 |
| Breathing Mechanism |
|
| The physical movements associated with the gaseous exchange are called breathing. The various structures that are involved in the breathing mechanism are the respiratory passage, the lungs, the outer and inner intercostal muscles, the diaphragm and the muscles attached to the diaphragm. |
| |
| The breathing movements are controlled by the three respiratory centres of the medulla oblongata and pons Varolli in the human brain. The three respiratory centres are the inspiratory, expiratory (in medulla oblongata) and pneumotaxic centres (in pons Varolli). The respiratory centres are stimulated by the carbon dioxide concentration of the blood. |
| |
| Thus, the breathing movements are involuntary, to a large extent. However, we can control the rate of breathing and extent of breathing but not for long. |
| |
|
| There are two types of physical movements associated with the gaseous exchange. They are inspiration and expiration. During inspiration, air is drawn in and during expiration, air is exhaled out. |
| |
| Inspiration or inhalation |
| |
| During inspiration the outer intercostal muscles contract which raises the chest cavity or the ribs. This is accompanied by the lowering of the diaphragm. Together these movements serve to increase the area of the thoracic cavity that reduces the pressure. The air from outside rushes into the lungs. |
| |
 |
| Breathing Movement - Inspiration |
| |
| After the internal respiration in the lungs, the impure air is expelled in the following manner: |
| |
| Expiration or exhalation |
| |
| The inner intercostal muscles contract bringing the ribs back to the original position and the diaphragm is also raised back by the action of the abdominal muscles. This reduces the space in the chest cavity and increases the pressure. This expels the air out of the lungs. |
| |
 |
| Breathing Movement - Expiration |
| |
| One respiratory cycle consisting of expiration and inspiration takes five seconds - two seconds for inspiration and three seconds for expiration. |
| |
| Demonstration of breathing mechanism |
| |
| Take a bell jar. Towards its rounded end, fix a 'Y' shaped glass tube and on the open ends of the two branches tie a balloon each. On its open end, tie a thin rubber sheet. The cavity of the bell jar acts as the thoracic cavity, the "Y" shaped tube as the trachea that branches into bronchi and the rubber sheet as the diaphragm. |
| |
 |
| |
| Demonstration of Breathing Mechanism |
| |
|
| There are totally about 700 million alveoli in the two lungs of an adult human being. This increases the surface area enormously. The total surface area of the lungs is 70 square metres - that is almost the size of the tennis court. It is nearly 100 times the surface of the human body (skin). |
| |
| Thus, the lungs can hold a lot of air, about 6000 ml. This lung capacity is defined as the maximum air that can be held in the two lungs at any time. |
| |
| However, during one breath in and out, the volume of gas exchanged is called the tidal volume. It is about 450ml during quiet breathing. |
| |
| The volume of air that can be drawn in after normal inspiration is about 1500 ml and is called the inspiratory reserve volume (IRV) or complemental air. |
| |
| The volume of air that can be expelled out after a normal expiration is about 1500ml and is called the expiratory reserve volume (ERV) or supplemental air. |
| |
| Even after forced expiration, some amount of air remains in the lungs. This is called residual air that is about 1500ml. |
| |
| Some amount of air remains behind in the various parts of the respiratory tract also. The air in the trachea and bronchi (where no diffusion occurs) is called dead space air (350ml). The air remaining in the alveoli or air sacs is alveolar air (150ml). |
| |
| The maximum volume of air that can be exchanged in one breath in and out is called the vital capacity. It is about 5000ml. |
| |
| The above figures are represented in the given graph |
| |
 |
| |
| The instrument that is used to measure the lung volume is called spirometer. |
| |
|
| In most cases, glucose is oxidised in the presence of oxygen to give carbon dioxide. Thus, for respiration, an organism has to take in oxygen and give out carbon dioxide. This is called gaseous exchange. The oxygen taken in is used to break down the respiratory substrate (e.g., glucose) and energy is released along with carbon dioxide. This whole process is called respiration. |
| |
| Gaseous exchange involves movements of some specialised structures. The mechanism of taking in oxygen and giving out carbon dioxide is called breathing. Thus breathing and respiration are not synonymous. Respiration involves the mechanical and the bio-chemical processes whereas breathing is only the mechanical or physical process of exchange of gases. |
| |
