In this last video about the blood vessels, I'm going to give you a reminder about your circulatory route that blood takes through the body, and then we'll talk about fetal circulation and how it's different from adult circulation. And you should know this systemic and pulmonary circuits by now. The systemic circuit is shown with black arrows, where oxygenated blood is pumped from the left atrium to the left ventricle, and then out into the aorta. From the aorta, blood passes into elastic arteries, and then into muscular arteries before entering arterioles. Blood in the arterioles enters into systemic capillaries of the head, the neck, and the upper limbs, and that's where we get exchange of gases and nutrients. Deoxygenated blood exits capillary beds into venules and then into veins. Deoxygenated blood is conducted to either the superior or the inferior vena cava, and enters into the right atrium of the heart. Now we're in the pulmonary circuit, indicated with yellow arrows. Blood flow, blood flows from the right atrium to the right ventricle and gets pumped into the pulmonary trunk. The pulmonary trunk conducts that deoxygenated blood into pulmonary arteries that travel to the lungs, the only arteries in the body that carry deoxygenated blood. At the lungs, the blood passes through smaller and smaller arteries before entering into the pulmonary capillaries for gas exchange. Oxygenated blood then exits the lung as a series of progressively larger veins that merge to form the pulmonary veins, and then the pulmonary veins drain into the left atrium, and the cycle completes and repeats. It's a little bit different for a developing fetus. The fetus receives oxygen and receives nutrients through the placenta, and that's served by the umbilical arteries and vein. The fetal lungs are not functional, so the pulmonary circuit is bypassed, and there are two ways that the pulmonary circuit is bypassed, through the foramen ovale and through the ductus arteriosus. The oxygenated blood is shunted away from the liver to prioritize the developing brain, and that's through a structure called the ductus venosus. And I'll show each one of these. Fetal circulation is more complex than post-natal, after birth circulation, because the objective of, of fetal circulation is to take blood to the placenta instead of to the lungs for gas exchange and nutrients. The first thing that's different about fetal circulation is the presence of the placenta, with the umbilical arteries and veins. So we'll take a look at that. The placenta is a temporary organ and it connects the developing fetus to the uterine wall to allow for nutrient uptake, thermoregulation, waste elimination, and gas exchange via the mother's blood supply. The placenta provides oxygen and nutrients to the growing fetus and attaches to the wall of the uterus, and the fetus' umbilical cord develops from the placenta. The placenta is mostly derived from the fetus, with a contribution from the lining of the mother's uterus. It's, at the placenta, the fetal blood and maternal blood come very close together, but not mixing, and this allows for gas exchange and nutrient exchange. The umbilical cord contains one umbilical vein which is carrying oxygenated blood, so it's shown in red, and it contains two umbilical arteries, which are shown in blue because they're carrying deoxygenated blood away. Another adaptation of fetal circulation are the two shunts that allow for bypassing of the pulmonary circuit. These shunts preferentially direct blood to the systemic circuit, and away from the pulmonary circuit. There is a hole in the wall between the right atrium and the left atrium. This is the foramen ovale, this allows blood to enter into the left atrium from the right atrium, and this would then go to the left ventricle and out to the aorta, so will be bypassing the pulmonary circuit. Some blood doesn't go through the foramen ovale, and it goes, enters into the right ventricle, and then we'll be headed out through the pulmonary trunk, and here's where we find a second shunt. This is the ductus arteriosus. The ductus arteriosus allows blood to flow from the pulmonary trunk into the aorta and bypass the lungs that way. Another structure of fetal circulation is the ductus venosus. The ductus venosus is a structure that carries oxygenated blood in the umbilical vein and lets it bypass the liver, and this is important for normal fetal circulation. Blood becomes oxygenated in the placenta and travels to the right atrium via the umbilical veins through the ductus venosus, and then to the inferior vena cava. This is to help preferentially shunt oxygenated blood toward the developing brain and away from the portal..the hepatic portal system which would use a lot of that oxygen. So here's an illustration of fetal circulation. We have oxygenated blood that's coming from the placenta, entering into the fetus by way of the umbilical vein, that's that red structure right there. The blood from the umbilical vein is shunted away from the liver and directly toward the inferior vena cava through the ductus venosus. Oxygenated blood from the ductus venosus mixes with deoxygenated blood coming from the liver in the inferior vena cava, then blood from the superior and inferior vena cava empties into the right atrium. Because the pressure is higher on the right side of the heart, most of the blood that's in the right atrium gets shunted into the left atrium by way of the foramen ovale. This blood then flows into the left ventricle and is pumped out through the aorta. A small amount of blood enters into the right ventricle and the pulmonary trunk, but much of that blood is shunted from the pulmonary trunk to the aorta, through that vessel detour called the ductus arteriosus. Blood then travels to the rest of the body, and deoxygenated blood returns to the placenta through a pair of umbilical arteries. Nutrient and gas exchange occurs at the placenta, and the cycle repeats. One thing to note is that in fetal circulation, um, veins are carrying both oxygenated blood from the placenta, and deoxygenated blood from the body, and these mix at the right atrium of the heart, and is then sent out to the body. So fetal arteries are carrying a mix of oxygenated and deoxygenated blood, but it's sufficient for the developing fetus. Then we see some changes after birth. The ductus arteriosus becomes the ligamentum arteriosum, the ductus venosus becomes the ligamentum venosum, the foramen ovale closes and becomes the fossa ovalis, the umbilical arteries become the medial umbilical...umbilical ligaments, and the umbilical vein becomes that round ligament of the liver, ligamentum teres that we saw with the digestive system. So at birth we get some big changes. Circulation changes with the first breath that a baby takes. When a baby inhales, the alveoli expands in the lungs, the pulmonary arteries dilate, and pressure in the right side of the heart drops. The umbilical vein and umbilical arteries constrict and become non-functional. The ductus venosus constricts and becomes the ligamentum venosum. That's this right here which we saw on the hearts and lab. The foramen ovale in the interatrial septum closes and leaves the fossa ovalis, and within 14 hours of birth the ductus arteriosus closes and becomes the ligamentum arteriosum that's right here. So at birth, or after birth, the ductus arteriosus becomes a ligament, the foramen ovale closes and leaves a depression, the ductus venosus becomes a ligament, the umbilical vein becomes a ligament, and the umbilical arteries become ligaments, and now we have the systemic and pulmonary circuit like you'd expect, and arteries carrying out fully oxygenated blood rather than partially oxygenated blood like they were before birth. There's a clinical application here! If you've ever heard of "blue babies", it's a result of that foramen ovale remaining open after birth. The foramen valley is open in the womb and allows blood to flow directly from the right to the left atrium and skip the pulmonary circuit. It should shut when the baby takes its first breath and form the fossa ovalis, that depression in the right atrium of the heart, but for some babies it fails to close, and for some this is a pretty serious condition where blood will not be routed to the pulmonary circuit. It allows for oxygenated and deoxygenated blood to freely mix and can give the skin a blue appearance because of this deoxygenated blood. In up to 25 percent of babies this hole doesn't close right away, and it may close during childhood or later in life, or it may never close. Most children who have this heart defect never experience symptoms, but in some cases it can cause breathing problems if a child...when the child enters adulthood, or it can be a risk factor for stroke and coronary heart disease in adults.