Describe inspiration and expiration.

What will be an ideal response?

During inspiration, the primary ventilatory muscles cause the size (volume) of the thoracic cage to increase, overcoming the elastic forces of the lungs and thorax and the resistance of the airways. As the volume of the thoracic cage increases, intrapleural pressure becomes more negative, resulting in lung expansion as the visceral pleura expands with the parietal pleura. Gas flows from the atmosphere into the lungs as a result of the transairway pressure gradient. During expiration, the muscles of inspiration relax. The elastic forces of the lung and thorax cause the chest to decrease in volume. Exhalation occurs as a result of the greater pressure at the alveolus when compared to atmospheric pressure.