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posted Sep 16, 2017, 7:42 PM by Upali Salpadoru   [ updated Sep 16, 2017, 9:59 PM ]


All living creatures must take in oxygen gas and give out carbon dioxide. This is generally referred to as breathing. Here’s how some groups carry out this most important function.

  1. Tiniest plants and animals (Eg. Protozoa)-- Gases simply diffuse in and out.

  2. Small animals--  In worms and frogs gases diffuse through the moist skin. Insects have a network of tubes.

  3. Fish-- They pass the water through the gills, where the blood exchange gases.

  4. Land plants-- Gases are exchanged through stomata in the leaves.

What about the man and the other Mammals?

All lung breathing animals take in air, inhale and give out air, exhale. It is wrong to say that we take in oxygen and give out carbon dioxide. Our nostrils have no way to extract oxygen from the air. It is the blood that takes oxygen from air. This happens inside the organ that we call the lungs.

 This is really the start of the respiratory process.

What is respiration?

Respiration includes a series of complex chemical changes that take place inside all living cells releasing energy from food.

How does the air come in?

Our thoracic cavity performs like the bellows of an accordion. As we increase the volume the air pressure inside decreases. Then the atmospheric pressure becomes greater than the air pressure inside, air rushes in.

Exhaling is done by compressing the chest cavity.

Demonstration 1.


Take a plastic bottle and cut off the bottom. Then get a thin tube and tie a balloon to one end. Keeping the balloon inside the bottle, fix the tube with play-doll as shown. Then take another balloon and obtain a sheet from that and cover the bottom of the bottle.

See what happens when you pull the sheet up and down.

This demonstrates how the air comes into our chest cavity.


Fig.1 A model to show breathing.


  What happens in the chest cavity?


The two lungs in the chest cavity are somewhat like two balloons. As we expand the chest by pulling the muscles on the ribs and pulling the diaphragm down air enters the lungs. Lungs are not exactly like balloons but like a sponge. There are tiny air sacs called alveoli. Atmospheric air gets into these air sacs and blows them up.

It is mostly the diaphragm, between the thoracic cavity and the abdominal cavity which pulls and pushes the air.  The rib cage attached to the sternum, and the abdomen also can assist this process.

A Demonstration to show there is more Carbon dioxide in Exhaled air than in ordinary air.

When Nelly breaths in,  air comes through the left  bottle bubbling in the lime water. Clear lime water (Calcium hydroxide solution) can detect the presence of Carbon dioxide gas. In presence of COthe clear solution becomes murky or milky due to the formation of Calcium carbonate.  When the girl blows bubbling will be in the right hand bottle.  It would be easy to find out which air has more CO2 gas.

1 Trachea 

This is the tube that connects the lungs with the throat. It is outlined with incomplete hoops of cartilage so that it may not squeeze in with the turning of the head or the decrease of pressure. This too is lined with mucus and cilia. These cilia push the dust particles and microbes upwards to the throat.

2 Bronchi

These are the branches of the trachea and they also have some cartilage. The smallest of these are called bronchioles. These supply air to microscopic air sacs called alveoli.

3 Alveoli   

The outside walls of these are well supplied with the blood capillaries. The walls are only 0.0001mm thick and consist of only one layer of cells. Inside is lined with a substance called ‘surfactant’ which facilitates the process of inflation and deflation and helps in diffusion and in destroying bacteria. Though alveoli are flexible they are not elastic. In the human lung there are about 700million alveoli.

Blood plasma does not leak into the alveoli but white cells may creep in. The total surface exposed by the alveoli is about 90m2.       

The high efficiency of diffusion in alveoli is due to the following factors.

  • Having a large surface area.
  • Very close contact with the blood in capillaries.
  • The capabilities of the blood.
  • Presence of surfactant.

A home Experiment


To find out the maximum volume of air one can blow out.


Take a large, empty, plastic oil can or a translucent bucket. Fill it completely with water and invert it over a basin of water as shown here. Then take a plastic tube and insert one end into the can and the other into your mouth.

Take a deep breath and start exhaling. Breathed out gas will bubble and get collected in the can. ( If air goes out after filling the can, your lung capacity is greater than the capacity of the can)

Results and calculation:

The full capacity of the can= ……………….....= V1 cm3

Volume of water after experiment =…………..= V2 cm3

Vital Lung capacity = V1 – V2 cm3 =………….cm3



Fig.2 Alveoli or air sacs in lungs.



  Fig.3 A single alveoluis.


            Blood meets air.

The gas exchange takes place by diffusion. There are minute blood vessels called blood capillaries on the surface of air sacs. There is a chemical compound called haemoglobin in the red blood corpuscles. These can combine with oxygen to form oxy-haemoglobin. At the same time carbon dioxide dissolved in the blood plasma is also released to the air-sacs.


Haemoglobin + Oxygen  =  Oxy-haemoglobin

The dissolved carbon dioxide due to blood pressure also escape as

H2CO3 =  H2O + CO2
There are 3 ways by which CO2 is taken to lungs.  1. Physically dissolved in the plasma..
2. Chemically combined with with water as carbonic acid. 3. Combined with haemoglobin .
Approximately 75% of carbon dioxide is transport in the red blood cell and 25% in the plasma.


How does the energy come?

The important part of the respiration is the release of energy. This occurs inside the cells.


1.0    Use the words here to fill in the blanks.

Contraction, Air, CO2 , removed, increasing, reduces, trachea, capillaries, alveoli, combine,

    1. In the case of humans …………. is taken in by…………… the volume of the thoracic cavity.

    2. The ………………. of the diaphragm and the lifting of the rib cage………….. the thoracic pressure.

    3. During inhaling air passes through the………..,bronchi and bronchioles to the ………...

    4. The hemoglobin in the blood ………….. that line the alveoli ………… with the diffused oxygen molecules to produce oxy-hemoglobin. 

    5. The blood plasma releases the dissolved ……….. gas which is ………….. during exhaling.                                                                     (2x10=20 marks)

2.0  Give the  number of the correct label.

 (4x5=20 marks) 

3.0  What substance or substances are directly responsible for these.

1.  Lime water turns milky.

2.  Taking oxygen into the blood.

3.   Supplying energy to living cells.

4.  On a cold day exhaled air come out as a mist.

5. Transport of CO2.

6. Inner surface of alveoli are lined with this substance.

                                                                                (5 x 6=30 Marks)

4,0   Describe these using only one word.

Words:  respiration, exhale, transport, diffusion,  photosynthesis, oxidation,

  1.  What mammals do by relaxing the diaphragm.

  2.  Gas molecules entering or leaving blood.

  3.  Oxygen molecules combining with hemoglobin.

  4.  Glucose combine with oxygen releasing energy.

  5.  Blood carries raw materials to supply energy and the waste products.

  6.  Opposite of respiration that take place in plants

                                                                (5x6 = 30 marks).
            For Answers visit  Answer page - Biology