Pregnancy physiology I

- You know, there's that saying that it takes a village to raise a kid. Well, I guess you could say that it takes an entire body, and I mean every single organ of a body, to make a baby. It's like the best example of teamwork that there is, all the organs in the body working together to support the growth and the development of the fetus, but also just as important, to make sure that mom's body isn't sacrificing its own needs to support the pregnancy. So in order to accomplish that, pretty much every organ system undergoes a significant amount of change. And I wanna go through what those changes are. So let's start with the cardiovascular system, so the heart and the blood vessels throughout the body. And, I guess you could say, at the very basic, most essential, level, the body needs more blood to carry oxygen and those nutrients to the fetus. So the blood volume increases by something like 40 to 50 percent throughout the pregnancy. And it's not just that there's more blood. The heart is also working harder to more efficiently supply that blood to the fetus. So, for example, the heart beats more quickly. It beats, like, 10 to 15 beats more each minute than usual. And the stroke volume, so the amount of blood that's pumped out with each heartbeat, increases, which means that the cardiac output, or the amount of blood that's pumped out of the heart each minute, also increases. So in summary, more blood is being pumped out of the heart to meet the demands of the fetus. So if you had to take a guess, what do you think happens to blood pressure during pregnancy? I'm gonna guess that you guys guessed that it increases, because that's certainly what I thought happened. But it actually decreases. And it decreases for a couple of different reasons. Firstly, there's a lot of progesterone floating around in the blood during pregnancy. And, if there's something that progesterone does really well, it relaxes smooth muscle. And that includes the smooth muscle that surrounds all of the blood vessels. So that relaxation causes dilation of the blood vessels, which then lowers the blood pressure. So that's a first cause, sort of, of the decrease in blood pressure through pregnancy. The second thing that contributes to the lower blood pressure is the placenta, which is an addition of an entirely brand-new blood vessel circuit to the circulatory system. It's like when you add a resistor in parallel, reducing the resistance of the entire circuit. I'm just joking. That doesn't help anyone understand it any better. I guess you can think of it kind of, kind of as a tall apartment building and what would happen to the pressure in the shower heads if you added a whole additional floor of apartments, with shower heads that are really leaky and let out a lot of water. The placenta is kind of like that. It's a really low-resistance circuit. And also, while we're talking about the cardiovascular system, there's a syndrome that's called Supine, it's called Supine .. "Supine" means when you're lying on your back, Supine Postural, so it's Supine Postural, "Postural" meaning related to posture, Supine Postural Hypotensive Syndrome, so Supine Postural Hypotensive Syndrome. And it's weird that I'm talk about a syndrome in a video that's talking about physiology during pregnancy. But I guess you could say, it's a syndrome, or something that goes wrong, due to normal pregnancy physiology. So anyways, what it refers to is when, late in the course of a pregnancy, the uterus becomes larger, right? And when the uterus becomes larger, it can compress the inferior vena cava. So that kind of looks like this. And since the inferior vena cava gathers the blood from the veins of the lower body and returns that blood to the heart, the compression of the inferior vena cava means that less blood is pushed back to the heart, meaning that less blood is pumped out with each heartbeat. And that leads to low blood pressure, or hypotension. Right? So that's where the "Hypotensive" in this name comes from. So the woman starts to feel light headed and like she's about to faint, especially when she's on her back, because that's the position, when she's on her back, that's the position in which the uterus is exerting the most pressure on the inferior vena cava. A really quick way to resolve that issue is for the woman to turn to her left side, and that tilts the uterus to the left and off of the inferior vena cava, allowing more blood to return to the heart. So that's all or most of the functional changes that occur with the cardiovascular system in pregnancy. And the growing uterus also shifts the heart to the left a little, so there are also some anatomical changes, too. Now, I know that's a lot of information, but the cardiovascular system undergoes lots of changes to support the pregnancy. So now let's move on to what changes occur in the respiratory system. So oxygen. Okay. Oxygen consumption increases in pregnancy. Right? The fetus uses oxygen, the mom is using more oxygen to support all the changes in the body. So that means that mom's body needs to bring in more oxygen into the blood. And that's mostly done by increasing minute ventilation. It's mostly done by increasing minute ventilation, or the volume of air that's taken in each minute. And it's not that pregnant ladies intentionally take deeper breaths, because that would get really uncomfortable very quickly. It's all that progesterone once again. So it's all that progesterone in the blood. And what that progesterone does is, it acts on the central respiratory centers in the brain to instruct the lungs to take in more air with each breath. So that's how you end up with more air being taken in with each breath during pregnancy. And a quick thing that needs to be mentioned is that, when you have more air being inhaled with each breath, more carbon dioxide is being exhaled with each breath. Right? Does that make sense? And carbon dioxide is an acid. So in order to keep the pH of the blood balanced, the body responds to that decrease in carbon dioxide, so that decrease in an acid, by increasing the secretion of bicarbonate, which is a base, from the kidneys. So you have increased secretion of bicarbonate from the kidneys. So what you end up with is either normal or a very slightly alkalotic, so a slightly basic, blood pH. Okay. And lastly, there are a couple of anatomical changes, too. So, the enlarging uterus pushes the diaphragm upwards, almost four centimeters through the course of the pregnancy. And that would really make it difficult to breathe. But the chest wall during pregnancy is also more mobile, it's more flexible. And your chest wall circumference is larger. So that works to make up for that upward shift of the diaphragm. Okay, so let's finish off down here by discussing the changes that occur with the kidney. So two things. First thing, we said that there's an increase in blood volume during pregnancy, right? And secondly, all of the arteries in the body are dilated during pregnancy, including the ones that supply the kidney. So if you add those two things up, you end up with having more blood flow to the kidney. And what that means is, you end up with an increase in the rate of filtration of blood through the kidney. It's kind of like, you know those water filters that you can attach directly to your faucet? Right? Imagine if you had one of those. And if your pipes and your faucet got much larger, they got much wider, and there's more water running through the pipes, the rate at which the water was being filtered through the water filter would increase drastically. Well, this is the exact same thing. The kidney is just like your water filter in that it filters all of your blood. Now, with regards to the bladder, there is a contentious topic of whether the bladder holds more or less urine in a pregnant woman. There's some thought that progesterone, which, remember, causes relaxation of smooth muscle, relaxes and increases the capacity of the bladder. And then there's other thought that the pressure of the uterus, the large uterus on the bladder, decreases the capacity of the bladder. So we're not entirely sure. But one thing is certain, and that is that pregnant women definitely urinate more frequently than normal. And that has to do with increased urine production, as well as that pressure on the bladder from the uterus. And that pressure from the uterus also leads to dilatation of the ureter. So the ureters become dilated. And that's really important. And it kind of looks like this, where the pressure from the larger uterus causes the ureters to become wider, to become dilated. And that uterus putting pressure here sort of acts as a road block. And urine builds up in the ureters behind that road block. That built-up stagnant urine acts as a medium for bacterial growth, right? Because we know urinary stasis is a risk factor for bacterial growth. And that's perhaps why pregnant women are more susceptible to developing pyelonephritis, or infection of the kidney, than are non-pregnant women. It's because of that urinary stasis that occurs as a result of the large uterus putting pressure on the ureters. All right. So those are some of the physiologic changes that occur in pregnancy, with the cardiovascular system, the respiratory system, and the renal system.