The fetal circulation

In the fetus the lungs are collapsed. Oxygen is provided by the placenta. The placenta allows the passage of oxygen and nutrients from the mother's blood through a membrane to the fetus. From here, it flows through the umbilical cord, through the ductus venosus into the fetal blood supply.

With the lungs collapsed, there is no need for the right side of the heart to send blood to the lungs, the blood is already oxygen-rich, from the placenta. Instead, there are two short cuts that allow the blood to by-pass the lungs. One is called the ductus arteriosus and the other is the foramen ovale. The ductus arteriosus steals blood normally directed to the lungs and lets it flow straight into the aorta and on to the rest of the body. The foramen ovale is a hole in the middle wall of the heart itself, allowing blood in the right side to flow through the wall into the left side and out, likewise, to the rest of the body. In both cases, the lungs are bypassed.

At birth, on cutting of the umbilical cord. Air pressure develops, and a reflex causes the neonate to gasp for air. With this gasp, the lungs expand for the first time and convert from a crimped up, solid block of tissue to soft, air-filled bags. It's the change in the consistency of the lungs that starts the process. When the lungs are in their unborn collapsed state, it takes a lot of pressure to try to pump blood through them. When they inflate at birth, this pressure falls so that it's easy for blood to flow into them. With this sudden fall in resistance, the path to the lungs becomes less resistant than the force needed to pump blood through the ductus arteriosus and foramen ovale. The flow in the heart becomes stronger on the left, which causes a one-way flap to slam shut over the foramen ovale, closing it. The large diameters of the pulmonary arteries to the lungs far out-measure that of the ductus. The laws of physics apply here: it is easier to flow to the lungs than through the ductus and it withers. That first gasping causes the ductus and the foramen to be bypassed, and the lungs that were previously bypassed, and the lungs are functional.

Ref: Pocock, G (2007) The Fetal Circulation.




http://www.wellesley.edu/Biology/cou...11/ductart.gif




http://www.health.uab.edu/15541/


http://www.embryology.ch/anglais/pca...tellung01.html

Discussion and debate regarding this glossary item can be carried out here:-
Fetal Circulation - Discussion and Debate

__________________
Rob (Co-founder & Technical Director)
StudentMidwife.NET
•••••••••••••••••••••••••••••••••••••••••
Have you just been offered a place? If so and you want a mentor please post in post this forum (also post here if you would like a mentee)