Content:- Alimentary canal Digestive glands. Digestion Absorption Assimilation

Alimentary canal

Alimentary canal of man measures 8-10 meters

in length. It starts with mouth and ends at the anus.

- Mouth

NoteNote – That during swallowing of food ,glottis remains closed .

Mouth is a transverse slit bounded two soft ,movable lips , the upper lip and the lower lip . The lips are covered with skin . Lips prevent the falling down of bitten food particles.

Mouth

Buccal cavityBuccal cavity

Buccal cavity is bounded by lips

in front .It contains cheeks ,upper

and lower jaws, gums ,teeth and

tongue .in man there are 16 teeth in

each jaws. there are four different

types of teeth .they are

(i) Teeth

In man two sets of teeth appear during life .in humans, the first set or milk teeth consists of 20 teeth . These are called deciduous or temporary teeth .these are completely replaced by the permanent teeth by about 12 years of age . Teeth help in cutting, tearing and grinding the food .

Types of teeth

Types of teeth

Shape Function Number of teeth

Incisors Chisel-shaped

Used for biting 4

Canines Dagger shaped

Used for tearing 2

Premolars Cuspid-crown

Used for grinding 4

Molars Cuspid –crown

used for grinding 6

Structure of a Tooth

Structure of a ToothStructure of a Tooth

The sensitive nerves and blood vessels at the centre of each tooth are protected by several layers of tissue, the outermost (the enamel) being the hardest substance in the body.

Under the enamel, surrounding the pulp from crown to root, lies a layer of bonelike dentin.

ContdContd:-Structure of a ToothStructure of a ToothA hard tissue called cementum separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing. Separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing.

(ii) Tongue(ii) Tongue It is a muscular ,sensory organ,

which is attached to the lower jaw. It bears taste buds .it helps in the

tasting the food it mixes food with

saliva and helps in swallowing. It also helps in speech and makes

it soft and slippery for chewing.

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Pharynx The buccal cavity opens into the pharynx . It is the common passage for food and air . It leads into the oesophagus . When food is swallowed , the glottis remains

closed (by eppiglottis ). If a small particle of food enters the tracheal

opening . It is throw out by coughing.

oesophagus

Oesophagus is a muscular tube about ten inches(25cm)long .

Food passes down the oesophagus by peristaltic movements of its muscular wall.

The rhythmic contraction and relaxation of muscles of alimentary canal to push the food downwards is known as peristalsis fig 1.15.

4- Stomach

The stomach is alarge muscular J shaped sac located below the diaphragm. It secrets gastric juice. The wall of stomach contains numerous branched and tabular gastric glands. These glands secrete gastric juice,which contains enzymes (Pepsin,Rennin )HCl and mucus. The sight , smell or thought of appetizing food can also initiate secretion of gastric juice.

Stomach

5-Small intestine Small intestine is about 6 meters in length and 2.5

cm I in diameter. It remains coiled in the abdominal cavity . The small intestine consists of three parts :

1. Duodenum. It is the first part (proximal) of intestine and is about25 cm(10 inches) long. It is U- shaped In appearance. The opening of common bile duct and pancreatic duct opens in it.

2. Jejunum. It is about 2.5metres long and narrower than duodenum.

3. Ileum . It is about 3.5metres long. It is distal in position.

Contd:-Small intestine Small intestine serves both for digestion and

absorption of digested food it has numerous tiny

finger like –projections called villi. The villi increases the inner surface area of

absorption of digested food . At the bases of villi are present intestinal glands . These secrete digestive enzymes for the digestion

of carbohydrates ,proteins and fats.

Large intestine Large intestine is wider than small intestine . It is about 1.5-

1.8 meters long . It is divided into caecum ,colon &rectum. Caecum is a small blind pouch situated at the junction of

small and large intestines its blind end is produced into vermiform appendix.in man it is a vestigial organ.

Colon is an inverted U-shaped tube about a meter long. Rectum is the last part of alimentary canal. It opens to the

exterior by anus. Rectum absorbs excess of water from the undigested food ,secretes mucous and helps in egestion of undigested food matteer.

Digestive Glands 1- salivary glands. Three pairs of salivary glands

open into the buccal cavity .These secrete saliva, which performs following functions:

It moistens and lubricates food and helps in swallowing.

It contains an enzyme ,salivary amylase, which carries out partial digestion of starch into maltose The salivary glands, especially the parotid, in front of each ear of human beings are affected by the disease called mumps.

liver Liver is the largest gland in the human body. It secretes bile juice, which is stored in the gall bladder before it is poured into duodenum through a common bile duct .Bile is alkaline due to the presence of bile salts and bile pigments. Bile helps in : Emulsification of fat (emulsification means breaking of fat into

small globules ). Activates the pancreatic and intestinal enzymes by making

the food alkaline. Stores glucose in the form of glycogen. Forms urea. Destroys dead and worn out red blood corpuscles.The bile secreted by the liver cells is normally stored in the gall

bladder until needed in the duodenum.

Pancreas

Pancreas is the second largest gland of the body . It secretes pancreatic juice , which carries out digestion of carbohydrate, proteins and fats. it also secretes insulin and glucagon hormone. pancreatic juice contains trypsin for digesting protein, pancreatic amylase for breaking down starch. Bicarbonate ions make the medium alkaline which is required for the action of pancreatic enzymes. It is secreted by duodenum wall. Both bile and pancreatic juice enter duodenum through common bile duct .

Digestion

The process of breakdown of complex food particles in simpler forms is known as digestion. It is necessary to digest so that food can be absorbed in the blood stream and can be utilized by the cells.

Process of digestion

1- digestion of food in buccal cavity.

2- digestion in stomach .

3- digestion in small intestine.

digestion of food in buccal cavity

In buccal cavity food is chewed and masticated to increase the surface area of food for digestion .salivary amylase (ptyalin) acts upon starch and breakdown into maltose .(raw bread tastes sweeter because starch has changed into maltose) No digestion occurs in pharynx and oesophagus.

digestion in stomach

In stomach gastric juice is released . The gastric juice is acidic due to the presence

of hydrochloric acid . Gastric juice contains pepsin and rennin. Pepsin works only in acidic medium . Pepsin enzymes break down the proteins of

the food into smaller molecules , which passes into intestine.

Rennin digests milk protein.

digestion in small intestine

In duodenum food is mixed with :- Bile from the liver Pancreatic juice from the pancreas and Intestinal juice (succus entericus).

Contd:- digestion in small intestine

The bile has no enzymes but makes food alkaline and emulsifies fats for easier digestion.

A-Pancreatic juice (in duodenum)digests carbohydrate ,proteins and fats.

Pancreatic amylase acts on starch and converts it into maltose.

Trypsin converts peptons to amino acids . Pancreatic lipase converts fats to fatty acids.

Contd:- digestion in small intestineB- succus entericus (intestinal juice, in ileum) also

digests carbohydrate, proteins ,fats :- Maltose acts upon maltose to form two glucose

molecules; lactase acts upon lactose to form glucose and galactose molecules ,sucrase acts upon sucrose to form glucose and fructose molecules .

Intestinal lipase also converts fat to fatty acids. Erepsin acts upon peptides to form amino acids.No digestion takes place in large intestine.

Absorption of food The final products of digestion are glucose (from

carbohydrates), amino acids (from proteins), and fatty acids and glycerol( from fats) .The glucose and amino acids enter the blood stream, while glycerol and fatty acids enter the lymph. The absorbed food is sent to different parts of the body.

The main site for absorption of food is small intestine because it is long and it has minute finger-like projections called villi, which increases the surface area of absorption. Excess of water is absorbed in large intestine and the contents become semi-solid faeces.

Assimilation The absorbed food is either used by the body to

produce energy or it can be stored in the body.Glucose is utilized in respiration to release enregy.

If in excess glucose is stored in the formof glycogen in lever and muscles, amino acids are converted in the form of proteins for synthesis of new cells whereas fatty acids and glycerol convert into fat and are stored in the adipose tissue.

Egestion The undigested waste is passed out

through anus.