What physical change occurs during severe hypoxia?

Severe hypoxia results in various physiological adaptations as the body attempts to restore adequate oxygen levels. One significant change that can occur during severe hypoxia is intercostal retraction. This physical response indicates that the muscles between the ribs are working harder to assist with breathing, as the body struggles to obtain sufficient oxygen.

Intercostal retraction occurs when the negative pressure in the thoracic cavity is not enough to facilitate normal inhalation, leading to a visible indentation of the intercostal spaces during breathing. This is typically a sign of respiratory distress and reflects the increased work of breathing that accompanies inadequate oxygenation.

In contrast, while hyperventilation may occur in response to low oxygen levels in some cases, it is not always present or visible as a specific physical manifestation like intercostal retraction. Stridor, which is a high-pitched wheezing sound due to airway obstruction, and bradycardia, or a slowed heart rate, do not specifically indicate the body's response to hypoxia in the same way that intercostal retraction does. Thus, the identification of intercostal retraction as the correct answer highlights the body's compensatory mechanisms during such critical situations.