Biology 12: Fetal Circulation
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Biology 12: Fetal Circulation
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Biology 12: Fetal Circulation
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Biology 12: Fetal Circulation
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A fetus doesn't breath air!
Of course you knew that. But if you think about it, that is a pretty good trick
considering our circulatory system. We have a systemic and a pulmonary circuit.
What would a fetus do with its pulmonary circuit?
Also, if the fetus doesn't breath air, it still has to be able to exchange gases. You know
that the fetus will get oxygen from its mother, and you may even know that has to
occur across the placenta, but how? You see, oxygen diffuses into our blood from the
air because there is more oxygen in the air than in our blood. But a pregnant woman
sends her blood to the placenta for gas exchange there, how does the fetus get enough
oxygen from its mother's blood?
The pulmonary circuit does two things in an adult:
1) it picks up oxygen from the air
2) it supplies the lung tissue with nutrients and picks lung tissue waste.
In a fetus the pulmonary circuit is not necessary for picking up oxygen from the air,
but it is still necessary for servicing developing lung tissue. In the adult, a lot of blood
goes off into the pulmonary circuit with every cardiac cycle. But in the fetus, that does
not need to be the case.
Therefore, there is no need to pump all the blood from the right atrium into the right
ventricle and into the lungs in the fetus. Therefore, it doesn't happen. Instead, the two
atria are connected via the foramen ovale, a hole between the atria. This foramen
even has a valve that helps to ensure that blood doesn't back up from the left atrium to
the right atrium.
That means that the blood that normally comes into the right atrium is the blood that
is being pumped out into the body. Where does the oxygen get in? The oxygen enters
the blood from the placenta, which the fetus accesses by sending its blood away from
the heart in the umbilical arteries. The blood returns from the placenta in the
umbilical vein, carrying oxygen. To connect this with what you know about blood
vessels, the left ventricle sends its blood into the aorta. The aorta has many smaller
arteries coming off it, including those that give rise to the umbilical arteries (one near
each hip). The oxygen is picked up through umbilical capillaries in the placenta, and
then oxygenated blood returns in the single umbilical vein. On its way back to the
heart (so that oxygenated blood can be sent out through the body again), some of the
Biology 12: Fetal Circulation
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blood detours to the liver, but the rest goes through the inferior vena cava and into the
right atrium. From there, the oxygenated blood spills over to the left atrium, and the
entire thing starts over. This is simplified, but it will do for now.
How is it possible for enough oxygen to diffuse from the mother's blood into the
fetus' blood? It has to be able to pick up lots of oxygen, since the fetal circulatory
system is not quite as efficient at sending as much blood for oxygen pick-up as the
adult one.
Fetal blood contains much more hemoglobin. In fact, if we have 100% of our
hemoglobin in our adult blood, a fetus has 150%! In addition, fetal hemoglobin is
different from adult hemoglobin. (That is not uncommon, by the way... there are other
proteins that are known to have different forms in the embryo than in the adult, like the
acetylcholine receptor). The thing about the fetal hemoglobin is that it is much better
at attracting oxygen than the adult hemoglobin.
The end result of having more and better hemoglobin as a fetus is that it can really
grab a lot of the oxygen off the maternal hemoglobin. So, it can certainly get enough.
Biology 12: Fetal Circulation
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Biology 12: Fetal Circulation
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Biology 12: Fetal Circulation
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On the diagram shown on the previous page , label these structures:
umbilical cord
ductus venosus
iliac arteries
aortic arch
umbilical arteries
ductus arteriosus
inferior vena cava
umbilical vein
foramen ovale
superior vena cava
Questions: answer these in short answers on a separate sheet.
1.
Why does fetal circulation have to be different from a adult’s circulation?
1.
What two shortcuts are made through the heart?
3.
Does oxygenated blood flow FROM the mother to the child in an artery or vein?
4.
Does de-oxygenated blood flow FROM the child to the mother in an artery or vein?
5.
Are the fetus’s blood and the mother’s blood mixed? Explain.
6.
How does oxygen get to the fetus’ blood?
7.
What enables a fetus to be more able to absorb oxygen?
8.
Why does the foramen ovale close when the baby takes it’s first breath?
9.
Newborn babies can have a condition called “blue baby” . What causes it? How is it fixed?
10.
Through which vessel(s) is fetal blood pumped into the umbilical arteries?
11.
Is blood going into the fetus’ inferior vena cava oxygenated OR de-oxygenated? Explain.
12.
Compare the concentration of Oxygen, glucose and carbon dioxide in the following blood
vessels or locations.
Umbilical vein
Right atrium
oxygen
glucose
Carbon dioxide
13.
Fill in the blanks in the following passage:
Iliac artery
Umbilical
artery
Biology 12: Fetal Circulation
Oval Opening = ____________________
Venous Duct = _____________________
Name:
Arterial Duct = _____________________
In the adult, oxygen-poor blood returns from parts of the body to the __________________ side of
the heart. This blood is then pumped to the lungs, oxygenated, then sent back to the
__________________ side of the heart, which then pumps it to the brain and the rest of the body
where the oxygen is needed. And then this process starts all over again. And again and again, seventy
or eighty times a minute, for seventy or eighty years.
But in the fetus, the lungs are collapsed. The most miraculous organ of all -the __________________
provides oxygen. The __________________, slapped up against the wall of the mother's uterus
(womb), allows the passage of oxygen and nutrients from the mother's blood through a
_______________ to the baby's blood. From here, it flows through the umbilical cord into the baby.
The right side/left side circulation of the born baby is not the case in the unborn baby. With the lungs
collapsed, there is no need for the __________________ side of the heart to send blood to the
__________________…the blood is already oxygen-rich, thanks to the __________________.
Instead, there are two short cuts that allow the blood to by-pass the lungs. One is called the
__________________ and the other is the __________________ .
The __________________ steals blood normally routed to the lungs and lets it flow straight into the
aorta on to the rest of the body. The __________________ is actually a hole in 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. Being born changes all of that.
With delivery, the lifeline of the umbilical cord is severed. Air hunger develops, and a reflex causes
the newborn child to gasp for air. With this gasping, 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 all of the magic.
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 __________________
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 __________________ and __________________. The
flow in the heart becomes stronger on the left, which causes a one-way flap to slam shut over the
__________________, closing it. The generous diameters of the pulmonary arteries to the lungs far
out measure that of the __________________. The laws of physics apply here: it is easier to flow to
the lungs than through the __________________ and it withers. That first gasping causes the
__________________ and the __________________ to be bypassed, and the lungs that were
previously bypassed, finally join the club.
Biology 12: Fetal Circulation
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