Jo explained that the heart and the blood vessels make a closed system of fluid (blood)-filled tubes in the body. As a consequence, the volume of blood, diameter of the vessels, and pressure are directly related. If blood volume increases, or vessel diameter decreases, pressure on the walls of the vessels increases. Likewise, if fluid volume decreases, or vessel diameter increases, pressure in the system decreases.
"Think of it like a water balloon,” said Jo. “If I have a balloon and let out water, the balloon is less stretched, there is less pressure; but if I add more water to the balloon, the pressure on the balloon walls increases.”
“Got it,” said Penny. “So if a person were to decrease their blood volume, say if they had a major cut and were bleeding, their blood pressure would drop?”
“Exactly! Additionally, the length of the vessel and the viscosity, or thickness, of the blood matters. Think about drinking out of a straw; which one is easier to use? One that is of typical length or one that is five feet long? The shorter straw is easier to use because there is less resistance. Likewise, what is easier to suck up through a straw, a thick milkshake or orange juice?”
“Orange juice, for sure!” said Penny. “Sometimes you really have to suck to get a thick milkshake to move in the straw.”
“That’s right. The cardiovascular system works much the same way; longer tubes and thicker blood increase resistance, which means the heart has to generate more force to move the blood, which increases blood pressure.”
"Wow, that’s amazing; I hadn’t realized how important blood pressure is for our health. Are there additional issues I should be aware of because I’m pregnant?”
“Unfortunately, yes,” said Jo. “A woman’s body goes through all sorts of physiological changes during pregnancy. In some cases, pregnant women develop high blood pressure and if that happens there can be risks to both the mother and the baby. I don’t know as much about specifics, though. We should consult Google Scholar and the American College of Obstetrics and Gynecologists webpage for information so that you’re prepared for your appointment in the morning. Sound okay?”
“Yes, that sounds great.”
Jo and Penny grabbed the computer and found helpful peer-reviewed publications that discussed the physiology of pregnancy. Jo was currently enrolled in a Master of Science program, so she had experience with credible scientific articles and helped interpret some of the complicated information for Penny.
They read about the maternal-placental (uteroplacental) and fetal-placental (fetoplacental) blood circulation. The mother and the fetus each have their own blood supply and that blood does not mix (at least typically). The placenta is the organ of gas, nutrient, and waste transfer. The fetus is connected to the placenta via the umbilical cord (houses the umbilical artery and vein) and the placenta is implanted into the mother’s uterus. As pregnancy progresses, the placenta becomes more developed and the mother’s vascular network within the placenta grows; there are new spiral arteries, intervillous spaces (funnel-shaped areas that aid in exchange with fetal blood), and veins. The remodeling of the utero-placental vasculature is typically complete by weeks 20–22 of pregnancy.
They also found typical cardiovascular changes that occur in the mother during pregnancy include:
• A 40–50% increase in plasma volume.
• A 25% increase in red blood cells (RBC), but a decrease in overall hematocrit and hemoglobin concentration.
• A decrease in vascular resistance driven by fetoplacental developmental changes, a decreased sensitivity of the mother to angiotensin, and increased production of nitric oxide and relaxin.
• An increase in arterial compliance.
• A 30–50% increase in cardiac output, driven by an increase in both stroke volume (about a 30% increase) and heart rate (about a 15% increase).
After their web browsing session, Jo and Penny felt that they had looked up enough information for one night and were finally able to fall asleep.
