Oxygen delivery to tissues.

For health, well perfused tissues, oxygen uptake is determined primarily by metabolic need rather than by oxygen supply. Tissue hypoxia supervenes when tissue oxygen tension (PO2) falls below a critical point, and the point where this occurs can be predicted from the systemic oxygen delivery or extraction ratio. A growing body of evidence suggest that tissue oxygen extraction may be impaired in adult respiratory distress syndrome (ARDS) and sepsis. In these syndromes the minimum oxygen delivery needed to maintain a normal oxygen uptake appears to be increased, as tissues become hypoxic despite high levels of delivery. However, controversy surrounds every phase of this observation, from its experimental basis, to potential causes, to its implications for patient care. In this review, we discuss the physiology of oxygen transport, the determinants of tissue oxygenation in normal and pathological states, and the therapeutic implications of oxygen transport.