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NEET Revision Notes

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Digestion

  • Process of conversion of complex food substances to simple absorbable forms is called digestion.
  • Intracellular: When the process of digestion occurs within the cell in the food vacuole, then it is termed as Intracellular Digestion.
    Examples: Protozoa, Porifera, Coelenterata and free-living Platyhelminthes
  • Extracellular: When the process of digestion occurs outside the cell, then it is termed as Extracellular Digestion.
    Examples: Coelenterates and phylum Platyhelminthes to phylum Chordata.

Digestion in vertebrates occurs in the digestive tract or alimentary canal. 

  • The various parts involved in digestion can be broadly grouped in two groups:
    (a) Digestive tract or alimentary canal
    (b) Digestive glands
  • On the basis of the embryonic origin, the alimentary canal of vertebrates can be divided into three parts:
    (i) Fore gut / Stomodaeum: Ectodermal. It includes buccal cavity / oral cavity, pharynx, oesophagus, stomach and small part of duodenum.
    (ii) Mid gut/Mesodaeum: Endodermal. It includes small intestine and large intestine.
    (iii) Hind gut/Proctodaeum: Ectodermal. It includes the anal canal and anus.

Human Digestive System

  • The human digestive system is a complex series of organs and glands that processes food. 
  • It converts ingested food so that it can be assimilated by the organism.
    The human digestive system consists of following parts:
     Notes | EduRev

1. Mouth

  • Mouth is also known as the oral cavity or buccal cavity
  • It is the first portion of the alimentary canal
  • Food and saliva are received by mouth. 
  • Mouth has inner lining of mucous membrane epithelium.
     Notes | EduRev
    Oral Cavity

2. Digestive Glands
(i) Salivary Glands

  • These are the exocrine glands that produce saliva.
  • These are the glands with ducts which also secrete amylase.
  • Amylase is an enzyme that breaks down starch into maltose.
  • Three types of salivary glands are:
    (a) Parotid Gland
    (b) Submandibular Gland
    (c) Sublingual Gland

(ii) Gastric Glands

  • The gastric glands (fundic gland) secrete acids and digestive enzymes.
  • Secretion of gastric gland is called gastric juice.
  • There are approximately 35 million gastric glands present in human stomach.

(iii) Intestinal Glands

  • Intestinal glands in mammals is a collective name for crypts of Liberkuhn (secretes alkaline enzymatic juice) and Brunner’s glands (secrete mucous).
  • Intestinal glands secrete intestinal juice or succus entericus.

Question 1:Brunner’s glands occur in

(iv) Pancreas
 Notes | EduRev

  • Pancreas has two different kind of tissue- exocrine and endocrine.
  • Pancreatic secretion is stimulated by cholecystokinin and secretin both.
  • Complete digestive juice is pancreatic juice as it contains amylolytic, lipolytic and proteolytic enzymes.
  • It produces several important hormones like insulin, glucagon, somatostatin and pancreatic polypeptide.

Question 2:Pancreas produces

(v) Liver

  • Liver is the largest digestive gland of the body, weighing about 1.2 to 1.5 Kg in an adult human.
  • It is situated in the abdominal cavity, just below the diaphragm and has two lobes (small left and large right lobe).
  • The liver has a wide range of functions to perform in the body:
    (a) It detoxifies various metabolites.
    (b) It helps in protein synthesis.
    (c) Various biochemical necessary for digestion are produced by liver.


3. Pharynx

  • It is the opening of oral and nasal cavities.
  • It is classified as:
    (i) Nasopharynx
    (ii) Oropharynx
    (iii) Laryngopharynx
     Notes | EduRev

4. Oesophagus

  • Oesophagus connects pharynx with stomach. Opening of oesophagus is regulated by gastro-oesophageal sphincter.
5. Stomach
  • It is a J-shaped, muscular, hollow and dilated part of the digestive system. It is located between the oesophagus and the small intestine. 
  • It has 1 liter capacity. It secretes protein-digesting enzymes (proteases) and strong acids which aid in food digestion.
     Notes | EduRev
  • The stomach has three parts:
    (i) Cardiac: The part of the stomach into which oesophagus opens.
    (ii) Fundus: It is the air-filled portion of stomach.
    (iii) Pyloric: The portion of the stomach that opens into the small intestine.
6. Small Intestine
  • It is the part of the gastrointestinal tract that comes after the stomach and is followed by the large intestine.
     Notes | EduRev
  • Small intestine distinguished into three parts:
     Duodenum: It is ‘U’ shaped first part of the small intestine.
     Jejunum: It is the longer, coiled middle portion.
     Ileum: Ileum is the highly coiled posterior part of the small intestine.
7. Large Intestine

Large intestine consists of three parts:
(i) Caecum: It is a small blind sac. Vermiform appendix is a finger-like blind tubular projection of caecum.
(ii) Colon: The Caecum opens into colon. Colon has three distinct parts-

  • Ascending colon
  • Transverse colon
  • Descending colon

(iii) Rectum: It is the final straight portion of the large intestine.

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