The relationship between arterial and tissue PO2 in the fetal lamb.

The relationship between arterial and tissue PO2 is not well understood in the adult, let alone in the fetus. Figure 1 shows the pathway for oxygen through the maternal lungs and pulmonary circulation to the systemic circulation. For the fetus, there is an additional step of transfer across the placenta to fetal blood, and thence via the fetal heart to systemic circulation and fetal tissues. It should also be noted that well oxygenated blood from the placenta is mixed with deoxygenated blood in the fetal circulation before it is distributed to fetal tissues thus further reducing the concentration of oxygen in fetal blood circulating to tissues. It can be assumed that both in mother and fetus there will be a marked drop in PO2 between arterial blood and tissues as oxygen diffuses to the oxygen consuming sites in the mitochondria. This cascade of oxygen is illustrated diagramatically in the lower part of Figure 1. The major difference which emerges between mother and fetus is the low PO2 of fetal blood compared with maternal blood. This is likely to impose an even lower tissue PO2 on the fetus than that found in the adult and values reported for fetal tissues have ranged from 1 to 20 mmHg (1-6).