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Digestive system

posted Dec 19, 2015, 12:56 PM by Upali Salpadoru   [ updated Dec 21, 2015, 1:06 AM ]

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The first attack, on whatever we eat, comes at the very start in the oral cavity / buccal cavity.

The 32 teeth will not spare a single hard morsel of food.

The digestive Process.

Buccal cavity.(Mouth)








1. Cut and chop.

2.Tear the food to shreds.


 3.Chewing and grinding.

Digestion is a two prong attack.

1. Physical - Pulverizes the food to obtain a bigger surface area.    

2. Chemical- Converts the insoluble food to soluble forms.

(Bigger molecules are broken down to smaller ones.) Example:- Starch to Glucose.     

These changes occur without the reagents (enzynes) being used up.  

So Enzymes are organic catalysts.






Tongue 2.jpg

“With a bitter back and sweet centre,

sour taste on the sides linger.

The tip takes the latest trend,

in licking a salty sweet blend.

Know how the buddies in the tongue,

taste the food of old and the young.”    

 This taste map, accepted earlier, does not seem to be right.

  Sensation of taste is not only from the tongue; the nose plays an important role.

Recognizes the food, mixes with saliva.

and is pushed to pharynx as  boluses.         


Salivary glands.


Deliver saliva. Saliva softens and lubricates the food . Physical change.

Saliva contains the enzyme Ptyalin (Amylase) that can chemically react with starch.

It also has a germicide, lysozyme.

1.5 L./day.




Function / Properties.


Esophagus Gullet.


A well lubricated tube.

Food is transported by the rhythmic contraction and relaxation of sphincter muscles.(circular muscles

Pushing the food is a reflex action. (in voluntary)

Chewing and gulping down are voluntary actions. (You can control them)  

Method of pushing food is peristalsis. This is involuntary.

Length= 25 to 30 cm.

Width 1.5 to 2 cm.


This guards the trachea. (Windpipe)

If an attempt is made to speak while you swallow, epiglottis cannot function. 

One can get choked.


Dilated part of the tract.

stomach 2.jpg

stomach 3.jpg

Three types of cells secrete gastric juice and mucus.

Only water, salts, alcohol and a little glucose are absorbed here.


Pepsinogen-Reacts with HCl and forms Pepsin.

Pepsin acts on proteins.

Cardiac sphincters control the inflow while the Pyloric sphincters release the chyme in an orderly manner.

Capacity =  4 to 5 litres

Time:- Approximately 2 hrs.

This will vary according to the food and the emotional conditions of the person.

. Small Intestine. 1. Duodenum. 2. Jejunum, 3. Ileum.

  1. Duodenum


This is is the first part of the small intestine.

Length;= 25 cm.

There are 2 glands pouring their secretions into the duodenum.

1. Pancreas:-  Gives Pancreatic juice. This contains 3 enzymes and bicarbonate ions.

Rate of flow 1.5 liters /day.

2. Gall bladder:- Releases bile which has been produced in the liver.

The alkaline salts in the bile emulsifies the lipids.

Bile is a greenish coloured liquid. The colour is due to the presence of degenerating matter from haemoglobin.

Capacity = approximately 30.cm3 .

                           Rate:-1 Litre//day.

2. Jejunum

Midsection of the small intestine, connecting the duodenum to the ileum.

Length about 2.5 m.

Contains villi.

3. Ileum

Highly enlarged view of the wall.

This is mainly responsible for the absorption;

The area of absorption is increased by a thousand fold due to tiny velvety like projections called villi.

The epithelial cells in these possess microvilli.

Crypt consists of younger cells which would replace the mature ones as they are discarded after a few days.

Length about 3.5m.

Absorption is not always passive. Often they have to be pushed against diffusion gradients. This has been termed active absorption


Glucose and amino acids straight away pass on to the blood capillaries in the villi. These capillaries form the hepatic portal vein which takes the food into the liver. The liver may store or alter any of the digestive products. Some of the fatty acids and glycerol may enter the blood capillaries, but a large proportion enters the epithelium to be converted back to lipids.  These products then pass on to lacteals. The fluid in these joins the lymphatic system which will join the blood stream later.

Large Intestine.   (Colon)


Absorbs the water.

Some salts may also be absorbed here.

Usually there are beneficial bacteria which form vitamin B and K.


The cellulose and vegetable fibres along with dead cells and the undigested matter form the faeces. or stool.

 Appendix This was earlier thought to be a vestgial organ. Now some researchers believe that it is a safe house for useful bacteria.

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1.0   Multiple Choice Questions..

1.1   Which answer describes the digestion in a meaningful manner.
        a. A physical change.  b. A chemical change. c. Converting food into a more nutritious form.
        d. Converting food into a form that can be absorbed by the blood.
1.2   In which organ does the chemical process of digestion start?
        a. Mouth.    b.  Gullet.   c. Stomach.  d. Small intestine.
1.3   Name the force that moves food along the food canal.
        a. Force of gravity. b. Peristalsis.  c Anti-peristalsis.  d. Support force.
1.4   Name the part that prevents food entering the windpipe.
        a. Tongue.  b. Epiglottis. c. Glottis. d. Larynx.
1.5   In which part of the tract, the chyme is most acidic.
        a.Mouth.  b. Esophagus. c. Stomach. d. Small intestine.

2.0 Supply short answers.

2.1  What is the advantage of having villi in the ileum, and how does it help?
2.2  What gastric juice neutralizes the Hydrochloric acid added to food.
2.3  What helps the release of partly digested food from the stomach? 
2.4  Why are the enzymes considered as catalysts?
2.5  Why does the stamach wall has 3 types of muscles?



Part A

Part B

Q 3.2.jpg

3.1 Name the numbered parts.


What enzymes are

present in the numbered regions.


2 to 3.


What is the function of ....



4.0   Fill in the blank spaces in this table.



Juice secreted


Enzymes present


Reaction on food

4.1 Salivary

4.2Gastric glands

4.3 Intestinal glands

Not required.

For the answers please CLICK :-  Answers-bio.