Digestion of Food



Mechanical Digestion:

  • Within the buccal cavity, digestion kicks off with mechanical processes aimed at breaking down food into smaller, more manageable particles. 
  • Key functions of the buccal cavity include mastication, or chewing, and facilitating swallowing. 
  • Teeth and the tongue, aided by saliva, meticulously grind and mix food, transforming it into a cohesive mass known as a bolus. 
  • Saliva, rich in mucus, lubricates and binds the masticated food particles together, forming the bolus.



  • The bolus is propelled into the pharynx and subsequently into the oesophagus through the process of swallowing or deglutition. 
  • This coordinated action ensures the smooth passage of food from the mouth to the digestive tract.



  • In the oesophagus, the bolus is further propelled downward by peristalsis, a series of rhythmic muscular contractions. 
  • Peristalsis ensures the continuous movement of food toward the stomach, overcoming gravity's pull.


Chemical Digestion:

  • Meanwhile, within the buccal cavity, the chemical digestion process commences with the secretion of saliva. 
  • Saliva contains electrolytes and enzymes, including salivary amylase and lysozyme, which initiate the breakdown of carbohydrates. 
  • Salivary amylase catalyzes the hydrolysis of starch into maltose, a disaccharide, beginning the process of carbohydrate digestion. 
  • Additionally, lysozyme acts as an antibacterial agent, safeguarding against potential infections.


Gastric Glands and Their Cells:

  • Within the mucosa of the stomach lie gastric glands, composed of three principal types of cells:

- Mucus neck cells: These cells secrete mucus, providing lubrication and protection to the stomach lining. 

- Peptic or chief cells: Responsible for producing the proenzyme pepsinogen, which later converts into the active enzyme pepsin. 

- Parietal or oxyntic cells: These cells secrete hydrochloric acid (HCl) and intrinsic factor, essential for vitamin B12 absorption.


Digestive Functions of the Stomach:

  • The stomach serves as a temporary reservoir for food, storing it for approximately 4-5 hours. 
  • During this time, food undergoes thorough mixing with acidic gastric juice, forming a semi-fluid mixture called chyme. 
  • Churning movements of the stomach's muscular wall aid in this mixing process.


Enzymatic Actions:

  • Pepsinogen, released by chief cells, transforms into its active form, pepsin, upon exposure to hydrochloric acid. 
  • Pepsin acts as the primary proteolytic enzyme in the stomach, breaking down proteins into smaller peptides like proteoses and peptones. 
  • The presence of mucus and bicarbonates in gastric juice plays a vital role in lubricating and protecting the stomach's mucosal lining from damage caused by the acidic environment. 
  • Hydrochloric acid provides an optimal acidic pH (pH 1.8) for pepsin activity. 
  • In infants, gastric juice contains rennin, a proteolytic enzyme aiding in the digestion of milk proteins. 
  • Additionally, small amounts of lipases, enzymes that break down fats, are secreted by gastric glands.


Muscularis Layer Functions:

  • The muscularis layer of the small intestine orchestrates various movements vital for thorough mixing of food with digestive secretions, facilitating efficient digestion. 
  • These movements ensure optimal contact between food particles and digestive enzymes.


Digestive Secretions:

  • Bile, pancreatic juice, and intestinal juice are among the key secretions released into the small intestine to aid digestion. 
  • Bile and pancreatic juice are discharged through the hepato-pancreatic duct, bringing vital enzymes and substances for digestion.


Pancreatic Juice Composition:

  • Pancreatic juice contains inactive enzymes such as trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, and nucleases. 
  • Activation of trypsinogen into active trypsin occurs with the assistance of enterokinase, an enzyme secreted by the intestinal mucosa.


Bile's Role:

  • Bile, devoid of enzymes, comprises bile pigments (bilirubin and biliverdin), bile salts, cholesterol, and phospholipids. 
  • Its primary function involves emulsification of fats, breaking them down into tiny micelles for better digestion. 
  • Additionally, bile activates lipases, enzymes responsible for fat breakdown.


Intestinal Juice and Brush Border Enzymes:

  • Intestinal juice, or succus entericus, consists of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, and nucleosidases. 
  • Goblet cells in the intestinal mucosal epithelium secrete mucus, providing protection and lubrication to the intestinal lining. 
  • Bicarbonates from the pancreas, along with mucus, create an alkaline environment (pH 7.8) conducive to enzymatic activity, shielding the mucosa from acid.


Brunner’s Glands Contribution:

  •  Sub-mucosal glands, known as Brunner’s glands, further assist in protecting the intestinal mucosa and maintaining an alkaline environment for enzymatic reactions.


Breaking Down Nutrients for Absorption:

These final steps of digestion occur in close proximity to the mucosal epithelial cells of the intestine, facilitating efficient absorption of nutrients.

By breaking down complex molecules into simple absorbable forms, the digestive system prepares nutrients for absorption into the bloodstream.


Protein Digestion:

  • Proteolytic enzymes such as trypsin, chymotrypsin, and carboxypeptidase from pancreatic juice act on proteins, proteoses, and peptones in chyme, converting them into dipeptides. 
  • Dipeptides are further broken down by the enzyme dipeptidase into individual amino acids, the simplest absorbable form.





Carbohydrate Digestion:

  • Polysaccharides like starch in chyme are hydrolyzed by pancreatic amylase into disaccharides. 
  • Enzymes in succus entericus, like maltase, lactase, and sucrase, then break down disaccharides (maltose, lactose, and sucrose) into their respective monosaccharides (glucose, fructose).





Fat Digestion:

  • Lipases, aided by bile, break down fats into di- and monoglycerides in the presence of bile. 
  • These are further hydrolyzed into fatty acids and glycerol by enzymes in succus entericus.




Nucleotide Digestion:

  • Nucleotides in chyme are acted upon by nucleotidase enzymes in succus entericus, converting them into nucleosides. 
  • Nucleosides are then broken down into bases and sugars by the same enzyme.


Digestive Breakdown:

  • Biomacromolecules undergo breakdown primarily in the duodenum region of the small intestine, catalyzed by various enzymes. 
  • This process yields simple substances that are absorbable by the body.


Absorption Zones:

  • The jejunum and ileum regions of the small intestine serve as absorption sites for the simple substances formed during digestion. 

Large Intestine Functions:

  • While the large intestine doesn't engage in significant digestive activity, it performs crucial functions: Absorption of water, minerals, and certain drugs. 
  • Secretion of mucus, aiding in waste particle adherence and lubrication for easy passage.


Faecal Passage:

  • Undigested and unabsorbed substances, termed faeces, enter the large intestine's caecum via the ileo-caecal valve. 
  • Faeces are temporarily stored in the rectum until defaecation.


Neural and Hormonal Control:

  • The activities of the gastrointestinal tract are intricately regulated by neural and hormonal mechanisms. 
  • Factors such as sight, smell, and the presence of food trigger saliva secretion, while neural signals stimulate gastric and intestinal secretions. 
  • Muscular activities along the alimentary canal are modulated by local neural mechanisms and the central nervous system (CNS). 
  • Hormonal regulation of digestive juice secretion is facilitated by local hormones produced by the gastric and intestinal mucosa.


Absorption of digestive products

  • Absorption is the process whereby end products of digestion traverse the intestinal mucosa into the bloodstream or lymphatic system. 
  • It employs passive, active, or facilitated transport mechanisms.


Passive Diffusion:

  • Small amounts of monosaccharides (e.g., glucose), amino acids, and electrolytes (e.g., chloride ions) are typically absorbed via simple diffusion, driven by concentration gradients.


Facilitated Transport:

  • Some substances, like glucose and amino acids, require carrier proteins for absorption, termed facilitated transport.


Water Transport:

  • Water absorption relies on osmotic gradients, with movement occurring passively depending on concentration differences.


Active Transport:

  • Against concentration gradients, active transport necessitates energy expenditure. 
  • Nutrients such as amino acids, glucose, and electrolytes like sodium ions are absorbed into the bloodstream via active transport.


Fatty Acid Absorption:

  • Insoluble fatty acids and glycerol are encapsulated within micelles, facilitating their movement into the intestinal mucosa. 
  • They reassemble into chylomicrons, small fat globules coated with proteins, which are transported into lymph vessels (lacteals) in the villi. 
  • Eventually, these absorbed substances are released into the bloodstream.


Absorption Sites:

  • Absorption occurs across various parts of the alimentary canal, including the mouth, stomach, small intestine, and large intestine.
  • However, the small intestine facilitates the maximum absorption due to its extensive surface area and specialized structures like villi and microvilli.



  • Absorption of certain drugs occurs when they come into contact with the mucosa of the mouth and the underside of the tongue. 
  • These substances are absorbed into the blood capillaries lining these areas.



  • Absorption of water, simple sugars, and alcohol occurs in the stomach.


Small Intestine:

  • The primary organ for nutrient absorption. 
  • Digestion is completed here, and final products such as glucose, fructose, fatty acids, glycerol, and amino acids are absorbed through the mucosa into the bloodstream and lymphatic system. 
  • Villi and microvilli increase surface area for enhanced absorption efficiency.


Large Intestine:

  • Absorption of water, some minerals, and drugs occurs in the large intestine. 
  • Functions include reabsorption of water from undigested food and the absorption of certain minerals and medications.



  • After absorption, nutrients are transported to tissues where they are utilized for various cellular activities. 
  • This process, known as assimilation, ensures that absorbed substances contribute to the body's functions and energy production.



  • Digestive wastes, solidified into coherent faeces in the rectum, initiate a neural reflex signaling the urge for removal. 
  • Defaecation, the process of expelling faeces through the anal opening, is a voluntary action. 
  • It is facilitated by a mass peristaltic movement, where coordinated muscular contractions propel the faeces out of the body.


Disorder of digestive system

Understanding digestive disorders highlights the importance of maintaining a balanced diet, proper hygiene, and seeking medical attention for timely treatment and management.


Intestinal Inflammation:

  • Common ailment resulting from bacterial, viral, or parasitic infections.
  • Parasites such as tapeworms, roundworms, threadworms, hookworms, and pinworms can infect the intestinal tract, leading to inflammation.



  • Liver disorder characterized by yellowing of the skin and eyes due to the accumulation of bile pigments.
  • Liver impairment affects bile metabolism, resulting in pigment deposition and discoloration.



  • Ejection of stomach contents through the mouth, controlled by the vomit centre in the medulla.
  • Preceded by a feeling of nausea, vomiting is a reflex action triggered by various factors, including toxins, infections, or motion sickness.



  • Abnormal increase in bowel movement frequency and liquidity of faecal discharge.
  • Reduces nutrient absorption, often caused by infections, food intolerances, or intestinal disorders.



  • Retention of faeces in the rectum due to irregular bowel movements.
  • Causes include inadequate fiber intake, dehydration, lack of physical activity, or certain medications.



  • Occurs when food is not properly digested, leading to a feeling of fullness and discomfort.
  • Causes include inadequate enzyme secretion, anxiety, food poisoning, overeating, and consumption of spicy foods.