怀孕生理学（一）

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 want to 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 and I guess you could say at the very basic most essential level the body needs more blood to carry oxygen and 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 beat 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 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 want to guess that you guys guess 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 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 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 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 apartment 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 called supine supine means when you're lying on your back supine postural so supine postural postural meaning related to posture supine postural hypo hypotensive syndrome so supine postural hypotensive syndrome and it's weird that I'm talking 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 is one 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 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 in this name comes from so the woman starts to feel light-headed like she's about to faint especially when she's on her back because 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 and off of the inferior vena cava allowing more blood to return to the heart so that's that's all or or most of the functional changes that occur with a cardiovascular system in pregnancy and the growing uterus also shifts the heart to the left a little so there are also some anatomical changes - 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 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 so it's mostly done by increasing minute ventilation or or the volume of air that's taken in each minute and and it's not that pregnant ladies intentionally take deeper breaths because I would get really uncomfortable very quickly it's all that progesterone once again so it's all that progesterone in the blood and 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 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 firstly 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 those water filters that you can attach directly to your faucet right imagine if if you had one of those and if your pipes and 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 and that it filters all of your blood now with regards to the bladder there's there's 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 relaxants 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 increase your in 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 you readers become dilated and that's really important and it kind of looks like this where where the pressure from the large uterus causes the readers to become wider to become dilated and that uterus putting pressure here sort of acts as a roadblock and your builds up in the ureters behind that roadblock that built up stagnant urine acts as a medium for bacterial growth right because we know urinary stasis is 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 then our non pregnant women it's because of that urinary stasis that occurs as a result of the large uterus putting pressure on the you readers all right so those are those are some of the physiologic changes that occur in pregnancy with a cardiovascular system the respiratory system and the renal system