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Digestion and Absorption





NUTRITION

Nutrition is the sum of all those activities which are concerned with ingestion, digestion, absorption of digested food into blood & oxidation of simple food to produce energy & egestion of undigested food.
TYPES OF NUTRITION
  1. Autotrophic (make their own food)
  2. Heterotrophic (derive their food from others)

Autotrophic:-
  1. Photosynthesis:- Ex. Green plants
  2. Chemosynthetic:- Ex. Sulphur or Iron Bacteria

Heterotrophic:-
  1. Holozoic:- Ex. Humans
  2. Parasitic:- Ex. Roundworms
  3. Saprophytic:- Ex. Yeast, Mushroom

DIGESTION

Intracellular:- Ex. Protozoan (Amoeba)
Intercellular (Extracellular):- Ex. Chordates

Intracellular
Intercellular or Extracellular
Digestion occurs inside a food vacuole in the cell & digested food is absorbed into cytoplasm.

It is less efficient
It is found in lower forms.
for ex. Protozoan
Digestion occurs in the alimentary canal outside the cells & digested food is absorbed into the cell.

It is more efficient
It is found in higher form.
For ex. chordates

Digestive System:

The digestion system is formed of two parts:-
  • Alimentary canal (Gut)
  • Digestive glands

Alimentary canal:

It is formed of three parts:-
  1. ForeGut (Ectodermal in origin & includes mouth & buccal cavity).
  2. Mid Gut (Endodermal in origin & includes parts from pharynx to colon).
  3. Hind Gut (Ectodermal in origin & includes rectum & Anus).

Digestive Glands:

These are exocrine glands & include
  • Gastric gland,
  • Intestinal gland,
  • Salivary gland,
  • Liver
  • Pancreas.

Human Digestive System

Digestive system of man consists of two parts:-
  1. Alimentary canal
  2. Digestive Glands

Alimentary Gland

It is a long sized tube of varying diameter & its complete.

Mouth:
  • The alimentary canal begins with mouth & opens out through the anus. The mouth leads to the buccal/ oral cavity. The oral cavity has number of teeth & muscular tongue.
  • Each tooth is embedded in jaw sockets i.e. they are thecodont.
  • The teeth are developed in two sets:-
    1. Milk teeth
    2. Permanent teeth
  • This type of dentition is called diphyodont.
Few things about Teeth
  • An adult human has 32 permanent teeth which are of four different types namely:
    Incisors (I), Canines (C), Pre- molars(PM), Molars (M)
  • The formula showing the number & arrangement of teeth in one half of each jaw is called dental formula.
    Dental Formula=
  • Teeth help in mastication of food.
  • The tongue in attached to oral cavity by the frenulum
  • The upper surface of tongue has papillae which bears taste buds.

PHARYNX
Oral cavity leads into a short pharynx which serves as a common passage for food & air. Lowest part of the pharynx has glottis that leads to the wind pipe & is closed by a cartilaginous flap called epiglottis.

OESOPHAGUS [FOOD PIPE]
It is the long tube which extends through the neck, thorax & diaphragm & leads to the stomach. It conducts the food to the stomach by peristalsis movement.
A muscular sphincter called gastro oesophageal regulates the opening of the oesophagus into stomach.

STOMACH
  • It is a J shaped organ located in the upper- left portion of the abdominal cavity. It is formed of three parts:-
    1. Cardiac: It receives the oesophagus.
    2. Fundic: middle part.
    3. Pyloric:- It opens into the duodenum of the small intestine.
  • Inner Mucosa of the stomach is raised into large no. of folds called gastric rugae. These rugae dilate the stomach to store the food & also increase the surface area of digestion.
  • Stomach helps in mechanical churning & chemical digestion of food.

SMALL INTESTINE
  • It is the longest narrow part of the alimentary canal (about 6m). It I divided into three parts:-
    1. Duodenum:- It is the widest C-shaped part of the small intestine in which the stomach is open & receives the bile- pancreatic duct.
    2. Jejunum:- Middle Part
    3. Ileum:- Longest part which opens in the caecum.
  • Inner Mucosa of the small intestine is raised into finger- like projections called villi. These cells have micro- villi.

LARGE INTESTINE
It is about 5m long which are around the small intestine. It is formed of three parts:-
Caecum:-
It is the small pouch-like structure which ends into a tubular structure called a vermiform appendix.
Colon:-
Caecum opens into colon which is differentiated into ascending, transverse & descending colon.
Rectum:-
Descending part of the colon opens into the rectum which opens out through the anus.

DIGESTIVE GLANDS

The digestion glands are associated with the alimentary canal includes:-
salivary gland, liver, pancreas & intestinal gland.

Salivary gland
Saliva is mainly produced by three pairs of salivary glands namely:
  • Parotids (cheeks),
  • Submaxillary/sub- mandibular(lower jaw)
  • Sub- lingual (under the tongue).
Their ducts open into the buccal cavity.
Saliva contains electrolytes (Na+, K+, Cl-, HCO3-), enzyme ptyalin (salivary amylase) and lysozyme (antibacterial).

Gastric glands
There are three types of gland cells in the gastric mucosa-
Mucus cells: secretes mucus.
Peptic cells (chief cells): secretes pepsinogen.
Parietal cells (oxyntic cells): secretes HCl.

Liver:
It is the largest gland of the body (1.2 to 1.5kg). The hepatic lobules are the structural & functional unit of liver containing hepatic cells. Each lobule is covered by Glisson’s capsule. The bile secreted by hepatic cells & stored in gallbladder.

Pancreas:
Both exocrine and endocrine gland.
Exocrine portion secretes alkaline pancreatic juice and endocrine portion secretes two hormones i.e. Insulin and Glucagon.
The duct of pancreas join the common bile duct to form hepato-pancreatic duct that opens into duodenum. The opening is guarded by sphincter of Oddi.

Intestinal gland:
Intestinal mucosa has goblet cells which secretes mucus and brush border epithelial cells secrete some enzymes. Secretion of both these glands are collectively called Succus entericus (intestinal juice).

Anatomy of Alimentary canal:
The wall of alimentary canal from oesophagus to rectum possess four layers namely-
Serosa
Muscularis
Submucosa
Mucosa
Serosa: Outer layer made up of mesothelium with some connective tissues
Muscularis: It is formed by smooth muscles arranged into an inner circular and Outer longitudinal layer.
Submucosa: It is formed of loose connective tissues containing nerves, blood vessels and lymph vessels.
Mucosa: It is the innermost layer. In stomach, this layer forms rugae and in small intestine it forms villi.





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