Process of Digestion

The process of digestion is accomplished by mechanical and chemical processes.

The buccal cavity performs two major functions, mastication of food and facilitation of swallowing. The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus. The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.

The gastro-oesophageal sphincter controls the passage of food into the stomach. The saliva secreted into the oral cavity contains electrolytes - (Na⁺, K⁺, Cl^–, HCO₃^–) and enzymes, salivary amylase and lysozyme.
The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide - maltose. Lysozyme present in saliva.

The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely –

(i) mucus neck cells which secrete mucus;

(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and

(iii) parietal or oxyntic cells which secrete HCI and intrinsic factor (factor essential for absorption of vitamin B₁₂. 

The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme. The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins into proteoses and peptones (peptides).

The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. HCI provides the acidic pH (pH 1.8) optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.

Various types of movements are generated by the muscular is layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.

The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. Pancreatic juice and bile are released through the hepato-pancreatic duct. The pancreatic juice contains inactive enzymes - trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the/pancreatic juice. The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.

The intestinal mucosal epithelium has goblet cells which secrete mucus. The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner's glands) also help in this.

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ilea-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored­in the rectum till defaecation. 

Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below: 

Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

Polysaccharides (starch)+Amylase →Disaccharides Fats are broken down by lipases with the help of bile into di and monoglycerides.

Fats +Lipase →Diglycerides → Monoglycerides Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides.

Nucleic acids +Nuclease → Nucleotides → Nucleosides

The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.

Dipeptides + Dipeptidases → Amino acids

Maltose +Maltase → Glucose + Glucose

Lactase + Lactase → Glucose + Caactose

Sucrose+ Sucrase → Glucose + Fructose

Nucleotides + Nucleotidases → Nucleosides → Sugars + Bases

Di and Monoglycerides +Lipase → Fatty acids + Glycerol

The breakdown of biomacromolecules mentioned above occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine. The undigested and unabsorbed substances are passed on to the large intestine. No significant digestive activity occurs in the large intestine. The functions of large intestine are:

(i) absorption of some water, minerals and certain drugs;

(ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage:

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defaecation.

Absorption of Digested Products

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It is carried out by passive, active or facilitated transport mechanisms.

Small amounts of monosacharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients. However, some of the substances like fructose and some amino acids are absorbed with the help of the carrier ions like Na⁺. This mechanism is called the facilitated transport.

Transport of water depends upon the osmotic gradient. Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosacharides like glucose, electrolytes like Na⁺ are absorbed into the blood by this mechanism.

Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. They are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream.

Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine.

There are two general pathways for the transport of materials absorbed by the intestine; the veins of the hepatic portal system which lead directly to the liver; and the lymphatic vessels of the intestinal area, which eventually lead to the blood by way of the lymphatic system and the thoracic duct. 

The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation.

The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

Absorption of amino acids and protein: It is probable that under normal circumstances the dietary proteins are almost completely digested to their constituent amino acids and that these end products of protein digestion are then actively transported from the intestine into the portal blood. Surplus amino acids are also withdrawn from portal blood by liver cells and deaminated into ammonia and keto acids. The ammonia is converted to urea and released into blood for excretion by kidneys, while the keto acids are converted to glucose or pyruvic acid and utilized for energy-production or for storage as glycogen and fat.

Absorption of carbohydrates: The products of carbohydrate digestion is absorbed from the intestine into blood of the portal venous system in the form of monosaccharides, chiefly the hexoses (glucose, fructose, mannose and galactose).

Absorption of fats: The dietary fat is digested, by the action of the pancreatic lipase present in the intestine, partially into glycerol and fatty acids and partially to split products such as mono acyl glycerols. These products of fat digestion enter the mucosal cells of the small intestine in the forms of micelles, fatty acids and glycerol.  

By the lacteals, the fat is carried to the cisterna chili (meaning 'the receiver of the chyle') and then by the thoracic (lymph) duct to the left branchiocephalic vein, where it enters the blood The lymph reaching the thoracic duct from the intestines contains an excess of fat giving it a milky appearance. It is called chyle. In this way, fatty ad and glycerol are eventually brought into the blood stream and so, by a circuitous route, to the liver In the liver, they are reorganized and recombine to form human fat.

Absorption of vitamins: Water-soluble vitamins like members of B complex (except B₁₂ and vita C readily diffuse across the walls of the intestin into the blood. The fat-soluble vitamins A, D, and K are dissolved in micelles, which enter the mucosal cells of the intestine, by simple diffusion The absorption of these fat-soluble vitamins i markedly decreased in the absence of bile.

Disorders of Digestive Sytem

Jaundice: The liver is affected skin and eyes turn yellow due to the deposit of bile pigments.

Vomiting: It is the ejection of stomach content through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling a nausea precedes vomiting.

Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.

Constipation: In constipation, the faeces are retained within the rectum as the bowel movemen occur irregularly.

Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.

+NOTE:

The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts. The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.

Gastric and intestinal secretions are also, similarly: stimulated by neural signals. The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.

Hormonal control of the secretion of digestive juices is earned out by the local hormones produced by the gastric and intestinal mucosa.

Oesophagus does not secrete any enzyme.

Taste of chilli is not real taste but it is burning sensation of nerves.

Salivary juice:

Amount: 1.0-1.5 litre/day

Chemical nature: Slightly acidic.

pH: 6.3 - 6.8

Control of secretion: Autonomic reflex parasympathetic nervous system increases salivation while sympathetic nervous system inhibit secretion)

Chemical composition; Water (99.5%), mucous acts as lubricant), salts (NaCl, NaHCO₃ etc.), enzymes (ptyalin, lysozyme) etc

 Gastric juice

Amount: 2-3 liters/ day.

Chemical nature: Highly acidic

pH: 1.0 - 3.5 (due to presence of HCI)

Control of secretion: By gastrin hormone:

Chemical composition: Water (99%), mucous, morganic salts, castle's intrinsic factor, HCI (0.5%, cone.) and enzymes prorennin and pepsinogen and gastrin lipase.

Succus entericus (intestinal juice)

Amount: 1.5 - 2.0 1/day.

Chemical nature: Alkaline.

pH: 7.6-8.3

Control of secretion: Nervous and hormonal (Enterocrinin, Duocrinin etc.)

Chemical composition: Water (99%), mucous, inorganic salts, enzymes etc.

Pancreatic juice

Amount: 1-1.5 1/day

Chemical nature: alkaline

pH: 7.1-8.2

Control of secretion: Hormonal and normal mechanism

Secretin hormones stimulate the production of more alkaline pancreatic juice but low in enzyme content. Pancreozymin or Cholecystokinin stimulates the production of enzyme rich pancreatic juice.

Chemical composition: Water (99%), enzymes and salts.

Bile/chole

Amount: 800-1000 ml daily. On the average about 700 ml.  

Source: Secreted by hepatic cells

Storage' site: Gall bladder

Colour: Greenish-blue

Chemical nature: Alkaline

pH: 7.6-8.6 

The Summary of Absorption in Different Parts of Dizestive Svstem:

Formula of Dental 

Gastrointestinal Hormones

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