Peripheral oxygen uptake and delivery in health and disease.

Peripheral O2 uptake is mandated to subserve the energy requirements of tissue and organ systems in an obligatory aerobe such as the human. In addition to this basic need, local O2 demand may also be increased by more general bodily conditions such as fever, increased catecholamine levels, or alkalosis. Blood flow, which brings O2 to the periphery, is closely regulated at the local level to meet O2 demand except where other functional requirements, such as filtering by the kidney, may intervene. In addition to regulation of total flow at the tissue level, the distribution of flow at the microcirculatory level is another important regulatory feature because it determines the capillary surface area available to extract O2. Dominant vasoconstrictor tone is modulated at the local level by a metabolically linked substance such as adenosine or possibly by a direct effect of PO2 on the microvasculature. The precapillary vessels that control distribution are more sensitive to local hypoxia than are the resistance vessels so that blood flow redistribution is a first-line defense. The total blood flow response is made more sensitive by activity of beta-adrenergic vasodilator receptors which are active in this counterpoised system of local blood flow control. Disruption of the microvasculature, such as by microembolization, makes O2 uptake more dependent upon O2 supply than is normally the case. This event is accompanied by a loss of autoregulatory ability. In view of the myriad potential effectors on microvascular smooth muscle, the true wonder is that blood flow and O2 delivery generally are so strongly meshed with O2 uptake at every level of functional organization in the body. It would hardly be surprising, then, if disruption in one part of the system, such as the pulmonary circulation, had a counterpart in the peripheral circulation. This was alluded to in the possible association of O2 supply dependency in the periphery and severe ventilation-perfusion abnormality in the lung during ARDS. It reminds us that deranged processes that are revealed by relatively routine measurements may not be restricted to a single target organ.