HUMAN DIGESTIVE SYSTEM:

DIGESTION:

The process of conversion of non-diffusible food molecules (complex food) into fusible food molecules (simple form) by the action of enzymes is called as DIGESTION. 

PROCESS/STAGES OF DIGESTION:

The entire process of digestion takes place in a long, coiled, muscular tube called ALIMENTARY CANAL which consists of the following:

  • Mouth cavity.
  • Oesophagus.
  • Stomach.
  • Small intestine.
  • Large intestine.
The process of digestion involves following different stages:
  • Ingestion.
  • Digestion.
  • Absorption.
  • Assimilation.
  • Egestion.

INGESTION: 

The process of taking of food into the mouth or body of an animal is called as ingestion.

DIGESTION OF FOOD IN MOUTH: 

  • The process of mechanical and chemical digestion begins in mouth.
  • Teeth are present in the mouth cavity.
  • The food is crushed down into small pieces with the teeth, and this process is called as mechanical digestion.
  • There are three pairs of salivary glands, which produce saliva.
  • Saliva contains enzymes called PTYALIN, which converts starch into maltose.
  • This process is called chemical digestion. Starch---------Ptyalin------------Maltose.
  • Now the partially digested food takes the form of a ball called BOLUS, which is pushed into oesophagus.
digestive system

OESOPHAGUS:

  • It is a long tube like structure, which keeps the food moist with the help of alkaline juices, secreted by its walls.
  • It also helps in transport of food into stomach.

DIGESTION OF FOOD IN STOMACH:

  • Stomach is a bag like structure, which is elastic in nature.
  • The gastric glands secrete gastric juices, and these juices perform chemical digestion.
  • Gastric juices are:
-HCL: It softens the food and kills germs.

-RENIN: It helps to curdle milk in infants.

-PEPSIN: It acts on proteins to break them into PEPTONES.

After few hours the food become a thick fluid called chyme, and is released bit by bit in small intestine. 

DIGESTION OF FOOD IN SMALL INTESTINE:

Small intestine is a narrow tube which is divided into two parts:

a) Duodenum.                                                                    b) Ileum.

DUODENUM:
small intestine

It is the first part of small intestine, which receives a common duct which formed by the fusion of a bile duct and pancreatic duct, and is called as hepato pancreatic duct.

(i) BILE DUCT comes from gall bladder of liver, and brings bile juice. Bile juice helps in breaking down large molecules of fats into small droplets. This process is called  EMULSIFICATION.

(ii) PANCREATIC DUCT comes from the pancreas, and contains sodium bicarbonate and many enzymes. Three important enzymes are as follows:

-AMYLASE breaks down starch into maltose.     Starch ------- Amylase ------- Maltose.

-TRYPSIN acts upon proteins into smaller peptides. Proteins-----Trypsin-----Peptides.

-LIPASE breaks fats into fatty acids and glycerol. 
                                                                  Fat droplets----Lipase---- Fatty acids+ Glycerol.

ILEUM:

  • This is the second part of small intestine, where food enters from the duodenum.
  •  Here the rest of the digestion is completed, by the enzymes present in intestinal juices secreted by the glands present in the walls of small intestine.
  • The enzyme amino peptidase breaks down peptides into amino acids.
                    Peptides---------------Amino peptidase--------------Amino acid.
  • The enzyme disaccharides break down maltose, lactose and sucrose into glucose.
                    Maltose/Lactose/Sucrose--------Disaccharides-------Glucose. 

ABSORPTION OF FOOD IN SMALL INTESTINE:

  • After the process of digestion, the digested food in the form of soluble molecules of glucose, fructose, amino acid, fatty acids and glycerol etc. is absorbed into the blood vessel through VILLI.
  • The digested food is now diffused in the blood and then taken to the liver.
  • From liver it is taken to the heart through blood.
  • From heart it is taken to the each and every cell of the body through blood.(circulatory system)

LARGE INTESTINE:

  • The undigested food (roughage) along with pass from the ileum to large intestine.
  • Large intestine is wide tube, consists of coecum, appendix, colon and rectum.
  • In large intestine absorption of water takes place, so the undigested food becomes slightly hard. 
  • During the process intestinal bacteria cause fermentation of the undigested matter, which is now termed as FAECES.

EGESTION:

  •  The FAECES are stored in the rectum,(the last portion of alimentary anal) where it is stored for some time.
  • From here the faeces pass outside the body through an opening called ANUS.