Unit04

ttp://www.zo.utexas.edu/faculty/sjasper/bio301L/digestion.html

Alimentary canal in man

 

In humans and other  vertebrates ,  the digestive system consists of a tubular  gastro intestinal tract  and accessory digestive  organs.             This is a one way  transport of food.
Mouth opens  to the buccal cavity .The pharynx which is the common passage of the oral and nasal cavities .The pharynx  leads  to the oesophagus, a muscular tube ,delivers food to the stomach.
From the stomach  food passes to the first  part of  intestine  where the digestive  enzymes  continues the digestive process.
The products then  pass across the wall of the small intestine into the blood stream.
Then the remains into the large intestine where water and minerals are absorbed.
The waste products then enter to the rectum and expelled  through the anus.
The accessory organs are the liver   -  produce bile gall bladder – stores and concentrates bile pancreas   -  produces pancreatic juice  which contain  digestive   enzymes and bicarbonates
Both bile and pancreatic juice first secreted into the first region of the small intestine and aid digestion
Human like all placental mammals lack a cloaca and have a seperate exit  from the digestive tract through rectum and anus 

Mouth
Humans are omnivorous, are specialized for eating both plants and animal food
Humans are omnivorous in front and herbivorous  in back of the mouth.

There are four types of teeth in the mouth.
Incisors- Four front teeth in the upper and lower jaws .
                They are sharp ,chaise – shaped for biting

 Canine - Each side of the incisors are sharp pointed teeth called  
               canine for tearing food .

Pre molars- behind pre molars are canine for crushing food.

Molar – with flattened rigid surfaces for grinding and crushing food

Each tooth is alive with a central pulp containing nerves and blood vessels.

The actual chewing surface is a hard layered enamel layer over the softer dentin which forms the body of the tooth.



 

Saliva

Inside the mouth tongue mixes food with a mucous solution , saliva. pH is 6.5 – 7.5
In humans three pairs of salivary glands, secreted saliva into mouth through ducts.
Saliva is a mixture of secretions of salivary glands and mucous glands of the buccal cavity  epithelium.
The secretion of the salivary glands are controlled by the nervous system.
The presence of the food  in the mouth triggers an increased rate of secretion.

Functions
Saliva moisten and lubricates the food  and this is easier to swallow the food .
Water  contained   in saliva wet the dry food.
Ions in saliva such as K+ , Cl -       
 It contains hydrolytic enzyme, salivary amylase. Cl  - activates  salivary amylase and converts   starch into maltose.

Polysaccharides       salivary amylase      Disaccharides
 Starch                                                            Maltose     

Mucus in saliva lubricates food.
Lysozymes present in saliva  destroy bacteria.
Saliva helps in taste reception.
Secretion of saliva is stimulated by parasympathetic  nervous system and inhibited by sympathetic nervous system.

Ingested food is fragmented through the tearing or grinding  action of specialized teeth. Series of pharyngeal muscle contractions  begins and then  food mixed with saliva is swallowed and enters the esophagus. Saliva does not abrade the tissue , it passes on its way through the esophagus.

Esophagus

Swallowed food enters the esophagus , which connects the pharynx to the mouth  .

The upper third is enveloped to skeletal muscle, for voluntary control of swallowing.

The lower two third is surrounded by involuntary smooth muscle.

 The food moves along the esophagus to the stomach  by peristalsis movements.

The food moves down to the stomach is controlled by a ring of circular smooth muscle  or a sphincter , that opens in response to the pressure extend by the food.

Contraction of this sphincter prevents food in the stomach from moving back into the esophagus.  The human esophagus  is closed off except during swallowing.

The tubular gastro vascular tract of a vertebrate  has a characteristic  layered structure. The tissue layers in the wall of alimentary canal are arranged in a basic pattern.

Basic plan consists of  four main layers of the gut wall.

From outermost  layer to innermost layer are,

Serosa – Consists of fibrous connective tissue  and are covered the external surface
                of the tract  with  peritonium.

Muscularis –  Just outside the sub mucosa. Consists of  double layer of smooth
                       muscles.The muscles in the outer layer arranged  longitudinally and 
                       the muscles  in the  inner layer have a circular muscles.
                       Auerbach’s plexus /  nerve plexus  are present between the  
                       longitudinal muscle layer and the circular muscle layer.

Sub Mucosa –Consists of loose connective tissue  .             
                       Meissner’s plexus are present between sub mucosa and circular muscle
                       layer.

 Nervous intertwined regions are called plexus, are located in the sub mucosa, help to regulate  gastro intestinal activities


Mucosa -   Consists of loose connective tissue , lined with an epithelium.



Major variations  are those seen in the basic plan occur in the mucosa.
They are associated with particular functions.

Tissue plan in small intestine

1.Outermost layer is serosa

2.The layer near to the outermost layer  is muscularis.  
   Circular muscle layer is thicken.

3.Next inner layer is submucosa.Bruner’s glands are 
   present in the sub mucosa.These glands secret the 
   mucous alkaline secretion that nutralize  the chyme and
   protects the deuodenum wall from the damage.

4.Innermost  layer is mucosa which consists number of  
   circular foldings  called Liberkun cripts . This increase the 
   surface area of the small intestine. 

5.The small intestine consists  mucosa, water, enterokinase
    and enteropeptidase.

6.Presence of villi.

7.Intestinal glands among the villi that are called enterocytes secretes entero gasteron.

8.Lymph noduleswhich are called Payers patches that helps to produce antibodies

9.Presence of network of blood capillaries.

10. Network of lymph vessels – Lacteals

11.Cells in the inner lining / epithelium have micro villi 

http://science.waltermack.com/smack/yr8biol/digestion2.gif

http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter48/