• Pulmonary ventilation has two parts – inspiration and expiration. It is a completely mechanical process that depends on volume changes in the thoracic cavity.

Boyle’s Law (P₁V₁=P₂V₂) is seen in ventilation as well.

• Inspiration : when the diaphragm moves down, the size of the thoracic cavity increases, which decreased the intrapulmonary pressure. Atmospheric pressure now being greater than the air pressure in the lungs, air rushes in. The lungs expand, aided by the intrapleural pressure (the pressure in the intrapleural space between the visceral and parietal pleura). When the pressure in the alveoli is relatively the same as atmospheric pressure, inspiration is over.
• Expiration : occurs when the diaphragm returns to its resting position and the lungs "recoil." The volume decreases in the thoracic cavity, which increases the intrapulmonary pressure. This would cause air to leave the lungs.
• The tidal volume, or the volume of air normally exchanged, is about 500cc. This corresponds to the amount of D V
• Except during collapse, the lungs are never completely emptied of air. Even after the most forceful expiration, about 1200cc of air remains. During forceful inspiration, up to 3000cc, in excess of tidal volume, may be brought in.
• 3000 (forceful inspiration) + 500 (tidal volume) + 1100 (expiratory reserve) = 4600cc. This is the vital capacity, or the max amount of air that can be exhaled after forceful inspiration. Total lung capacity would be 4600 (vital capacity) + 1200 (residual volume) = 5800cc.
• Atelectasis is lung collapse, usually due to a puncture wound to the chest. Air enters the pleural cavity. Normally, the serous fluid in the cavity holds the visceral and parietal pleura together, creating the intrapleural pressure that aids in inspiration. When a pneumothorax (air in the pleural cavity) occurs, the lung’s airspaces are not held open, and the lung collapses due to its natural tendency to recoil. Until the pneumothorax is dealt with, no air can enter the collapsed lung.