Digestion and Absorption | Zoology Optional Notes for UPSC PDF Download

Digestion Process

1. Mouth and Oral Cavity:

   • Mouth: Oval-shaped cavity for eating and speaking.
   • Oral Cavity: Includes tongue, palate, and teeth.
   • Tongue: Movable muscular organ with papillae and taste buds.
   • Teeth: Humans have two sets - deciduous (temporary) and permanent (32 teeth).

2. Pharynx:

   • Passage for food to the esophagus and air to the trachea during respiration.
   • Epiglottis prevents food from entering the trachea during swallowing.
3. Stomach:
   • Located in the upper-left abdominal cavity.
   • Divided into cardiac, fundus, body, and pyloric parts.
4. Small Intestine:
   • Longest part of the alimentary canal.
   • Divisions: Duodenum, Jejunum, and Ileum.
5. Large Intestine:
   • Connected to the small intestine through the Ileum.
   • Divisions: Caecum, Colon, and Rectum leading to the anus.

Digestion and Absorption

• The digestive system converts complex food into simple, absorbable forms.
• Biomacromolecules (carbohydrates, proteins, fats, vitamins, minerals) are broken down.
• Alimentary canal and associated glands make up the digestive system.

This process allows the body to extract energy and essential nutrients from the ingested food. Each part of the digestive system plays a crucial role in ensuring effective digestion and absorption.

Digestive Glands

1. Salivary Glands:

   • Produce saliva in the mouth.
   • Classified as parotid glands, submandibular glands, and sublingual glands.
2. Liver:
   • Located in the abdominal cavity, the largest gland.
   • Divided into left and right lobes.
   • Filters blood from the digestive tract, distributing it to the body.
   • Works with the gallbladder, pancreas, and intestine.
3. Pancreas:
   • Divided into endocrine and exocrine glands.
   • Exocrine glands produce enzymes for carb, fat, protein, and acid breakdown in the duodenum.
   • Endocrine glands secrete insulin and glucagon, regulating blood glucose levels.

Digestion and Absorption Process

1. Mechanical Digestion:

   • Starts in the oral cavity through chewing.
   • Continues in the stomach where churning and segregation occur.
   • Peristalsis: Involuntary contraction and relaxation of muscles in the esophagus, stomach, and intestines during food breakdown.

2. Chemical Digestion:

   • Takes place in the small intestine.
   • Enzymes are secreted to break down fats into fatty acids, carbohydrates into monosaccharides, proteins into amino acids, and nucleic acids into nucleotides.

Digestion Initiation:

• Begins in the oral cavity.
• Various digestive enzymes act at different stages of the process.

Mechanical Digestion:

• Involves chewing, churning in the stomach, and segregation in the small intestine.
• Peristalsis aids in the involuntary breakdown of food.

Chemical Digestion:

• Occurs in the small intestine.
• Enzymes break down macromolecules into their simpler forms.

This process is vital for the body to absorb essential nutrients from food, enabling proper functioning and energy production.

Absorption of Digested Products

1. Absorption Mechanism:

   • Occurs when end digestive products pass through the intestine into the bloodstream or lymph.

2. Facilitated Transport:

   • Fructose and amino acids are absorbed using special carriers like ions (Na+), known as facilitated transport.

3. Insoluble Components:

   • Fatty acids and glycerol are converted into micelles before entering the intestinal mucosa.

4. Chylomicron Formation:

   • Micelles reform into chylomicrons, a protein transported to the lacteals in the villi.

5. Assimilation:

   • Tissues utilize absorbed substances for their activities.

6. Faecal Removal:

   • The removal of faeces through the anus is a voluntary action carried out by peristaltic movement.

Digestive System Disorders

1. Jaundice:

   • Results in yellowing of the skin, sclera, and mucous due to high bilirubin levels.

2. Vomiting:

   • Not a disease but a symptom of various illnesses.
   • Controlled by the medulla oblongata and often associated with stomach uneasiness.

3. Diarrhoea:

   • Frequent discharge of loose, watery stools.
   • Reduces nutrient and fluid absorption.

4. Constipation:

   • Bowel movements become less frequent, and faeces are retained in the colon.
   • Caused by changes in routine, inadequate water intake, or lack of fiber.

5. Indigestion:

   • Upset stomach causing discomfort, abdominal pain, and a feeling of fullness.
   • Results from inadequate enzyme secretion, food poisoning, anxiety, overeating, or spicy food intake.

These disorders highlight the importance of a healthy digestive system for overall well-being.