Thursday, July 24, 2008

Changes In Fetal Circulation At Birth

the other day i mentioned that there was a muscle in the newborn infant that closes like a fist around the umbilical artery at birth. I thought i read this somewhere and tried later to find it. I couldn't find where i read it but i did find my physiology text and it describes fetal circulation pre and post birth.

the story, as usual, is a lot more complicated and subtle. there are half a dozen places where fetal circulation has to be rerouted, and of course blood vessels all have smooth muscle walls already, so these are the muscles responsible for closing things off.


main points:

the lungs in the fetus do not function, so no need for blood to flow in and out of them.

The mother's liver takes on most of the liver function so not much blood needs to pass through the fetal liver either. (note that SOME blood passes through the usual routes in the liver and lungs and no doubt this flow of blood mechanically and chemically is necessary to modulate the development of those organs.)

i don't see any info on fetal kidneys. do they function? I imagine the mother's kidney's function for the fetus? where would fetal urine go? i see a tube called allantois coming out of the urinary bladder going into the umbilical cord, makes no sense?

fetal blood flows from internal iliac artery to umbilical arteries to the placenta. the blood which has been enriched in nutrients and oxygen and has been cleansed of wastes and CO2 flows back in the umbilical vein. some goes to the fetal liver, through the liver into the inferior vena cava. the rest goes through a bypass called the ductus venosus around the liver into the inferior vena cava. the veins from the intestines which WILL send nutrients to the body through the liver also connect to this bypass.

from there to the right atrium. from the right atrium it has 3 choices:

1) the usuall route into the right ventricle and to the lungs. the fetal lungs are collapsed and not much blood takes that route.

2) a shunt to the aorta called the ductus arteriosus.

3) flow to the left atrium via a flap covered opening, the foramen ovale in the interatrial septum.

most of the highly oxygenated blood from the placenta flows across to the foramen ovale into the left atrium and to the aortic arch. this supplies the brain which is the organ in the fetus that needs the most oxygen.

separation from the placenta causes oxygen to drop and carbon dioxide to build up. this induces the infant to take its first breath. expansion of the lungs allows blood to flow from the right ventrical to the pulmonary arteries. blood is now returning from the lungs to the left atrium and increases pressure there. this causes the flap of the foramen ovale to close. thus the blood from the right atrium now has only two choices. (eventually the foramen ovalle is closed with connective tissue.

increased pressure in the left side of the heart and reduced pressure in the right side mean more pressure in the aorta and less coming from the right ventrical to the ductus arteriosis into the aorta. the ductus arteriosis now constricts. It's got smooth muscle around it right? WHAT IS THE SIGNAL to constrict? within weeks it is obliterated by fibrous connective tissue.

my text doesn't mention any particular constricting on the part of the umbilical arteries, just that they fill in. curious. i don't see why there would be any less blood pressure in them after birth, they come off the bottom of the aorta, internal iliac artery.

ditto the umbilical vein gradually fills with connective tissue.

the text does mention that the ductus venosus (from umbilical vein to vena cava) does constrict. This causes the blood from the infant's intestines to flow into the hepatic portal vein into the liver.

diagram from here

the anatomy from:
"Human anatomy and physiology" 2nd ed. Alexander P. Spence and Elliott B. Mason.