Physiology of Digestion

Digestion is the process of conversion of complex food into simple (absorbable) form.

This involves two parts:

(i) Mechanical part - Cutting and chewing of food to increase its surface area for enzymatic action,

(ii) Chemical part - Enzymatic hydrolysis of food i.e. breaking at molecular level i.e., actual digestion.

It could be both extracellular and intracellular, but secures optimal efficiency of extracellular digestion.

Discovered by Beaumont (1833).

It begins from mouth and continue upto small intestine, in herbivorers digestion also takes place in the caecum but with the help of bacterial enzymes.

In Mouth

With chewing of food the saliva is secreted (= 1.5 – 2 litres in 24 hrs. in man).

Saliva contains - 98% water, and rest is mucin and enzyme, ptyalin (amylase or zymase) and lysozyme, pH is 6.8 to 7.2.

Ptyalin is present in the saliva of man and pig but absent in ruminants and carnivores.

Functions of Saliva

Moistens and softens the food and lubricates its passage.

Facilitates speaking and free movement of tongue by keeping it moist.

The lysozyme kills bacteria; amylase breaks starch into dextrin and maltose.

After treatment in mouth the food called bolus passes into stomach.

In Stomach

The activity begins under both neuronal and hormonal controls.

The hormone gastrin is secreted by stomach wall upon entry of food.

Medulla oblongata in brain is the centre for neuronal control through parasympathetic (vagus) nerves of ANS in both following conditions :

(a) Upon entry of food : through stretch receptors in stomach wall.

(b) Without entry of food : through sight, smell and taste of food and also as conditioned reflex.

I.P. Pavlov (1932) discovered the phenomenon of conditioned reflex. One of the experiments was made through “mucosal pouch” connecting it externally to the stomach of dog which

showed all the changes occurring in stomach. This pouch is known as Pavlov’s pouch.

Secretion of HCl

Parietal cells secrete 0.2 – 5% HCl (1.5 to 2 litre in 24 hrs).

Stomach wall is protected from HCl by mucous coat.

Functions of HCl : (in order of importance)

Converts bolus medium to acidic (pH = 2.5 – 4) for the action of gastric enzymes.

Activates enzymes like Pepsinogen to Pepsin and Prorennin to Rennin.

Precipitates protein from rest of food (i.e. curdling of milk).

Kills germs (or bacteria) and other living organisms in the food.

Dissolves the inorganic substances like bones, stones.

Secretion of Enzymes

1. Pepsin (a type of endopeptidase) breaks protein into peptones, proteoses and polypeptides

2. Rennin (chymosin) converts caesin (milk protein) into calcium paracaesinate (precipitate)

3. Gastric Lipase (weak enzyme, only in human), breaks lipid into glycerol + fatty acid.

In stomach mainly protein and only a trace amount of lipid are digested.

Mouth and stomach are the site of partial digestion.

After treatment in stomach the food called chyme enters duodenum.

In duodenum

Stimulated by acidic chyme a number of hormones is secreted from the duodenal wall to control various processes.

Hormones Stimulates secretion of         

1.   Cholecystokinin  →  Bile from gall bladder or liver

2.   Pancreozymin  → Pancreatic juice (enzymes)

3.   Secretin → Pancreatic juice with higher salt ratio.

4.   Enterocrinin → Succus entericus from intestinal wall.

5.   Enterogastrone → Inhibits the activity of stomach.

Secretion of Bile

0.5 to 1 litre (in 24 hrs.) bile (pH = 9.0) is released into duodenum through hepatopancreatic duct by relaxing sphincter of Oddi.

It is secreted by liver and stored in gall bladder but has no enzyme for digestion of food.

Function:

Converts the medium of acidic chyme into alkaline for the action of pancreatic and intestinal enzymes.

Emulsification (saponification) of fat by reducing surface tension of fat droplet thus helps in digestion and absorption of fat.

Helps in absorption of Vit A, D, E; kills germs/bacteria thus sterlizes the food.

Pancreatic Secretions

Watery fluid with enzymes and other organic and inorganic substances mainly HCO3– and Cl– , Na+ and K+.

Enzymes and their actions are as follows :

1. Trypsin a proteolytic enzyme, is the oldest known enzyme and the first enzyme to evolve in animal kingdom; acts as both exopeptidase and endopeptidase; breaks protein into amino acids and polypeptides.

Ssecreted as inactive trypsinogen and activated by the enzyme, enterokinase (enteropeptidase) from intestinal wall.

2. Chymotrypsin first precipitates the milk protein (caesin) and also breaks it as amino acids and polypeptides

3. Amylopsin breaks carbohydrates into disaccharides and monosaccharides

4. Steapsin (Pancreatic lipase) hydrolyses lipid into glycerol + fatty acid

About two third of fat are digested this way.

Other enzymes like collagenase and elastase (peptidase of broad specificity) are also present.

Carboxypeptidase hydrolyse the terminal peptide bond of peptide chain at the carboxy terminal.

In ileum :

Site of final digestion; succus entericus is a mixture of many enzymes, as follows :

1. Erepsin is a group of proteolytic enzyme converts all the left out protein into amino acids.

2. Glycosidases, mainly as disaccharidase are of 3 types

(a) Sucrase (or Invertase) breaks sucrose into glucose + fructose

(b) Lactase breaks lactose into galactose + glucose

(c) Maltase breaks maltose into glucose + glucose

3. Intestinal Lipase breaks lipid into glycerol + fatty acids.

In old age due to shortage of lactase milk is not digested properly. This results into formation of intestinal gases, alcohols and acids to cause flatulence, intestinal cramp and diarrhoea.

Enzymes like nucleosidase, nucleotidase are also present in the small intestine.

In Large Intestine

Part of food passing into colon is referred as chyle. This is degraded into faeces by bacteria. The absorption of water electrolytes and ions takes place here.

In herbivores cellulose is digested in caecum but by bacteria living in vermiform appendix.