Respiration

Respiration is also a type of combustion. The red blood cells in blood pick up oxygen from the lungs and send it to other cells in different tissues. There, oxygen combines with glucose to release energy. Carbon dioxide and water are formed as a result of this reaction. The resulting products, water and carbon dioxide, are expelled when we exhale.

The red blood cells absorb only 4 - 5% of oxygen form the inhaled air. The percentage of carbon dioxide given out is more than 100 times the percentage of carbon dioxide taken in. The percentage of water vapour given out is also much higher. The quantity of nitrogen remains unaltered.

An easy experiment proves that the exhaled air contains more water vapour than inhaled air. When we blow against a glass mirror it becomes misty, showing that exhaled air has more water vapour than inhaled air.

To prove that the exhaled air has more carbon dioxide than the inhaled air, you would have to set up apparatus as shown in fig. 5.12.

Close the end of the tube at B. Keep your mouth at C, and suck in a breath of air. The air from the atmosphere enters through A, passes through the limewater, and comes into your mouth. Air cannot get in through B as that end is closed.

Then open end B, and close end A, and breath out the air from your lungs. The exhaled air cannot escape out of A, as it is closed. So it has to pass through the limewater in B, and get expelled through B.

You will notice that the limewater in B turns milky, much faster than in A. This proves that the percentage of carbon dioxide is higher in the exhaled air than in the air inhaled.