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digestion and absorption

 


What is Digestion?

Digestion is breakdown of food by both chemical and mechanical processes into small and simple chemical compounds which absorbed into the blood stream.

Breaking down of larger food molecules into small molecules for use by body cell is called digestion.



1. Mechanical Breakdown (chewing)

2. Chemical breakdown by Enzyme


What is Absorption?


Absorption is the process by which digested food passes from the gastrointestinal tract into the cardiovascular (circulatory system) and lymphatic system for distribution.


1. Digestion in the Mouth


After ingestion food - taking food into the alimentary track - process of digestion start in the mouth.


Mechanical Digestion

Mechanical digestion occurs- 

When food taken into mouth,

Mastification or chewed by teeth,

Moved round the mouth by the tongue and muscles of the cheeks,

Mixed with saliva and form bolus,

Ready for deglutition or swallowing


Chemical Digestion

Chemical digestion involve -

Salivary amylase or ptyalin enzyme secreted by salivary gland - Initiated breakdown of starch.


Carbohydrates are either Monosaccharides, Disaccharides, Polysaccharides 

Amylase breakdown of chemical bound.

Saliva contains lingual lipase secreted by glands in tongue.

Start digestion of dietary triglycerides into fatty acids and monoglycerides.


Digestion in the Stomach



Mechanical Digestion

Mechanical digestion occurs when-

Through esophagus food reaches the stomach. Stomach causes mixing and liquification of bolus of food into rhyme. Breakdown bolus and mix it with gastric juice. Mixing waves pass over stomach every 15-25 sec. 

Propel the stomach content toward pyloric sphincter,

Food remain in fundus1/2 an hours, digestion of starch.



Chemical Digestion


Chemical digestion involves digestion of proteins.

Pepsin secreted by stomach - Pepsin breakdown certain peptide bonds between amino acids making up proteins. 

Principal chemical activity of stomach begin digestion of protein. Digestion of fat, gastric lipase enzyme of stomach splits short chain triglycerides molecule.


Digestion of Carbohydrate


Infant stomach secrete Rennin - Important in digestion of milk.


Absorption in the Stomach


Absorption in stomach occurs only small quantity of water, alcohol, glucose, electrolytes, asprin absorbed through gastric mucosa.



Digestion in the Small Intestine


When acidic chyme enters into small intestine. It mixed with pancreatic juice, bile and intestinal juice.

In small intestine digestion of all nutrient completed.

Carbohydrates to monosaccharides

Proteins to amino acids

Fats to fatty acids and glycerol


Mechanical Digestion


Movement of small intestine -

1. Segmentation 

2. Peristalsis


1. Segmentation

Segmentation is major movement of small intestine. It mixes chyme with the digestive juice and bring the particles of food into contact with mucosa for absorption.

Segmentation depends mainly on intestinal distension which initiates nerve impulses to the central nervous system.

Returning parasympathetic impulses to increase motility.

Sympathetic impulses decrease intestinal motility.


2. Peristalsis

Peristalsis is a progressive type of movement.

Propelling material from point to point in GastroIntestinal track (GI).

Peristalysis propels chyme through the intestinal track

Chymes move through small intestine at rate 1cm/ minute

Chyme remains in small intestine for 3 to 10 hours.

Peristalsis initiated by mucosal stretching of longitudinal and circular muscle layers when chyme enters a segment of intestine.

Peristalsis is controlled by autonomic nervous system.


Chemical Digestion


In mouth - salivary amylase converts starch to maltose.

In stomach - Pepsin convert protein to peptides.

Chyme entering small intestine.


Chemical Digestion in the small intestine involve - 

1. Digestion of carbohydrate

2. Digestion of proteins

3. Digestion of lipids

4. Digestion of Nucleic acid


1. Digestion of Carbohydrate

Pancreatic amylase enzyme in pancreatic juice act in small intestine convert all digestible polysaccharide (starch).

Brush border enzymes α-dextrin act on the a-dextrins (breaks into glucose).


There are 3 brush border enzyme digest the disaccharides into monosaccharides inside the cell of villi -


1. Maltase

Maltase splits- maltose and maltotriose into two or three molecules.

2. Sucrase

 Sucrase breaks - sucrose into a molecules of glucose and a molecules of fructose.

3. Lactase

 Lactase digest lactose into a molecules of glucose and molecules of galactose.


2. Digestion of Protein

Protein digestion starts in the in the stomach.

10-20% of total ingested protein - digested - by gastric pepsin into peptides.


Enzyme found in pancreatic juice -

Trypsin

Chemotrypsin

Elastase

Carboxypeptidase


1. Trypsin, Chymotrypsin and Elastase


Trypsin, chymotrypsin - convert some polypeptides to amino acids.

Trypsin, Pepsin, chymotrypsin , elastase - all convert whole proteins into peptides.


2. Carboxypeptidase

Carboxypeptidas act on peptides - breaks the peptide bond that attaches the terminal amino acid to the carboxy (acid) end - split off amino acid from carboxyl end.


3. Peptidases

Protein digested completed by peptidase produced by small intestinal cell.

1. Aminopeptidase

 Act on peptides - break peptide bonds that attach amino acid.


2. Dipeptidas

Cleaved dipeptides into amino acids that can be absorbed.

Protein food contains -

Large molecule polypeptides

Smaller polypeptides

Smaller peptides

Amino acids


3. Digestion of Lipids


Almost all lipid digestion occurs in the small intestine.


1. Emulsification

Emulsification is first step in digestion of lipid. Process of transformation of large molecules into smaller droplets.

Emulsification process occurs in small intestine, bile is secreted by liver and stored in gall bladder, passes into the duodenum along with the pancreatic juice.

1. Bile salts, sodium taurocholate and sodium glycocholate break the fat into droplets.

2. Pancreatic lipase enzyme found in pancreatic juice, it hydrolysis or convert fat molecules into fatty acids and monoglycerides.

3. Intestinal lipase enzyme secreted by gland of small intestine, it splits natural fat to fatty acids and glycerol. This enzyme is present in intestinal mucosal cells.


4. Digestion of Nucleic Acid


1. Nucleases 
Pancreatic Juice contains Nucleases that digest RNA and DNA into Nucleotides.
Ribonuclease act on Ribonucleic acid. 
Deoxyribonuclease acts on Deoxyribonucleic acid.

2. Nucleosidases and Phosphatases
Brush border enzymes digest the nucleotide into pentoses, phosphates and nitrogenous bases.

Absorption of material in small ingesting through villi and depend on -

a. Diffusion
b. Osmosis
c. Active Transport

Glucose, protein, fat and water soluble vitamin are absorbed primarily in jejunum.
Ileum - site of bile salt absorption and reserve to absorb glucose, protein, fat, water, soluble vitamin.
Vitamin B12 absorption takes place in ileum.
Iron absorption - in duodenum and jejunum.
Absorption of nutrients through epithelial cells of the villi.
Absorption of nutrients into the blood and lymph.

Absorption in Small Intestine

1.Carbohydrates

2. Lipids

3. Water

4. Electrolytes

5. Proteins 

6. Vitamins



1. Absorption of Carbohydrates


All carbohydrates are absorbed as monosaccharides.
Glucose and galactose - transported into epithelial cell of the villi by active sodium transport.
Fructose - transported by facilitated diffusion.
Enters the capillaries of villi and then they are transported into bloodstream to the liver via hepatic portal system then to heart and finally enter general circulation.

2. Absorption of Proteins

95%-98% proteins absorbed in small intestine.
Proteins are absorbed as amino acids.
Protein absorption - mostly in duodenum and jejunum
Amino acid transport into epithelial cells of villi an active transport.
Move out by diffusion
Enters to the blood stream.

3. Absorption of Lipids

Adult about 95% of lipid present in small intestine are absorbed.
Infants, about 85% of lipid are absorbed.
As a result of emulsification and fat digestion, neutral fat (triglycerides)- breaken down- into triglycerides and fatty acid.
These pass into epithelial cells by diffusion to enter blood stream.

4. Absorption of Water

About 8 liters of water is absorbed in the small intestine.
Remaining about 1 litre passes into large intestine.
Absorption of water occurs by osmosis from lumen of intestine through epithelial cells and 
Into blood capillaries in the villi.
Normal rate of absorption is about 200-400ml/hour.


5. Absorption of Electrolytes


Absorbed by small intestine.

Constituents of gastrointestinal secretion.

Component of ingested foods and liquids.


Sodium

It is able to move into the absorptive epithelial cells by diffusion and secondary active transport.

Negatively charged bicarbonate, chloride, iodine, and nitrate ions- actively transported.

Calcium ions actively transported.

Iron, potssium, magnesium and phosphate-move by active transport.


6. Absorption of Vitamins
Fat-soluble vitamin( A, D, E and, K) are absorbed.
Water soluble vitamins (B,C) are absorbed by diffusion.
Vitamin B12 absorption - in ileum
Surface area through which absorption take place - greatly increased by circular folds of mucosa  and submucosa, and - by large number of villi present in small intestine.

Digestion in Large Intestine


Only 10% of all absorption of nutrients occurs in stomach and large intestine.
Mechanical Digestion
Passage of chyme from ileum into cecum is regulated by action of ileocecal sphicter.
When substance enter into colon, movement of colon starts.
Food passes through ileocecal sphincter, it fills cecum and accumulates in ascending colon.







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