Digestion and Absorption - Notes | Class 11 | Part 1: Human Alimentary Canal

16. DIGESTION AND ABSORPTION

• Nutrition is the getting and utilization of energy rich nutrients (food) by an organism.
• Food consists of carbohydrates, proteins, fats (lipids), vitamins, minerals and water.
• Food provides energy for life activities, materials for growth, maintains body temperature and repairs tissues.
• The water plays an important role in metabolic processes and prevents dehydration of the body.

I. ALIMENTARY CANAL (GUT)

It includes the following parts:

·   Mouth: To receive the food (ingestion).

·   Buccal cavity (oral or mouth cavity):

-   Consists of palate (roof), teeth & muscular tongue. 

-   Palate has anterior hard palate and posterior soft palate.

-  Tongue is a muscular organ attached to the floor of oral cavity by the frenulum. Tongue has small projections called papillae. Some papillae bear taste buds.

-   At the back, on either side of tongue tonsils present.

·   Pharynx:

-  Common passage for digestive and respiratory systems.

- When food materials pass through the pharynx, the cartilaginous epiglottis closes the glottis (opening of larynx) and prevents the entry of food into trachea.

·   Oesophagus:

-   Muscular tube (30 cm) that conducts food into stomach.

-  Posterior part of the oesophagus has gastro-oesophageal sphincter (a circular muscle). It controls the opening of oesophagus into stomach.

·  Stomach:

• ‘J’ shaped structure for storage and digestion of food.
• 4 parts: a cardiac portion into which the oesophagus opens, a fundic region, body (main central region) and a pyloric portion (antrum).
• Pyloric stomach leads to small intestine by an opening called Pylorus, guarded by pyloric sphincter muscle.
• Inner wall of stomach bears rugae (longitudinal folds).

·   Small intestine:

• Longest part of gut (7 m long and 2.5 cm diameter).
• Consists 3 parts, namely duodenum (C shaped first part), Jejunum (middle part) and Ileum (terminal part).
• Finger-like villi are seen at the mucosa. Each villus has a brush-bordered columnar epithelial layer provided with microvilli. Villus consists of a capillary network and a small lymph vessel (lacteal).

·   Large intestine:

• 1.5 m long. Consists of caecum, colon and rectum.
• Caecum is well-developed in herbivores but very small in man. Arising from the caecum is a finger-like vestigial organ, the vermiform appendix.
• The colon consists of ascending colon, transverse colon, descending colon and Sigmoid colon.
• Pelvic colon leads to rectum that opens out by anus. Anus is guarded by anal sphincter (circular voluntary muscles).
• In some herbivores, the large intestine consists of several cellulose digesting bacteria.

TEETH

Nature & mode of arrangement of teeth is called dentition.
Human dentition is Thecodont, Heterodont & Diphyodont.

• Thecodont: It means teeth are placed in the jaw sockets.
• Heterodont: It means different kinds of teeth are present. They are incisors (I) for cutting, canines (C) for tearing, premolars (PM) & molars (M) for mastication. Premolars & molars are collectively called as cheek teeth which have cusps. 
• Diphyodont: It means teeth appear twice in the lifetime. They are milk (deciduous) teeth and permanent teeth.

Milk teeth (20 in number) are erupted at 6-7 months of birth. They are replaced by permanent teeth (32 in number) at the age of 6-7.

Last 4 molars (wisdom teeth) appear only at the age of 18.

The hard chewing surface of teeth is made up of enamel.

Dental formula: It explains the kinds and number of teeth.

Human dental formula (of permanent teeth):  

I ²/₂  C ¹/₁  PM ²/₂  M ³/₃  x 2 = 32   

Dental formula of milk dentition:

I ²/₂  C ¹/₁  PM ⁰/₀  M ²/₂  x 2 = 20  

Histology of human gut (Transverse section)

Human gut is formed of 4 layers:

1. Mucosa: Innermost, moist epithelial layer. Contains secretory and absorptive cells.
2. Submucosa: Soft connective tissue layer just outside the mucosa. Nerves and blood vessels are present.
3. Muscularis: Outer to submucosa. Smooth muscle layer (inner circular & outer longitudinal muscles).
4. Serosa: Outermost fibrous layer.