Experiment to Show that Carbon Dioxide is produced during Respiration

We know that carbon dioxide gas turns lime-water milky. The fact that carbon dioxide is produced during respiration can be shown by demonstrating the effect of inhaled air and exhaled air on lime-water. The apparatus to demonstrate the effect of inhaled air and exhaled air on lime-water.

The apparatus consists of two boiling tubes A and B fitted with two-holed corks. The boiling tubes A and B are connected through a special type of glass tube C. The left arm of glass tube C is short which goes in the boiling tube A.

The right arm of glass tube C is long and dips in lime-water in boiling tube B. The boiling tube A has another bent glass tube D whose longer side dips in lime-water contained in it. The boiling tube B has also another short, bent tube E in it which does not dip in lime-water.

To perform the experiment, we put the top end of the tube C in mouth and 'breathe in' and 'breathe out' gently. When we breathe in, then the inhaled air (fresh air) enters the glass tube D and passes through the lime-water in boiling tube A. And when we breathe out, then the exhaled air (coming from our lungs) passes through the lime-water in boiling tube B. We continue to breathe in and breathe out for about five minutes.

We will find that the lime- water in boiling tube A (in which inhaled air is passed) turns milky only slightly but the lime-water in boiling tube B (in which exhaled air is passed) turns milky appreciably.

This shows that less carbon dioxide is present in inhaled air but much more carbon dioxide is present in exhaled air. From this observation we conclude that carbon dioxide is produced during respiration (which comes out in exhaled air).

The air which we 'inhale' is a mixture of gases and the air which we 'exhale' is also a mixture of gases. The only difference in the inhaled air and exhaled air is that they contain different proportions of oxygen, carbon dioxide and water vapour. (The proportion of nitrogen gas in the inhaled air and exhaled air remains the same, 78 per cent, because it is neither used up in respiration nor produced during respiration). The proportions of oxygen, carbon dioxide and water vapour in 'inhaled air' and 'exhaled air' are given below:

We can see from the above figures that the air which we inhale contains a greater proportion (21 per cent) of oxygen. Now, some of the oxygen of inhaled air is used up in breaking down glucose food during respiration, so the exhaled air which comes out after the process of respiration contains a lower proportion (16.4 per cent) of oxygen.

The air which we inhale contains a lower proportion (0.04 per cent) of carbon dioxide. Now, during respiration, when oxygen breaks down glucose food, then a lot of carbon dioxide is produced, so the exhaled air which comes out after respiration contains a much higher proportion (4.4 per cent) of carbon dioxide.

Again, the air which we inhale contains only a little of water vapour. Now, when glucose food is broken down by oxygen during respiration, then water is also produced (along with carbon dioxide). So, the exhaled air contains a lot more water vapour than inhaled air.

Rate of Breathing :

The process of breathing pumps in oxygen into our body (and removes carbon dioxide). Breathing occurs involuntarily (on its own) but the rate of breathing is controlled by the respiratory system of brain. The average breathing rate in an adult man at rest is about 15 to 18 times per minute.

This breathing rate increases with increased physical activity. For example, if we do some physical exercise (like sit-up exercise), then our breathing rate goes up considerably. This is because when we do some physical exercise, then our body needs more energy. And to produce more energy through respiration, our body requires more oxygen gas.

Rapid breathing supplies more oxygen to body cells for producing more energy required for doing physical exercise. Thus, we breathe faster after exercise so as to produce more energy to compensate the loss of energy suffered by our body in doing exercise.

We all breathe through nose. We can, however, not breathe inside water when we are diving. This is because water does not have free air or oxygen for us to breathe (and we do not have gills like the fish to utilise oxygen dissolved in water). So, the deep sea divers carry oxygen gas cylinders with them for breathing when they go under the sea.

We have just studied that oxygen required for breathing and respiration (release of energy) is carried by hemoglobin present in our blood. The normal range of hemoglobin in the blood of a healthy adult person is from 12 to 18 grams per deciliter (12 to 18 g/dL) of blood. The deficiency of hemoglobin in the blood of a person reduces the oxygen-carrying capacity of blood resulting in breathing problems, tiredness and lack of energy. The person looks pale and loses weight.

Many times we have heard of carbon monoxide poisoning. This happens as follows. Carbon monoxide gas (CO) is formed whenever a fuel burns in an insufficient supply of air. For example, if coal (or charcoal) is burned in a closed space (like a room with closed doors and windows), then a lot of carbon monoxide is formed. Carbon monoxide is also produced when petrol burns in a car engine.

Now, we know that haemoglobin present in our blood carries oxygen to all the parts of our body. Haemoglobin has mort affinity (or attraction) for carbon monoxide than oxygen. So, if carbon monoxide gas is inhaled by a person then this carbon monoxide binds very strongly with haemoglobin in the blood and prevents it from carrying oxygen to the brain and other parts of the body.

Due to lack of oxygen, the person cannot breathe properly. If carbon monoxide is inhaled for a long time, then the person becomes unconscious and can even die due to oxygen starvation.

The persons having breathing problems (or respiratory problems) are given oxygen masks to facilitate breathing. In serious cases, the patient is put on a machine called 'ventilator' in which a tube is inserted directly into the trachea (or wind pipe) of the patient to help him in breathing comfortably.