A clinician's musings on oxygen: Too little or too much with life in the balance.

The most abundant element on Earth, oxygen (O) is essential for all complex, eukaryotic life. Because under- (hypoxia) or overexposure (hyperoxia) to O can be detrimental, achieving the ideal balance is crucial for human survival. In this perspective, we discuss the fundamental role of hemoglobin in O transport and tissue oxygenation. We also discuss the role that O can play in oxidative stress, sometimes initiating inflammatory cascades in vulnerable individuals, such as those with deficiencies in antioxidant defenses or with immune dysregulation. Preterm newborns may be especially prone to such oxidative injury.