Introduction The human digestive system is a complicated organ system where different organs of the digestive system work in coordination with different digestive glands. Digestive glands secrete chemical enzymes into different organs of the digestive system which specifically act on either carbohydrates or proteins or fats of our diet. The purpose of the digestion process is to make the food taken by our body into an easily absorbable form of nutrients and that can be utilized by our body for its life activities.
From simple microscopic animal-like Amoeba to human nutrition process occurs in five steps. They are ingestion, digestion, absorption, assimilation, and egestion.
IngestionThe intake of food into mouth is known as ingestion. The teeth in the mouth help in breaking down food into small pieces. The saliva secreted in the mouth help in swallowing the food.
DigestionThe breaking down of bigger molecules of food into an easily diffusible form of nutrients with the help digestive enzymes at various parts of the digestive canal is known as digestion.
AbsorptionThe inner lining of digestive canal has many blood vessels in its walls. These absorb the digested nutrients in liquid form and supply to all parts of the body.
AssimilationThe digested food absorbed into bloodstream is finally diffused into the cells of our body. Then the cells use it to derive energy for their growth and repair. This process is known as assimilation.
EgestionThe elimination of undigested food in the form of faeces through the anus of the alimentary canal is known as egestion.
Digestion process in different parts of human digestive system
Digestion in the mouth or Buccal cavityThe space present between the upper and lower jaw of the mouth is known as the buccal cavity. The buccal cavity is an organ of ingestion, chewing and swallowing of food.
The four different types of teeth present in the mouth break down the food into smaller and smaller pieces. Incisor teeth help in cutting and bitting the food, canines for tearing the food, molars and pre-molars help in grinding and chewing the food. This process of breaking down food into smaller pieces with the help of teeth is known as mechanical digestion.
The three types of salivary glands present in the mouth pour saliva into the mouth. Saliva contains a slimy material mucus that helps to aggregates the food particles present in the mouth to form into a ball-like structure called 'bolus' that can easily be swallowed. Salivary amylase or ptyalin enzyme present in saliva will help in the digestion of starch into maltose sugar. So this is the first step of chemical digestion that occurs in the mouth. Saliva helps to keep the mouth lubricated and also helps to dissolve sugars present in the diet. The tongue present in the mouth helps to rotate the food in the mouth for proper breaking down of food with the help of teeth.
Digestion in StomachThe food which gets swallowed in the form of the bolus from mouth moves through a short narrow tube called pharynx into the esophagus. The esophagus is a long narrow tube that connects the pharynx to the stomach. Pharynx and esophagus have no digestive glands present in them and so in these parts, no digestion of food occurs. These structures are meant to transport food from the mouth into the stomach. The wall of the esophagus is lined with smooth muscles and that allows it to show rhythmic contractions called peristaltic movements. Due to these peristaltic movements (alternate contraction and expansion of the wall of the esophagus), the food will be little by little is pushed into the stomach. The cardiac sphincter present at the junction of the esophagus and stomach controls the backflow of food.
The stomach is an elastic bag-like structure that stretches as the food collects into it. The main function of the stomach is to store the food until it gets partially digested. The food finally gets changed into a fine paste in the stomach which gets pushed into the small intestine little by little. The pyloric sphincter present between the stomach and the small intestine controls the movement of food in a phased manner.
The inner surface of the stomach wall is lined by a large number of tiny glands called gastric glands. These glands secrete a digestive juice called gastric juice. This gastric juice is mainly composed of dilute HCl, pepsin and renin enzymes. Dilute HCl present in gastric juice kills the germs present in food and prevents the fermentation of food in the stomach. Pepsinogen and prorenin are two inactive enzymes present in gastric juice. These inactive enzymes get activated into active pepsin and renin in the presence of dilute HCl. Pepsin enzyme is a protease and acts on proteins to convert them into a soluble compound called peptides. In children, the renin enzyme acts upon soluble milk protein caseinogen and converts into insoluble casein. The mucus layer secreted by the cells of the stomach wall helps it to protect itself from the action of dilute HCl.
The rhythmic peristaltic movements of the stomach wall about once every 20 seconds will churn the food into a fine paste with the help of gastric juice called 'chyme'. The food remains for about three hours in the stomach. The wall of the stomach absorbs water and other soluble digested nutrients. The pyloric sphincter present between the stomach and small intestine will regulate the entry of partially digested food from the stomach into the first part of the small intestine little by little.
Digestion in small intestineThe contraction of the stomach muscles helps the chyme to be pushed into the duodenum which is the first part of the small intestine. The bile duct from the liver and the pancreatic duct from the pancreas pour their secretions, viz, bile juice and pancreatic juice into the duodenum by a common duct called hepatopancreatic duct.
Bile juice secreted by the liver has no digestive enzymes present in it but it still helps in the process of digestion. The salts present in the bile juice helps in the process of emulsification of fats. The breaking down of bigger molecules fat into smaller molecules by bile is known as emulsification. This process helps in easier digestion of smaller fat molecules in the further steps of digestion. The bile juice also helps to change the acidic condition of food into alkaline that facilitates the enzymes of pancreatic juice to work on the digestion of carbohydrates, proteins, and fats.
Pancreatic juice includes three enzymes pancreatic amylase, trypsin, and lipase or steapsin present in it. These enzymes work only when the food is in an alkaline condition. The protein-digesting trypsin is secreted in an inactive form called trypsinogen which is made active by another enzyme called enterokinase. Pancreatic amylase works on starch and converts into maltose sugar. Trypsin enzyme works on proteins, proteases, and peptones into peptides and amino acids. Steapsin (lipase) acts on emulsified fats and converts them into fatty acids and glycerol.
The partially digested food from the duodenum moves through jejunum to reach ileum. No digestion occurs in the jejunum region. The ileum is the last part of the small intestine where the completion of digestion and absorption of digested food occurs. The inner surface of the small intestine includes two types of glands-Crypts of Lieberkuhn and Burner's glands. They secrete digestive enzymes and mucus. The inner surface of the small intestine also includes folds of finger-like projections called villi which increase the surface area of absorption of digested food.
The intestinal juice secreted by the glands of the small intestine is known as succus entricus. It includes various enzymes in it like Erepsin/peptidase, Maltase, Sucrase, Lactase, Lipase, etc. Peptidase/Erepsin enzyme acts on the peptides of proteins and converts them into amino acids. Maltase enzyme acts on maltose sugar and converts into glucose. Sucrase enzyme acts on sucrose sugar and converts into glucose and fructose. Lactase enzyme acts on lactose sugar and converts into glucose and galactose. Lipase enzyme acts on emulsified fats and converts them into its final end products fatty acids and glycerol.
The final end products of digestion amino acids and simple sugars (glucose, fructose, galactose) are absorbed into the blood through the blood capillaries present in villi. Similarly, fatty acids and glycerol are absorbed into lymph capillaries which are located in villi along with blood capillaries. Through lymph vessels, they ultimately mix with blood circulation. During blood circulation, the digested nutrients are transported into cells and tissues of our body. The undigested food is then pushed from the small intestine into the large intestine.
Formation of faces in large intestineThe caecum is the first part of the large intestine. The fluids and salts present in the undigested food which comes from the small intestine are absorbed by the wall of the caecum part of the large intestine. The inner wall of caecum consists of a thick mucus layer which helps in the absorption of the fluids and salts.
The colon is the second part of the large intestine. The materials which reach colon include undigested matter, cellulose, roughage, mucus and dead cells from the alimentary canal. No digestive enzymes are secreted by any part of the large intestine. But the bacteria present in the colon digest part of the fiber to form fatty acids. The wall of the colon absorbs these fatty acids along with salts and water present in the undigested food and transforming the remaining waste into faces.
The rectum is the last part of the large intestine where the fecal matter gets stored for some time before it gets expelled through the anus. The act of expelling of feces is called egestion or defecation.
Various steps of chemical digestion in human digestive system:-
Here is a video on the process of human digestion:-
This is a nice article by the author detailing the process of digestion in the human alimentary canal. It is very true that alimentary canal is one of the most important organs of the body and this particular organ is unique in the sense that it is the interface between the outside food and creation of internal energy and constituents for the running of the body system. So alimentary canal is like a manufacturing and finished products hub where many inputs in the shape of various edible items are being fed to it, it processes them step by step, takes out the ingredients as per the use and requirement of the body and sends them to the bloodstream from where they are given to each and every part of the body as per the need. The un-utilised and useless items are then excreted from the body at the end of this canal through the anus.
It is really nature's miracle that the alimentary canal works on a relentless basis providing the body everything it needs to run itself effectively and to that extent, the role of the alimentary canal is not only basic but very important also. It is said that if the condition of alimentary canal is good and its functions are working effectively then it helps a person to keep a good health. At the same time any problem in the digestion system would cause a number of deficiencies or malfunction of the body and then it would be needed to supplement the body through drips or other 'through injection' ways. It is imperative that we should take care of this vital organ and should not load it with junk and other unhealthy food which would slowly ruin its normal functioning and then it would not be successful in its attempts to provide healthy and essential ingredients to the various body parts which are waiting for that eagerly. A good food habit and lifestyle would definitely keep the alimentary canal in a good shape which in turn would keep our health in a much better shape.