Page 25, Early Development, Devel. Physiol., Dr. D. Penney

The Umbilical Vessels:

Fetal blood has a lower saturation with oxygen than adult blood. In some vessels the level of saturation is much lower than that of comparable adult blood.

Umbilical vein blood is approximately 80% saturated (Figure 2.14). This contrasts with values of 97-98% saturated for arterialized blood post-birth, and is low considering the higher affinity of fetal blood for oxygen. However, when one considers the low partial pressure of oxygen in this vessel (P02 = 20-25 mmHg, as opposed to 100 mmHg postbirth), it is remarkable that a value as high as this is achieved.

Umbilical arterial blood has an even lower oxygen saturation, usually less than 50%, as it returns to the placenta for re-oxygenation.

See Misconception 21

In contrast, venous blood mixing with umbilical vein blood is only 26-27% saturated. As a result, the blood reaching the heart through the inferior vena cava is only 67% saturated. Through the action of the vena caval-right atrial structures discussed above, the blood eventually reaching the heart, head, etc. is approximately 62% saturated, while the blood traversing the descending aorta is 58% saturated. This or a slightly lower level is the degree of saturation of blood returning to the placenta. This contrasts with a value of 73-75% saturation of mixed venous blood in the adult.

The placenta is a complex structure in which gases and other substances are transferred between maternal and fetal circulations, but in which the maternal blood and fetal blood never come into direct contact.

To review the oxygen dissociation curve

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