The major effects of breathing dense gas during physical exercise in hyperbaric environments can be traced to high airway resistance during inspiration and expiration and especially to an increased tendency for lung airways to become "choked" during expiration. The body's responses to the high resistance include decrease of alveolar ventilation, which leads to CO2 retention. This hypoventilation is aggravated by poor mixing in the lung because of low diffusivity of gases in the dense environment. Also, there is a tendency for the person to let the end-expired volume of the lung enlarge; this causes a marked increase of work against elastic recoil of pulmonary structures. Because the elastic work occurs during the inspiratory phase of a breath, there is a disproportionate increase of the work of the inspiratory muscles that may lead to fatigue of inspiratory musculature and consequent aggravation of the hypoventilation.