BREATHING AND EXCHANGE OF GASES:MECHANISM OF BREATHING:-Respiration occurs in 2 stages. 1 Inspiration: during which atmospheric air is drawn in and 2 Expiration: in which the alveolar air is released out
The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. When the air pressure within the alveolar spaces falls below atmospheric pressure, air enters the lungs, resulting in inspiration there is a negative pressure in the lungs with respect to atmospheric pressure, provided the larynx is open. Similarly expiration takes place when the air pressure within the alveoli exceeds atmospheric pressure; air is blown from the lungs. The flow of air is rapid or slow in proportion to the magnitude of the pressure difference because atmospheric pressure remains relatively constant, flow is determined by how much above or below atmospheric pressure, the pressure within the lungs rises or falls. Alveolar fluctuations are caused by expansion and contraction of lungs resulting from tensing and relaxing of the muscles internal intercostals muscles between ribs diaphragm. Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero -- posterior axis the contraction of external intercostals muscles lifts up the ribs and sternum causing an increase in the volume of thoracic chamber in the dorso ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases intra -- pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs inspiration.

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