The foramen ovale (fuh-RAY-men oh-VAL-ee) is a normal opening between the upper two chambers (the right atrium and left atrium) of an unborn baby's heart. The foramen ovale usually closes 6 months to a year after the baby's birth. Show When the foramen ovale stays open after birth, it's called a patent (PAY-tent, which means "open") foramen ovale (PFO).
A PFO usually causes no problems. If a newborn has congenital heart defects, the foramen ovale is more likely to stay open.
Before birth, the foramen ovale allows blood flow to bypass the lungs (a fetus gets the oxygen it needs from the placenta, not the lungs). That way, the heart doesn't work hard to pump blood where it isn't needed.
When newborns take their first breath, a new flow direction happens. The blood now needs to go to the baby's lungs. This new flow helps push the patent foramen ovale closed. The blood can no longer flow directly between the upper two heart chambers. Instead, it flows from the right side of the heart into the baby's lungs to pick up oxygen, and then the left side of the heart sends the oxygen-rich blood out to the body.
In most people, the flap that closes off the foramen ovale gradually seals itself in place so it's permanently closed. In babies, kids, and adults with a PFO, the flap remains unsealed. What Are the Signs & Symptoms of a PFO?A PFO usually causes no problems, so most babies who have one don't show symptoms. Many active adults have a PFO and don't know it. Sometimes having a PFO is helpful. Babies born with serious heart problems or pulmonary hypertension (high blood pressure in the lungs) and a PFO may have less severe symptoms because the PFO lets blood from the two sides of the heart mix. pWhat Causes a Patent Foramen Ovale?A patent foramen ovale is normal until birth. The flap that closes it usually doesn't completely do so until a baby is at least several months old. Why the flap doesn't seal in some people is unknown. Who Gets a Patent Foramen Ovale?Everyone has them at birth, but the hole usually closes. PFOs that do not close are common, and found in 1 of every 4 adults. PFOs are more likely in newborns who have a congenital heart defect. How Is a Patent Foramen Ovale Diagnosed?A patent foramen ovale most often is seen on an echocardiogram (ultrasound of the heart) being done for other reasons. How Is a Patent Foramen Ovale Treated?PFOs usually aren't treated unless there's another reason for heart surgery or someone's risk for blood clots or stroke is higher than average. A PFO may increase the risk of strokes because tiny blood clots elsewhere in the body can break loose and go to the heart via the blood. These tiny clots are usually filtered out of the blood by the lungs. In a person with a PFO, the clot can slip from the right atrium to the left atrium. From there, the clot goes to the left ventricle, which sends the clot out to the body or the brain, where it can affect organs that are much more sensitive to injury than the lungs. When a blood clot blocks blood flow to part of the brain, the result is a stroke. Even in a person who has had a stroke, treatment usually focuses on preventing clots rather than closing the PFO. If closure is required, cardiac catheterization can be used to place a device through a long, thin tube guided through blood vessels to the heart to close the foramen ovale. Looking AheadPFOs aren't likely to cause trouble and need no special treatment for most people. But kids and adults should know that they have one if it is diagnosed.
By the end of this section, you will be able to:
In a developing embryo,the heart has developed enough by day 21 post-fertilization to begin beating. Circulation patterns are clearly established by the fourth week of embryonic life. It is critical to the survival of the developing human that the circulatory system forms early to supply the growing tissue with nutrients and gases, and to remove waste products. Blood cells and vessel production in structures outside the embryo proper called the yolk sac, chorion, and connecting stalk begin about 15 to 16 days following fertilization. Development of these circulatory elements within the embryo itself begins approximately 2 days later. You will learn more about the formation and function of these early structures when you study the chapter on development. During those first few weeks, blood vessels begin to form from the embryonic mesoderm. The precursor cells are known as hemangioblasts. These in turn differentiate into angioblasts, which give rise to the blood vessels and pluripotent stem cells, which differentiate into the formed elements of blood. (Seek additional content for more detail on fetal development and circulation.) Together, these cells form masses known as blood islands scattered throughout the embryonic disc. Spaces appear on the blood islands that develop into vessel lumens. The endothelial lining of the vessels arise from the angioblasts within these islands. Surrounding mesenchymal cells give rise to the smooth muscle and connective tissue layers of the vessels. While the vessels are developing, the pluripotent stem cells begin to form the blood. Vascular tubes also develop on the blood islands, and they eventually connect to one another as well as to the developing, tubular heart. Thus, the developmental pattern, rather than beginning from the formation of one central vessel and spreading outward, occurs in many regions simultaneously with vessels later joining together. This angiogenesis—the creation of new blood vessels from existing ones—continues as needed throughout life as we grow and develop. Blood vessel development often follows the same pattern as nerve development and travels to the same target tissues and organs. This occurs because the many factors directing growth of nerves also stimulate blood vessels to follow a similar pattern. Whether a given vessel develops into an artery or a vein is dependent upon local concentrations of signaling proteins. As the embryo grows within the mother’s uterus, its requirements for nutrients and gas exchange also grow. The placenta—a circulatory organ unique to pregnancy—develops jointly from the embryo and uterine wall structures to fill this need. Emerging from the placenta is the umbilical vein, which carries oxygen-rich blood from the mother to the fetal inferior vena cava via the ductus venosus to the heart that pumps it into fetal circulation. Two umbilical arteries carry oxygen-depleted fetal blood, including wastes and carbon dioxide, to the placenta. Remnants of the umbilical arteries remain in the adult. (Seek additional content for more information on the role of the placenta in fetal circulation.) There are three major shunts—alternate paths for blood flow—found in the circulatory system of the fetus. Two of these shunts divert blood from the pulmonary to the systemic circuit, whereas the third connects the umbilical vein to the inferior vena cava. The first two shunts are critical during fetal life, when the lungs are compressed, filled with amniotic fluid, and nonfunctional, and gas exchange is provided by the placenta. These shunts close shortly after birth, however, when the newborn begins to breathe. The third shunt persists a bit longer but becomes nonfunctional once the umbilical cord is severed. The three shunts are as follows:
Chapter ReviewBlood vessels begin to form from the embryonic mesoderm. The precursor hemangioblasts differentiate into angioblasts, which give rise to the blood vessels and pluripotent stem cells that differentiate into the formed elements of the blood. Together, these cells form blood islands scattered throughout the embryo. Extensions known as vascular tubes eventually connect the vascular network. As the embryo grows within the mother’s womb, the placenta develops to supply blood rich in oxygen and nutrients via the umbilical vein and to remove wastes in oxygen-depleted blood via the umbilical arteries. Three major shunts found in the fetus are the foramen ovale and ductus arteriosus, which divert blood from the pulmonary to the systemic circuit, and the ductus venosus, which carries freshly oxygenated blood high in nutrients to the fetal heart. Self CheckAnswer the question(s) below to see how well you understand the topics covered in the previous section.
Glossaryangioblasts: stem cells that give rise to blood vessels angiogenesis: development of new blood vessels from existing vessels blood islands: masses of developing blood vessels and formed elements from mesodermal cells scattered throughout the embryonic disc ductus arteriosus: shunt in the fetal pulmonary trunk that diverts oxygenated blood back to the aorta ductus venosus: shunt that causes oxygenated blood to bypass the fetal liver on its way to the inferior vena cava foramen ovale: shunt that directly connects the right and left atria and helps to divert oxygenated blood from the fetal pulmonary circuit hemangioblasts: embryonic stem cells that appear in the mesoderm and give rise to both angioblasts and pluripotent stem cells umbilical arteries: pair of vessels that runs within the umbilical cord and carries fetal blood low in oxygen and high in waste to the placenta for exchange with maternal blood umbilical vein: single vessel that originates in the placenta and runs within the umbilical cord, carrying oxygen- and nutrient-rich blood to the fetal heart vascular tubes: rudimentary blood vessels in a developing fetus |