by • 26/07/2011 • Old Pattern Biology NotesComments (0)605


Breathing occurs in two phases:

1. Inspiration

2. Expiration

·        Inspiration:

1. During inspiration, the dome-shaped diaphragm contracts and becomes flat some what and thereby lowering the floor of the thoracic cavity.

2. The external inter-costal muscles contract raising the ribcage. A combined action of these two events expands the thoracic cavity, which in turn expands the lungs.

3. The air pressure within the lungs decreases.

4. Thus air from the environment outside the body is pulled into the lungs to equalize the pressure of both sides.

·        Expiration:

1. The diaphragm relaxes and assumes dome like shape. During expiration, the external inter-costal muscles relax and the internal inter-costal muscles contract as a result of which ribcage drops.

2. The combined action of these two event decreases the volume of the thoracic cavity which in turn decreases volume of lungs.

3. The air pressure with in the lungs increases.

4. The air is thus forced out of the lungs.

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