Before a baby is born, it has a lot of adaptations that allow it to receive oxygen and nutrients from the mother and successfully transport them to various cells in the body who need them. So now I’m going to draw one picture with all the adaptations we see before birth. These things happen while the fetus is still in the womb.
So, this structure here is the placenta. It has a part formed by the mother and a part formed by the fetus. Mother’s blood is poured into this space (maternal part of the placenta) and the baby part, in essence, sticks the capillaries into that space with the mother’s blood.
And we see that the purple blood goes in and the red blood goes back out. And I want to show that the blood is oxygenated there. And this oxygenated blood that I draw in red. it gets into that vessel here. So this is the navel, and this is the umbilical vein.
Thus, the umbilical vein carries oxygenated blood towards the liver. And this is actually the first of the adaptations I’ve talked about. I’ll make a list of them here on the side. The first is: Umbilical vein Once the blood passes through the umbilical vein, it reaches a crossroads and can go right or left.
When she goes left, she gets into the liver and it will take her some time to get through it because it must pass through all the small hepatic capillaries (capillaries). But there’s a shortcut, and I’m circling it … right here. And it’s called a venous duct (ductus venosus) It allows blood from the umbilical vein to pass through it, so it’s actually a short vessel.
And on the other hand, it added to the inferior vena cava. This is the inferior vena cava, I’ll write the IVC here. This is a large vein that carries blood from both the left and right legs. So the blood from the inferior vena cava meets the blood from the venous duct which is strongly oxygenated So this blood I’m drawing now is basically pink.
Because there is some oxygen in it, but not as much as it is in the blood in the venous duct, because it is mixed with blood from the inferior vena cava. And this blood gets into the right atrium. And at the same time, blood from the upper vena cava enters the right atrium. In short SVC. It draws blood from the head and hands.
So in the right hall, he mixes. And I’ll draw it darker purple, because it’s mixed blood. So the second adaptation is the venous duct. It is, as I said, basically an abbreviation from the umbilical vein to the inferior vena cava. So now there’s blood in the right atrium. And it has two options for where to go next.
First, he can simply continue down to the right ventricle. And part of the blood really goes that way. And then it is expelled by the heart into the pulmonary artery. And we know that the pulmonary artery branches off and the blood from it goes to the right and left lungs.
But now we need to realize what’s going on with the blood in our lungs. I’ll show it here in the picture. We have these alveoli, but they are not aerated in the fetus. Because in a baby in the womb, the cellars are full of fluid. And blood brings small pulmonary arteries (arterioles) to the alveoli Since the cellar is full of fluid, drive a minimum of oxygen. The process that takes place here is called hypoxic pulmonary vasoconstriction.
This means that the cellars literally help to contract the supply arteries. The arteries have smooth muscle and because there is no oxygen in the cellar, it causes the artery to contract. so then it looks more like this And by being narrower, the resistance of that artery essentially increases.
And if this happens in all of the billions of the alveoli so the whole lung will put a lot of resistance as a result. From the fact that the lungs are very resistant, we can deduce several things. First: The blood pressure in the pulmonary artery, I will mark both branches here, will be very high Simply because the flow of blood faces great resistance, which he must fight. We now know that there is high pressure in the pulmonary artery and remember where the pulmonary artery comes from. It comes from the right ventricle
So for the blood to flow forward, there must also be high pressure in the right ventricle. And if there must be high pressure in the right ventricle, there must also be high pressure in the right atrium. Thus, there is high pressure in both right heart compartments because the lungs offer great resistance.
And this high pressure, especially the pressure in the right atrium It reaches such a level that it becomes higher than the pressure in the left atrium. This creates a stream of blood flowing from the right atrium through the foramen ovale (oval window) It is here and allows blood to pass through it from hall to hall.
And so part of the blood can go to the right ventricle and part can go through the septum to the left atrium And this is very useful because a relatively large amount of blood passes through the foramen ovale and at the same time relatively little blood returns from the lungs through the pulmonary veins And the reason is that it is difficult to have high lung flow when there is so much resistance.
And so blood comes from the pulmonary veins and from the right atrium to the left atrium. From the left atrium, blood goes down to the left ventricle and from there it is expelled into the aorta by contraction So the left ventricle expels blood into the aorta and it distributes it further into the body.
Before I tell you what’s going on with the aortic blood I will return to my list here on page The third adaptation is the foramen ovale allowing blood to flow from the right to the left atrium. The fourth adaptation, which I haven’t talked about yet, is the one I’m circling now Such a small connector, basically a small catheter, because blood passes through it between the pulmonary artery and the aorta.
And it’s called ductus arteriosus. The ductus arteriosus allows blood to flow from the pulmonary artery to the aorta. And why would blood actually go that way? Remember that there is high blood pressure in the pulmonary artery due to high resistance in the lungs Of course, the blood goes from high pressure areas to lower pressure areas Which in this case means that it goes from the pulmonary artery to the aorta.
In the direction I draw the arrow. The ductus arteriosus is thus the fourth adaptation of the fetus. This basically explains why so little blood returns through the pulmonary veins. Because a lot of blood gets from the pulmonary artery directly into the aorta and doesn’t get into the lungs at all due to the great resistance in them.
So now let’s finish it. The blood is now down in the aorta and flows into the legs. But it also flows into these two arteries called the internal iliac arteries. These are the ones I drew here. There are a lot of branches coming out of them, but the important one that I would like to emphasize now This is what I’m coloring now.
And she calls it the umbilical artery. And it actually brings the blood back to the placenta. And why does so much blood actually go into the placenta? That is the right question. Why isn’t he going somewhere else? There is a branch to the bladder and there are branches to other places.
Why does it mainly go to that placental branch, the umbilical artery? It turns out that the placenta, and it is very clever, has a very low resistance. Like the lungs, they have great resistance and thus redirect blood away from them so the placenta has low resistance and thus directs the blood to itself.
So we see that it is a really brilliant system. We have these five adaptation mechanisms. Umbilical vein Umbilical arteries Venous duct Oval foramen a ductus arteriosus This is how blood flows through the fetus.
