Breathing Mechanism

The physical movements associated with the gaseous exchange are called breathing. They are controlled by the respiratory centre of the medulla oblongata in the human brain. Thus, the breathing movements are involuntary to a large extent. However, we can control the rate of breathing and the extent of breathing but not for a long time. The respiratory centre is stimulated by the carbon dioxide concentration of the blood.
There are two types of physical movements associated with the gaseous exchange. They are:

• Inspiration or inhalation

• Expiration or exhalation

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, which reduces the pressure. The air from outside rushes into the lungs.

Inspiration After the internal respiration in the lungs, the impure air is expelled in the following manner:

Expiration

Expiration or Exhalation

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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.

Demonstration of Breathing Mechanism

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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.

Experiment Demonstrating Inspiration To demonstrate inspiration, pull the rubber sheet down. The balloons get inflated. This shows how the lungs are filled with air when the diaphragm moves down.

How are the Balloons Inflated?

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When the rubber sheet is pulled down, the volume of the cavity increases. This brings the pressure down. Low pressure inside the jar brings the air rushing in through the tubes and inflates the balloon.

Experiment Demonstrating Expiration To demonstrate expiration, the rubber sheet is allowed to go back to its original position. The balloons get deflated. This shows how the lungs are deflated when the diaphragm comes back to its original position.