Absorption of food

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.

• Takes place, mainly in jejunum and ileum through villi and microvilli by both active and passive processes. It first enters mucosal cells and then passes into submucosa.
• No absorption occurs in stomach except that of ethanol (alcohol) & aspirin.
• Water is absorbed in small intestine and large intestine.
• Absorbed food is transported through two pathways : the hepatic portal system, directly to liver and through lymphatic channel it is drained into subclavian veins via thoracic duct.

Absorption of Glucose and Amino acids

Both are absorbed mainly by active transport as this process is many thousand times faster than passive transport.

Entry of glucose in the cell is coupled with Na+ where it is immediately phosphorylated.

Galactose is absorbed like glucose, some disaccharides are absorbed directly.

Fructose and mannose are absorbed by facilitated diffusion.

From submucosa these pass out through blood capillaries.

Absorption of Lipid

Glycerol and fatty acids in the cell generally combine to form lipid again. Thus lipid is absorbed in both forms:

(a) Directly as lipid :

Lipid enters mucosal cells as chylomicrons and micelles by pinocytosis in the crypts between villi.

Chylomicrons are lipid droplets of  < 1 µm diameter formed by emulsification. With nonlipid substance like salts, amino acids etc. it forms micelles.

In submucosa it enters lacteals and move with lymph.

(b) As glycerol and fatty acid :

Glycerol is soluble in both aqueous and lipid phases hence easily pass through cell membrane.

Small soluble fatty acids enter into cell as the membrane component, while large insoluble fatty acid after combining with alkali (of bile) forms soluble micelle.

Egestion (Efaecation or Defaecation) is the process of elimination of faeces.

Stercobilin causes the brown/yellowish color of faeces.

Foul odor of faeces is due to indole and skatole formed by decarboxylation of amino acids by bacteria in colon.

Autocoprophagy (Pseudorumination or Refecation)

Rabbit consumes its own faeces for recycling of cellulose and some useful substance left in it.

Flatulence

The gases in gastrointestinal tract inflates its different parts, which is released by belching and flatus.

In stomach most gases are N₂ and O₂.