Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the lymph and blood plasma. 

Digestion in human is a long process so for the convenience of study it is divided into these 5 steps, as mentioned below:

a) Digestion in Buccal Cavity: 

Structure of buccal cavity-

Structure of Buccal Cavity in Human

In the buccal cavity teeth, tongue and saliva make pulp of ingested food. Lisozym which are found in saliva disinfect food. Ptylin enzymes digest a partial amount of complex or polysaccharide carbohydrates like starch into disaccharides like sucrose.

digestion of starch in buccal cavity

So after a long time chewing food bread give a sweet test. Then it is swallowed in the esophagus.

b) Digestion in Stomach:

Structure of stomach-

Structure of Stomach in Human

When the pulp of food reaches into the stomach from a buccal cavity, gastric glands of the stomach secret gastric juice which contains about-

Composition of gastric juice-

Water- 99.45%,   Inorganic substances- 0.15% (HCl, NaCl, KCl, etc.)
Organic substances- 0.40% (pepsinogen, gastric lipase, prorenin, mucin, etc.)

Here HCl makes medium acidic and disinfects food from the harmful germ.
Pepsinogen is converted by HCl into proteolytic enzyme pepsin which partially digests protein into polypeptide and peptones.

Digestion of Protein in stomach

In infants, HCl converts prorenin into renin enzyme which digests milk protein casein into calcium paracaseinate.

Digestion of Milk Protein(casein) in stomach

Gastric lipase partially digests fat into triglycerides.

Fat digestion in stomach

After 2-3 hours of digestion and mixing food converted into chyme. This form of food is forwarded into the duodenum through the pyloric valve.

c) Digestion in Small Intestine:

It can be divide into three parts-

Parts of Small Intestine

Digestion in the duodenum:

When food reaches the duodenum in the form of chyme from the stomach gall bladder releases bile and the pancreas releases pancreatic juice.
Composition of Bile-

Water- 98%,   Bile salts- 0.7%,   Bile pigments- 0.2 (bilirubin & biliverdin)
Fats- 0.51% (cholesterol, fatty acids, & lecithin)
Rest- other substances.

Works of bile:

1)-It makes acidic food of stomach alkaline so that pancreatic enzymes act on it.
2)- It emulsified fat particles by this process increased the area of the surface to help pancreatic enzyme action and absorption of it in the small intestine.

Composition of pancreatic juice-
Water- 98.5%
Organic substances-
Proteolytic enzyme( trypsin, chymotrypsin, and carboxypeptidase)
Amylolytic enzyme- pancreatic amylase
Lipolytic enzyme- pancreatic lipase and phospholipase
Neucliolytic enzyme- nucleotidase
Inorganic substances- various ions

Works of pancreatic juice:

Pancreatic juice plays a very important role in the process of digestion. It can be considered as mentioned below-

Action of pancreatic enzymes of food in duodenum

Function and Structure of Duodenum in Human

Digestion in jejunum:

Jejunum begins at the duodenojejunal flexure and ends at the ileum. It appears red in color due to a large number of blood vessels. Jejunum makes up about two-fifths of the small intestine.
The main function of the jejunum is the absorption of nutrients like carbohydrates, fatty acids, amino acids, etc. which is an extract of digested food of the duodenum. In this part of the small intestine, the digestion process is negligible.

Digestion in ileum:

It is the final and longest part of the small intestine. First It is specified for the absorption of vitamins B12 and the reabsorption of conjugated bile salts. After that, it also absorbs nutrients gained through final digestion occurs in itself through intestinal juice.
When food reaches in ileum intestinal glands secret digestive juices named succuss enterics. It is an alkaline solution (ph value 7.5-8.3) that contains various enzymes for the digestion of food. These name and functions are mentioned below:

1- Actions of amylolytic enzymes:

Carbohydrates digestion in ileum

2- Action of Lipolytic enzymes-

Fat digestion in ileum

3- Actions of Proteolytic and Nucleolytic enzymes-

Digestion of Protein and Nucleoprotein

d) Absorption of Nutrients:

the process of absorbing nutrients starts from the duodenum and ends in the large intestine. finger-like structure “villi” are found in the inner surface of the small intestine for the increasing area of absorption. These villi are also segmented microvilli for this purpose. Blood and lacteal vessels are found in these villi and microvillus for the absorption of nutrients.

Internal Structure of Small Intestine in Human
Mainly iron absorb into the duodenum, vitamin B12 and conjugated bile salts absorb in the jejunum and other nutrients like simple carbohydrates (monosaccharides: glucose, fructose, galactose, etc), fatty acids, glycerol, amino acids, phosphate, nitrogenous compounds absorb in the ileum. fatty products absorb in lacteal vessels and all other nutrients absorb in blood vessels.

The large intestine performs the vital functions of converting food into feces, absorbing essential vitamins, minerals, and water.


e) Assimilation of Absorbed Nutrients:

After absorption of nutrients in the blood and lacteal vessels, all of these nutrients carry to the liver through the Hepatic portal vein. Where some of these store in modified form and carry to body cells through the blood and lacteal vessels. Body cells assimilate these nutrients into their protoplasm to execute biochemical activities according to need.

The Process of Assimilation

f) Fecal excretion or Egestion:

After absorption of nutrients and water, the waste parts of the food convert into feces through the large intestine. The anus timely egests this feces.

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