As previously mentioned the feto-placental circulation is independent of maternal circulation, the blood of both circulation is essentially not mixed with each other. This time I will explain the regulating factors of feto-placental circulation.

As you might imagine, the research on human feto-placental circulation is difficult. Therefore the research is mainly based on the results of pregnant sheep and they are applied to human feto-placental circulation.

Therefore the following evidences are the results of pregnant sheep.  Mean blood pressure is calculated from one third of subtract diastolic blood pressure from systolic blood pressure. While mean blood pressure in adult is around 95mmHg, that in the fetus is around 45-50mmHg. The peripheral region of umbilical artery in the placenta changes into capillary and the blood pressure in the capillary is around 10mmHg. In the region of this capillary fetus takes oxygen from maternal circulation in the retro-placental blood pool and excretes wastes into maternal circulation. The blood supplied with oxygen in the capillary in the placenta flow out into umbilical vein and return to the fetus. The diameter of capillary is only 5-7 micron (1 micron is the one thousand millimeter). Since the number of capillary is too much, the total cross section is reached to 700 hundred hold than artery.  The blood volume in the feto-placental unit increases with advancing gestation. At term sheep pregnancy weighing 40Kg (duration of sheep pregnancy is around 140 days) the volume in the feto-placental circulation reaches to 600ml/min. The placental circulation occupies the 40-50% of this circulation.

Therefore the placental circulation effects largely on the fetal blood pressure.

In my previous lecture I have described the 3 main whole body factors, which regulate blood pressure: (1) hormonal and (2) nervous systems and local factor is (3) autonomous system. Let us show you the involvement of the 3 main factors in the feto-placental unit.

Firstly autonomous system is shown: the change of blood flow in the aorta and placenta by ligation of fetal big artery, which locates in the inferior aorta of the fetus, was examined in relation to the degree of ligation. The ligation of aorta results in the increase of blood pressure. The placental blood flow decreased in parallel with the degree of ligation of aorta. This evidence show clearly autonomous system is not involved in in the feto-placental circulation. (Ref.1) The brain circulation is the example of autonomous system control of blood pressure. In the brain the change of blood pressure between 60-140 mmHg, does not effects on blood flow in the brain and blood flow does not change in the range of the above blood pressure.

Next hormonal and nervous systems are described in my previous blog. Our visceral organs including vessels are influenced under the autonomic nerve systems. While adrenaline is released from sympathetic nerves and contract vessels, acethylcholine is released from parasympathetic nerves by stimulation of each nerves. Angiotensin and vasopressin are very active hormones for contraction of vessels. When norepinephrine (similar to adrenaline) and angiotensin were administered into sheep fetus, the change of blood flow and resistance (blood pressure) in the umbilicus were examined. In this experiment while norepinephrine does not effect on both of them, angiotensin decreases blood flow and increases resistance (blood pressure) in the umbilicus. These experiments show that feto-placental circulation is regulated by angiotensin and other peptide hormones (described in my previous blog), but by not nervous systems and autonomous system.

  

Ref. Anderson DF, Faber JJ. Regulation of fetal blood flow in the lamb. Am J Physiol 1984;247:R567-R571