What is the structure found in a fetus that allows blood to flow directly from the right atrium into the left atrium?

By the end of this section, you will be able to:

  • Describe the development of blood vessels
  • Describe the fetal circulation

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:

  • The foramen ovale is an opening in the interatrial septum that allows blood to flow from the right atrium to the left atrium. A valve associated with this opening prevents backflow of blood during the fetal period. As the newborn begins to breathe and blood pressure in the atria increases, this shunt closes. The fossa ovalis remains in the interatrial septum after birth, marking the location of the former foramen ovale.
  • The ductus arteriosus is a short, muscular vessel that connects the pulmonary trunk to the aorta. Most of the blood pumped from the right ventricle into the pulmonary trunk is thereby diverted into the aorta. Only enough blood reaches the fetal lungs to maintain the developing lung tissue. When the newborn takes the first breath, pressure within the lungs drops dramatically, and both the lungs and the pulmonary vessels expand. As the amount of oxygen increases, the smooth muscles in the wall of the ductus arteriosus constrict, sealing off the passage. Eventually, the muscular and endothelial components of the ductus arteriosus degenerate, leaving only the connective tissue component of the ligamentum arteriosum.
  • The ductus venosus is a temporary blood vessel that branches from the umbilical vein, allowing much of the freshly oxygenated blood from the placenta—the organ of gas exchange between the mother and fetus—to bypass the fetal liver and go directly to the fetal heart. The ductus venosus closes slowly during the first weeks of infancy and degenerates to become the ligamentum venosum.

What is the structure found in a fetus that allows blood to flow directly from the right atrium into the left atrium?

Figure 1. The foramen ovale in the interatrial septum allows blood to flow from the right atrium to the left atrium. The ductus arteriosus is a temporary vessel, connecting the aorta to the pulmonary trunk. The ductus venosus links the umbilical vein to the inferior vena cava largely through the liver.

Chapter Review

Blood 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 Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

  1. All tissues, including malignant tumors, need a blood supply. Explain why drugs called angiogenesis inhibitors would be used in cancer treatment.
  2. Explain the location and importance of the ductus arteriosus in fetal circulation.

Glossary

angioblasts: 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

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.

When the foramen ovale stays open after birth, it's called a patent (PAY-tent, which means "open") foramen ovale (PFO).

What is the structure found in a fetus that allows blood to flow directly from the right atrium into the left atrium?

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.

What is the structure found in a fetus that allows blood to flow directly from the right atrium into the left atrium?

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.

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What 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 Ahead

PFOs 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.