Digestion is the process by which food is broken down into smaller molecules that can be absorbed and used by the body. It begins in the mouth, where enzymes in saliva begin to break down carbohydrates. The food then passes through the esophagus and into the stomach, where it is mixed with acidic gastric juices that help break down proteins. From there, the partially digested food enters the small intestine, where enzymes from the pancreas and the lining of the intestine further break down carbohydrates, proteins, and fats. The nutrients are then absorbed into the bloodstream and transported to the liver, where they are processed and distributed to the rest of the body. Waste products are eliminated through the large intestine and rectum. Digestion is a complex process that involves the coordinated action of multiple organs and enzymes, and disruptions in any of these steps can lead to digestive disorders and nutrient deficiencies. The physiology of Digestion will be described in detail here.

Digestion is accomplished by mechanical and chemical processes.

Digestion in Buccal / Oral cavity; Mouth

• The buccal cavity performs two major functions, mastication of food and helping in 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 esophagus by swallowing and down through the esophagus by successive waves of muscular contractions called peristalsis.
• The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase, and lysozyme.

                                               Starch  +   Ptyalin (enzyme)   -> >>  Maltose

• The chemical process of digestion is initiated in the oral cavity by enzyme, the salivary amylase. About 30 percent of starch is hydrolyzed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose. Lysozyme present in saliva acts as an antibacterial agent that prevents infections.

Digestion in Stomach

The mucosa of the 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 HCl and intrinsic factor
(factor essential for the 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 proteases and peptones (peptides).

                                 Proteins + Pepsin(enzyme)  ->>>>> Proteases and Peptones

• 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. HCl provides the acidic pH (pH 1.8)
  optimal for pepsins.
• Rennin is a proteolytic enzyme found in the gastric juice of infants which helps in the digestion of milk proteins.

                 Casein (milk protein) + Renin(enzyme)  ->>>>> Paracaesin

                            Paracaesin + Calcium ->>>>> Calcium paracaseinate

                        Calcium paracaseinate + pepsin (enzyme) ->>>> Peptones

• Small amounts of lipases are also secreted by gastric glands which helps in digesting fats into fatty acids.

Fat + Lipase (enzyme) ->>>>> Fatty acid + Glycerol

Digestion in Small Intestine

• The bile, pancreatic juice, and 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.

• The bile released into the duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol, and phospholipids but no enzymes. Bile helps in the emulsification of fats, i.e., breaking down the fats into very small micelles. Bile also activates lipases.
• The intestinal mucosal epithelium has goblet cells that secrete mucus.
  The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice.
• This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 8) for enzymatic activities.
• 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.

Digestion in Large 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 ileocaecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defecation.

Absorption

• 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.
• Water moves by osmosis; small fat-soluble substances, e.g. fatty acids and glycerol, are able to diffuse through cell membranes; while others are generally transported inside the villi by other mechanisms.

Passive transport:

• Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion.
• The passage of these substances into the blood depends upon the concentration gradients.

Active transport:

• Active transport occurs against the concentration gradient and hence requires energy.
• Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
• Some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called the facilitated transport.
• 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. The absorbed substances
  finally reach the tissues which utilise them for their activities. This process is called assimilation.  

 Defecation

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

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