Digestion and Absorption | Plus One | Exam Capsule Notes (Web and PDF)

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DIGESTION AND ABSORPTION: CHAPTER AT A GLANCE 

HUMAN DIGESTIVE SYSTEM

1. Alimentary Canal

Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach (cardiac → fundic → body → pyloric) → Small intestine (Duodenum → Jejunum → Ileum) → Large intestine (Caecum → Colon → Rectum) → Anus.
  • Gastro-oesophageal sphincter: Between oesophagus stomach.
  • Pyloric sphincter: Between Stomach & small intestine.
  • Anal sphincter: Guards anus.
  • Rugae: longitudinal folds in stomach wall.
  • Villi: Finger-like structures at the mucosa of small intestine. It has capillary network and lacteal (lymph vessel).
  • Vermiform appendix: finger-like structure arising from the caecum.




Human dentition is Thecodont, Heterodont & Diphyodont.
  • Thecodont: Teeth are placed in the jaw sockets.
  • Heterodont: Different kinds of teeth - incisors (I), canines (C), premolars (PM) & molars (M).
  • Diphyodont: Teeth appear twice in lifetime - milk (deciduous) teeth (20) and permanent teeth (32).

Human dental formula (of permanent teeth): 2123/2123.

2. Digestive Glands 
  1. Salivary glands: Parotids, Submaxillary & Sublingual → Saliva. Saliva contains Salivary amylase (Ptyalin) and Lysozyme.
  2. Gastric glands: Secretes Gastric juice.
    • Mucus neck cells: Secrete mucus.
    • Chief (peptic) cells: Secrete pepsinogen & prorennin.
    • Oxyntic (parietal) cells: Secrete HCl & intrinsic factor.
  3. Liver: Secretes Bile juice. No enzymes. Contains Bile pigments, Bile salts, Phospholipids and  Cholesterol.
  4. Pancreas: Secretes Pancreatic juice. Contains Trypsinogen, Chymotrypsinogen,  Procarboxypeptidase,  Pancreatic amylase, Pancreatic lipase & Nucleases.
  5. Intestinal glands: Secretes intestinal juice (Succus entericus). It contains Dipeptidase, Maltase, Lactase, Sucrase, Lipase, Nucleotidase, Nucleosidase.

Transverse section of human gut: 


Bile is transported from liver to duodenum as follows:

Bile → hepatic duct → gallbladder → cystic duct → common bile duct → common hepato-pancreatic duct → duodenum.

Hepato-pancreatic duct is guarded by sphincter of Oddi.

DIGESTION OF FOOD

Digestion in Buccal cavity:


Digestion in Stomach:


Rennin digests milk protein in infants.

Chyme: Acidic pasty food formed in stomach.

Digestion in Small intestine:

Role of Bile juice: 

Emulsification of fats (fat → micelles). It increases surface area for the action of lipase.
Bile also activates lipase.

Role of Pancreatic juice:


Role of Intestinal juice:


ABSORPTION OF DIGESTED PRODUCTS 

Absorption is 2 types- passive and active.

a) Passive absorption (Passive transport):

    Higher concentrated region to lower concentrated region. It includes osmosis & diffusion.

    Diffusion is 2 types:
  1. Simple diffusion: E.g. glucose, amino acids, Cl-.
  2. Facilitated diffusion: Diffusion with the help of carrier proteins. E.g. glucose, amino acids etc.
b) Active absorption (Active transport): 

    Absorption against concentration gradient. 
    E.g. absorption of amino acids, monosaccharides like glucose, electrolytes like Na+ etc.

Absorption of lipids:

Bile salts & phospholipids convert lipids to water-soluble droplets (micelles) → small protein coated fat globules (chylomicrons) → transported into lacteals in the villi → lymph → blood.

Absorption in different parts of alimentary canal:
  • Mouth: Certain drugs.
  • Stomach: Water, simple sugars, some drugs & alcohol.
  • Small intestine: All nutrients. Ileum is the chief area of absorption due to villi, its great length and coiled nature.
  • Large intestine: Water, some minerals & drugs.
Absorbed nutrients are incorporated into tissues (assimilation).

Undigested substances form faeces. It enters caecum through ileo-caecal valve. 

DISORDERS OF DIGESTIVE SYSTEM 
  1. Jaundice: Skin and eye turns yellow due to the deposition of bile pigments. It indicates liver damage.
  2. Vomiting: Ejection of stomach content through mouth.
  3. Diarrhoea: Frequent elimination of watery faeces. It reduces the absorption of food.
  4. Constipation: Infrequent elimination of dry stool. It is due to decreased peristalsis in colon.
  5. Indigestion: Condition leading to feeling of fullness due to improper digestion.

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