Abstract The exchange of the respiratory gases in the multivillous stream system of the human placenta is examined on the basis of the laws of diffusion. The results thus obtained can most clearly be represented by graphs: 1. The exchange rates of O 2 and CO 2, together with the influence of the Bohr effect and Christiansen-Douglas-Haldane effect of the blood, determine the course of the “effective O 2 and CO 2 dissociation curves,” which are especially valid for the placenta. The effective dissociation curves for the maternal and fetal blood can be reproduced in O 2 and CO 2 diagrams. 2. The arterialization in the placenta depends on the ratio of the maternal-fetal blood flow Q̇ M / Q̇ F and on the ratio of the O 2-diffusing capacity to the fetal blood flow D F / Q̇ P . The dependence of the O 2 and CO 2 pressures in the fetal mixed capillary-end blood on Q̇ M / Q̇ F and D P / Q̇ F is reproduced in an alignement nomogram. From this, the exchange-determining factors can be determined as Q ̇ M Q ̇ F = 1 and D P Q ̇ F = 8×10 −3 [1/mm Hg] by taking into account the measured values obtained at the exact moment of delivery.