When the arterial blood reaches the peripheral tissues, its PO2 in the capillaries is still 95 mm Hg. Yet, as shown in Figure 1, the PO2 in the interstitial fluid that sur rounds the tissue cells averages only 40 mm Hg. Thus, there is a large initial pressure difference that causes O2 to diffuse rapidly from the capillary blood into the tissues—so rapidly that the capillary PO2 falls almost to equal the 40 mm Hg pressure in the interstitium. Therefore, the PO2 of the blood leaving the tissue capillaries and entering the systemic veins is also about 40 mm Hg.
Fig1. Diffusion of oxygen from a peripheral tissue capillary to the cells. (PO2 in interstitial fluid = 40 mm Hg, and in tissue cells = 23 mm Hg.)
Increasing Blood Flow Raises Interstitial Fluid PO2. If the blood flow through a particular tissue is increased, greater quantities of O2 are transported into the tissue and the tissue PO2 becomes correspondingly higher. This effect is shown in Figure2. Note that an increase in f low to 400 percent of normal increases the PO2 from 40 mm Hg (at point A in the figure) to 66 mm Hg (at point B). However, the upper limit to which the PO2 can rise, even with maximal blood flow, is 95 mm Hg because this is the O2 pressure in the arterial blood. Conversely, if blood flow through the tissue decreases, the tissue PO2 also decreases, as shown at point C.
Fig2. Effect of blood flow and rate of oxygen consumption on tissue PO2.
Increasing Tissue Metabolism Decreases Interstitial Fluid PO2. If the cells use more O2 for metabolism than normally, the interstitial fluid PO2 is reduced. Figure2 also demonstrates this effect, showing reduced interstitial fluid PO2 when the cellular oxygen consumption is increased and increased PO2 when consumption is decreased.
In summary, tissue PO2 is determined by a balance between (1) the rate of O2 transport to the tissues in the blood and (2) the rate at which the O2 is used by the tissues.
