Contents
Evolution, a key concept in Biology Topics, explains the development of species over time.
Mechanism of Breathing – Inhalation and Exhalation Process
We can live without food and water for many days but we cannot live for more than a few minutes without air. This is because air is necessary for breathing. During breathing, we take air into our lungs through the nose, and then expel it. The ordinary air which we take into the lungs is rich in oxygen but the air expelled from the lungs is rich in carbon dioxide. We can now define breathing as follows : Breathing is the process by which air rich in oxygen is taken inside the body of an organism and air rich in carbon dioxide is expelled from the body (with the help of breathing organs). Different organisms have different organs for breathing depending on their structure and oxygen requirements. The breathing in human beings takes place through the organs called ‘lungs’ (see Figure).
The taking in of air rich in oxygen into the body during breathing is called ‘inhalation and giving out (or expelling) the air rich in carbon dioxide is called ‘exhalation.. Both, inhalation and exhalation take place regularly during breathing. A breath means ‘one inhalation plus one exhalation’. We know that air contains oxygen. So, when we breathe in air, it is actually the oxygen gas present in air which is utilised by our body (to break down food and produce energy). Thus, we ‘breathe in’ air to supply oxygen to the cells of our body (for the breakdown of food to release energy), and we ‘breathe out’ to remove waste product carbon dioxide from our body (which is produced during the breakdown of food in the cells).
We normally breathe air through our nose (or sometimes even through the mouth). Now, if we close our nostrils and mouth tightly, we soon start feeling uneasy and cannot hold our breath even for one minute ! (see Figure).
This shows how essential breathing of air is to keep us alive. And when we release the breath after holding it for some time, we have to breathe deeply. This deep breathing is to supply the extra oxygen to our body to make up for the oxygen which was not available during the time we held our breath. Breathing is a continuous process which goes on all the time and throughout our life and that of other animals as well as plants. The mountaineers carry oxygen cylinders with them for breathing because the amount of air available to a person for breathing at high altitude is much less than that available on the ground.
Breathing Rate
We have just learnt that a breath means ‘one inhalation plus one exhalation’. The number of times a person breathes in one minute is called breathing rate. On an average, an adult human being at rest breathes in and out 15 to 18 times in a minute. So, the average breathing rate in an adult human being at rest is 15-18 times per minute. Women breathe slightly faster than men. Breathing rate of a person is not constant always. The breathing rate of a person changes according to the oxygen requirements of the body.
For example, the breathing rate of a person is the slowest when he is sleeping because minimum energy is required by the body during sleep which can be provided by a slow rate of breathing. The breathing rate of a person increases with increased physical activity (like exercise, running, weightlifting, etc.) (see Figure).
When the breathing rate increases, greater amount of air goes into the lungs. With more air going into the lungs, the blood can absorb oxygen at a faster rate. Thus, faster breathing supplies more oxygen to the body cells for producing more energy (by the rapid breakdown of food) needed for doing heavy physical exercise, etc. During heavy physical exercise, the breathing rate of an adult human being can increase up to 25 per minute (or even more).
For example, if a person runs a distance, his breathing rate increases. Running makes a person breathe faster because the faster breathing rate supplies more oxygen to the body cells for the speedy breakdown of food for releasing more energy required for running. When we run (or do some other physical exercise), not only do we breathe faster, we also take ‘deep breaths’ so as to inhale more air (and get more oxygen) for the speedy release of energy from food.
It is a common observation that an athlete breathes faster and deeper than usual even after finishing a race. This is because during the race, the leg muscles of athlete have produced extra energy by doing anaerobic respiration (without using oxygen). By breathing faster and deeper, the athlete is giving back oxygen to the muscles which it could not give earlier at the time of running (due to lack of oxygen at that time).
Please note that whenever a person needs extra energy, he breathes faster than usual. Due to faster breathing, oxygen is supplied to our body cells at a faster rate. The faster supply of oxygen increases the speed of breakdown of food due to which more energy is released rapidly. It is a common observation that we feel hungry after doing a heavy physical exercise (or any other heavy physical activity). This is because to provide extra energy for doing heavy physical exercise, the food has broken down very rapidly (by faster breathing) and made us feel hungry. Another point to be noted is that the breathing rate in children is higher than that of adults. Children breathe about 20 to 30 times per minute.
How Do We Breathe
We will now learn the mechanism of breathing. That is , we will now learn how air from outside is sucked into our lungs during inhaling (breathing in), and how air from our lungs is pushed out during exhaling (breathing out). The process of breathing takes place in our lungs. Lungs are connected to our nostrils (holes in the nose) through nasal passage (or nasal cavity) and windpipe. When we inhale air, it enters our nostrils, passes through nasal passage and windpipe, and reaches our lungs.
Our two lungs hang in an airtight space in our body called ‘chest cavity’. Around the sides of the chest cavity is the rib cage with sheets of muscles between the ribs. The rib cage encloses the lungs in it [see Figure (a)]. At the bottom of the chest cavity is a curved sheet of muscle called diaphragm [see Figure (a)], Diaphragm forms the floor of chest cavity. Breathing involves the movements of the rib cage and the diaphragm. This happens as follows :
(a) Breathing in. When we breathe in (or inhale), then two things happen at the same time :
(i) the muscles between the ribs contract causing the rib cage to move upward and outward, and
(ii) the diaphragm contracts and moves downward [see Figure (a)], The upward and outward movement of rib cage, as well as the downward movement of diaphragm, both increase the space in the chest cavity and make it larger [see Figure (a)]. As the chest cavity becomes larger, air is sucked in from outside into the lungs. The lungs get filled up with air and expand.
(b) Breathing out. When we breathe out (or exhale), even then two things happen at the same time :
(i) the muscles between the ribs relax causing the rib cage to move downward and inward, and
(ii) the diaphragm relaxes and moves upward [see Figure (b)]. The downward and inward movement of rib cage, as well as the upward movement of diaphragm, both decrease the space in our chest cavity and make it smaller [see Figure (b)]. As the chest cavity becomes smaller, air is pushed out from the lungs.
The process of breathing in of air and breathing out goes on continuously all day and all night. The breathing movements in our body can be felt easily as follows : Place one hand on the middle of your chest and the other hand on the abdomen (as shown in Figure). Take a few deep breaths and release them. We can feel the up and down movements of our chest and abdomen caused by ‘breathing in’ and ‘breathing out’ of air. If we observe carefully, we can also see the up and down movements of the abdomens of animals such as cows, dogs and cats, etc., as they breathe.
The size of our chest increases during the process of breathing in of air (because the chest cavity becomes bigger). Different people can expand their chest by different amounts We can measure the maximum expansion in the chest size as follows : First measure the size of chest with a measuring tape while holding the breath, after exhaling normally (see Figure). Then take a very deep breath (so as to cause maximum expansion in chest), hold the breath for a while and measure the size of the chest in this expanded position. The difference in the two readings of the measuring tape will give the maximum expansion in the size of the chest which can he brought about by us.
Activity 1
To Demonstrate the Mechanism of Breathing
We can demonstrate the mechanism of breathing by performing a simple activity as follows : The apparatus required for this activity is shown in Figure (a). A big bell jar B is taken (The bell jar is a glass jar whose bottom is open). A glass tube T branching into two smaller tubes at its lower end is fitted in the mouth of the bell jar with the help of a cork. Two balloons are tied at the two ends of the tube T as shown in Figure (a). A thin rubber sheet S is tied around the open bottom of the bell jar. In this apparatus, the space inside the bell jar represents the chest cavity, the balloons represent the lungs whereas the rubber sheet represents the diaphragm.
(i) In order to show the process of ‘breathing in’ air, we pull the rubber sheet downwards [see Figure (b)]. In this case, the space in the bell jar increases, lowering the air pressure inside the bell jar. The air from outside rushes in through the tube T into balloons due to which the balloons get inflated (their size increases) [see Figure (b)]. This shows the action of diaphragm during inhaling of air.
(ii) In order to show the process of ‘breathing out’ of air, we release the rubber sheet [see Figure (c)]. In this case, the space in the bell jar decreases. The air from inside the balloons goes out through the tube T due to which the balloons get deflated (their size decreases) [see Figure (c)]. This shows the action of diaphragm during exhaling of air.
The action of rubber sheet in this activity shows how we inhale and exhale air during breathing with the help of the downward and upward movement of diaphragm in our body. When the diaphragm moves downward during inhaling, the lungs get filled with air. But when the diaphragm moves upward during exhaling, then the air is forced to go out of the lungs. This activity, however, does not show the action of rib cage during breathing.
What Happens to the Air Which We Breathe During Respiration
The air which we breathe in from the atmosphere is called ‘inhaled air’ and the air which we breathe out (from our lungs through nose or mouth) is called ‘exhaled air’. We know that air is a mixture of gases such as nitrogen, oxygen, carbon dioxide and water vapour, etc. Now, the air which we ‘breathe in’ is a mixture of gases and the air which we ‘breathe out’ 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 percent) because it is neither used up in respiration nor produced during respiration. We will now discuss what causes changes in the proportions of oxygen, carbon dioxide and water vapour in the inhaled air and exhaled air.
(i) The Case of Oxygen. The air which we inhale contains a higher proportion 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 of oxygen. Thus, exhaled air contains less oxygen than inhaled air.
(ii) The Case of Carbon Dioxide. The air which we inhale contains a lower proportion of carbon dioxide. Now, during respiration when oxygen breaks down glucose food, then some carbon dioxide is produced. So, the exhaled air that comes out after the process of respiration contains a higher proportion of carbon dioxide. Thus, exhaled air contains more carbon dioxide than inhaled air.
(iii) The Case of Water Vapour. The air which we inhale contains only a little of water vapour. When glucose (food) is broken down by oxygen during respiration, then water is also produced (alongwith carbon dioxide). So, the exhaled air contains a lot more water vapour than inhaled air.
From the above discussion we conclude that during the process of respiration, when glucose (food) is broken down to release energy, then some of the oxygen of inhaled air is used up, whereas carbon dioxide and water are produced. This is why, the exhaled air contains less of oxygen but more of carbon dioxide and more of water vapour. The amount of oxygen, carbon dioxide and water vapour in inhaled air and exhaled air is given below :
Inhaled Air
Oxygen : 21%
Carbon dioxide : 0.04%
Water vapour : A little
Exhaled Air
Oxygen : 16.4%
Carbon dioxide : 0.04%
Water vapour : A lot
It is a common observation that if we exhale air on a mirror through our mouth, then a patch of moisture is formed on the mirror surface. This is because the exhaled air coming from our mouth contains a lot of water vapour. This water vapour condenses on the mirror surface to form tiny droplets of water (which appear as a patch of moisture). We will now describe an activity to show that carbon dioxide gas is formed during the process of respiration in our body.
Activity 2
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 is shown in Figure. 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 (see Figure). 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 activity, 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).