胎盤の構造図 

 

 

When the fetus exposed to hypoxia, the fetus redistributes its blood flow to survive. In this case, the fetus increase both angiotensin and vasopressin actively. It looks like contradictive in survival, since both angiotensin and vasopressin contract actively the vessels of the feto-placental unit. It is assumed that the more increased of fetal angiotensin and vasopressin, the more contraction will come to of the vessels of the feto-placenta. Therefore in this time and next I will answer to this contradictive question.

The vital organs for the fetus are essentially similar to the grown-up adults: brain, liver, kidney and placenta. These organs are rich in capillary.

As I explained in the previous blog No.5, the placental circulation occupies 40-50% of the feto-placental circulation reaching to 600ml/min.

In the next blog, I would show you the anatomy of capillary vessels of human placenta. It is known that the one aorta branch off repeatedly and finally reach to the capillary. The diameter of the capillary is about 8 micron (1/1000 mm) and the number of the capillary is about 50 hundred million.

The wall of capillary is very thin and devoid of both elastic fiber and smooth muscle cells. The wall of capillary is consisted of only endothelial cells.

Let’s consider the anatomy of human placenta. For your understanding,

the figure of the human placenta which is typically shown in the textbook.

Umbilical cord is a navel string and shown at the bottom of left corner of the figure. The blood from the fetus (left side) - which is poor with oxygen and rich in waste - pour into the placenta via 2 umbilical arteries (UA).  The placenta is floating in the retro-placental blood pool. The center of the figure is the placenta and it looks like a Christmas tree.

The background of the tree is the retro-placental blood pool. The face of a wall at the right side of the figure is the smooth muscle of the uterus.

The almost all of the placenta is composed of the leaves of the tree. The periphery of the umbilical artery changes to the capillary (leaf). The mean arterial pressure of the capillary such as in glomerulus of the kidney is around 20 mmHg. The mean arterial pressure of the capillary of the placenta is known to be around 10mmHg. The fetus takes oxygen and excretes waste via the capillaries of the placenta from the retro-placental blood pool filled with maternal arterial blood. Maternal uterine artery (abbreviated UTA) gush out arterial blood (rich in oxygen) into the retro-placental blood pool. Fetal blood now rich in oxygen returns to umbilical vein (UV) and reaches to the fetus.

Although the diameter of the capillaries is only about 8 micron, the number of the capillaries is extraordinary abundant. Therefore, the total cross section reaches about 7 hundred than artery. Both angiotensin and

vasopressin exert effect of their hormonal action by binding to the receptors located in smooth muscle that surrounds vessels (tree branch) and contract them. However the capillary (leaf) is devoid of smooth muscle, therefore, both angiotensin and vasopressin do not work at the capillary of the placenta.

At this point, I will end this blog and continue to the next.