Fetal Blood Pressure and Vascular Resistance:
Maximum arterial blood pressure in the fetus is considerably less than that in the mother (i.e. 50-70 mmHg) (Figure 2.15).
From the heart, blood pressure falls gradually as it travels through the pulmonary trunk and ductus arteriosus, to the descending aorta, to the umbilical artery, and finally to the placenta. This is due to the low level of vascular resistance in these vessels.
An abrupt fall in pressure to 30-35 mmHg takes place in the tertiary arterioles of the placenta, indicating a site of high resistance. Pressure then falls gradually again from the fetal villi, to the collecting veins, to the umbilical vein.
Another large pressure drop of 20-25 mmHg occurs at the ductus venosus, thus the ductus venosus is also the site of high resistance. Keep in mind that resistance is a function of perfusion pressure divided by flow rate. The pressure of the blood reaching the heart through the inferior vena cava is only 5-10 mmHg.
To review vascular resistance
Return to Index