oral cavity, a little proportion of the carbohydrates is digested by the enzyme alpha-amylase or ptyalin in saliva. In the oral cavity, the major role in digestion is of saliva that is a slimy liquid and is composed of:
Water and mucous
Sodium bicarbonate and some other salts
Carbohydrate that is starch digesting enzyme, amylase or ptyalin.
Saliva is produced by the salivary glands. These are:
Water and mucous make a slimy liquid which serves to moisten the food and converts the food into bolus form along with grinding by teeth so that the bolus is easily accepted by the esophagus and is passed on smoothly.
Sodium bicarbonate and some other salts have an antiseptic effect that they destroy the micro-organisms which enter along food but main function is pH stabilization of saliva.
The carbohydrate digesting enzyme performs its functions by digesting the starch and glycogen into maltose.
Esophagus is a muscular structure and is not meant for digestion. It only acts as a bridge between the oral cavity and stomach for the transfer of food. Its main purpose is only transportation.
In stomach, the main digestion is of proteins. This is carried out by the gastric juice which is secreted by the gastric glands. The composition of the gastric juice is such that:
Hydrochloric acid (HCl)
Pepsinogen (activated into pepsin by the action of HCl)
Mucous and water
When you even smell the food, your stomach starts producing gastric juice and when the food is high in protein there is more and more gastric juice production.
Hydrochloric acid is secreted in the concentrated form, has an antiseptic effect. It adjusts the stomach pH to acidic ranging from 2-3 because the enzymes of the stomach can work only in the acidic environment.
As, stomach is itself a protein body so there is a chance that it may be digested by the gastric enzymes. To protect the stomach lining from being corroded, mucous makes a layer over it that prevents the pepsin to act on the stomach lining.
Pepsin is secreted in the pepsinogen form being activated by the HCl. It hydrolyzes the proteins to yield peptones and polypeptides.
The main digestion is carried out in small intestine and all of the absorption too. The small intestine has 3 parts:
Duodenum (the part which receives the pancreatic secretion)
Jejunum (main site for digestion of lipids and proteins along with carbohydrates)
Ileum (it promotes only the process of absorption. There is almost no digestion there)
Pancreatic juice contains many enzymes. Such as:
Pancreatic amylase (for carbohydrates digestion)
Pancreatic lipase (for fat digestion)
Trypsinogen (for protein digestion; it is released in inactivated form but is activated by the intestinal enterokinase into trypsin)
In duodenum, there is not much digestion. By the action of pancreatic juice a small proportion of starch is hydrolyzed into maltose, proteins into peptones and polypeptides and fats into fatty acids and glycerol.
Jejunum carries out the most of digestion. The enzymes act on the substrates and convert them into products:
Amino peptidase converts polypeptides into dipeptides
Erypsin converts dipeptides into amino acids
Lipase converts fats into fatty acids and glycerol
Maltase converts maltose into glucose
Lactase converts lactose into glucose and galactose
Ileum is the site for absorption. All the digested carbohydrates, proteins and fats are absorbed into the blood stream through the lacteals of intestinal villi especially fats in the form of lipoproteins or some by direct absorption e.g glucose and amino acids by active transport.
The main purpose of large intestine is to form the fecal material. Here, there is no digestion but only the absorption of salts and water. When large amount of water is absorbed, it leads to constipation. But when due to any microbial infection there is a huge loss of water out of the body leading to diarrhea and dehydration.
So, digestion is a very important process that makes the nutrients available to our body bound in the food molecules