The embryo’s heart is the first organ that forms in its tiny body,
and like most complex instruments, it begins with some simple structures.
Let’s go back to 18 days after conception......
Looking in the mother’s uterus,you can see the embryo surrounded by its yolk sac and amnion.
Let’s take a look inside.
Here’s a diagram of the embryo seen from a side view.
Right now, it’s about the size of a raisin.
There’s the head region
and that red-colored area slightly above it
contains two tubes that will form the embryo’s heart.
Here’s what the tubes look like from a front view.
On day 21, we see that the primitive heart tubes
have moved below the embryo’s developing head region.
And by day 22, the tubes have fused together,
and have moved to the area that will eventually be our embryo’s thoracic, or chest cavity.
It’s also about this time that the heart begins to beat for the first time...
Here’s what it looks like from the front.
Now let’s go back to day 18 and watch this happen from a different viewpoint.
Here are two tubes in our embryo’s chest region seen from a front view.
Watch this... Over the next two days, these tubes fuse together.
Here’s another amazing part: the tube now starts bending and twisting
and over the next 8 days it forms a simple version of the heart.
By the time the embryo becomes a fetus at two months,
the heart bears a close resemblance to what it will look like after the baby’s born.
But the resemblance is only superficial.
On the inside of the heart, things are much different in both form and function.
Here’s a newborn heart on the left. Let’s take a closer look.
There’s the right atrium
right ventricle
left atrium
and left ventricle.
The two major blood vessels are the aorta
and the pulmonary artery.
The pathway of blood in the newborn heart works like this:
oxygen-poor blood from the body enters the right atrium, then goes to the right ventricle.
From the right ventricle, the blood is pumped to the lungs where it becomes oxygen rich.
Then the blood flows back to the heart filling the left atrium
and from there on to the left ventricle.
The left ventricle pumps the oxygen rich blood through the aorta,
which carries it to the rest of the newborn’s body.
You can see the fetal heart has the same basic components as the newborn heart,
but there are a couple important differences.
Because the placenta is providing all of the oxygen the fetus requires
its lungs are not needed to perform this task,
and therefore much of the fetus’ blood is detoured away from the lungs
through two openings or connections.
They are the foramen ovale,
which connects the right and left atria,
and the ductus arteriosus which connects the aorta and the pulmonary artery.
As blood enters the heart into the right atrium
some of the blood flows into the right ventricle as in the newborn,
but also notice that some blood flows directly into the left atrium through the foramen ovale.
This blood will pass directly into the left ventricle
and be pumped out to the body without ever having gone to the lungs.
In addition, some of the blood that did enter the right ventricle,
and would normally go to the lungs, never reaches the lungs.
Here lets watch.
As blood is being pumped out of the right ventricle
towards the lungs through the pulmonary artery
some of that blood escapes into the aorta through the ductus arteriosus
bypassing the lungs as it does.
These two important connections will remain open up until the time of birth.
Within thirty minutes after the baby’s first breath,
the ductus arteriosus will completely close,
and the flap of the foramen ovale will shut off like a valve.
This happens because of an increase in pressure on the left side of the heart,
and a decrease on the right side.
These changes in the heart anatomy cause the blood to flow to the lungs,
which will take over their lifelong job of supplying oxygen to the body.
It’s incredible to think that this complex organ
started off as a couple of tubes only 2 1/2 weeks ago.