Fossa Ovalis

1. Overview

The fossa ovalis is a depression in the interatrial septum of the heart, located between the right and left atria. It is a remnant of the foramen ovale, an opening that allows blood to bypass the fetal lungs during fetal development. In a fetus, the foramen ovale allows blood to flow directly from the right atrium to the left atrium, thus bypassing the lungs, which are not yet functional for oxygenation. After birth, the foramen ovale typically closes, leaving behind the fossa ovalis as a small, oval-shaped indentation. The fossa ovalis has little to no function in adults but is an important structure during fetal circulation. In some individuals, the fossa ovalis may remain patent (open), leading to certain clinical implications.

2. Location

The fossa ovalis is located in the interatrial septum, which is the wall that separates the right and left atria of the heart. It is found in the lower part of the septum, near the opening of the inferior vena cava (IVC) into the right atrium. In terms of anatomical landmarks, the fossa ovalis is situated in the region known as the oval fossa, which is typically on the posterior wall of the right atrium. It is visible during an echocardiogram or other imaging techniques, especially when there is a persistent foramen ovale or an atrial septal defect (ASD).

3. Structure

The fossa ovalis is a shallow, oval-shaped depression in the interatrial septum. Its structure is simple, consisting of the following components:

• Shape and Size: The fossa ovalis is typically a small, oval-shaped depression measuring about 1-2 cm in diameter. Its shape may vary slightly between individuals, but it is generally located in the lower part of the interatrial septum.

• Border: The fossa ovalis is bordered by a raised ridge called the limbus fossae ovalis, which is a fibrous tissue ring that demarcates the boundary of the depression. This ridge represents the site where the foramen ovale was previously located during fetal development.

• Remnant of the Foramen Ovale: The fossa ovalis is the remnant of the foramen ovale, which was an opening in the fetal heart that allowed blood to flow from the right atrium to the left atrium, bypassing the nonfunctional fetal lungs. After birth, the foramen ovale typically closes, leaving the fossa ovalis as a vestigial structure.

4. Function

In the adult heart, the fossa ovalis has no direct function, but it is a key anatomical feature of the interatrial septum. During fetal development, the foramen ovale allows blood to bypass the lungs, as they are not yet functional for oxygenation. The opening helps direct oxygenated blood from the placenta to the left atrium and subsequently to the left ventricle and body. Once the baby is born and begins breathing air, the lungs become functional, and the foramen ovale normally closes within the first hours to days after birth, leaving the fossa ovalis as a small depression in the septum.

• Fetal Circulation: In the fetus, the foramen ovale permits blood to flow from the right atrium to the left atrium, bypassing the lungs that are filled with fluid and nonfunctional. This shunt allows oxygenated blood from the placenta to be delivered directly to the left side of the heart, from where it is pumped to the rest of the body, including vital organs such as the brain.

• Postnatal Role: After birth, the fossa ovalis no longer serves a functional purpose, as the foramen ovale typically closes and the pulmonary circulation becomes fully operational. The closure of the foramen ovale helps to establish normal circulation through the lungs, where oxygenation occurs before blood is pumped to the body.

5. Physiological Role(s)

The primary physiological role of the fossa ovalis is during fetal circulation. In a normal healthy heart, this structure has little to no impact on adult heart function, but its role during fetal life is critical:

• Facilitating Fetal Circulation: The fossa ovalis represents the site where the foramen ovale existed during fetal development. The foramen ovale allowed blood to bypass the lungs, facilitating the delivery of oxygenated blood from the placenta directly to the left atrium, left ventricle, and the systemic circulation. This was crucial, as the fetus relied on placental oxygenation rather than pulmonary oxygenation.

• Closure After Birth: After birth, when the infant begins breathing air and the lungs become functional, the foramen ovale closes, and the fossa ovalis is formed. This marks the end of the bypass mechanism and the beginning of normal pulmonary circulation.

6. Clinical Significance

The fossa ovalis itself is generally not associated with any health problems, but its role as a remnant of the foramen ovale can be clinically significant in certain situations. Some of the key clinical conditions related to the fossa ovalis include:

• Patent Foramen Ovale (PFO): In some individuals, the foramen ovale does not close properly after birth, leaving a small opening between the right and left atria. This condition is known as a patent foramen ovale (PFO). While most people with a PFO do not experience symptoms, it can lead to health issues such as paradoxical embolism, where a clot from the veins can travel from the right atrium to the left atrium and then to systemic circulation, potentially causing a stroke or other complications. PFOs are often detected incidentally during imaging studies like echocardiograms, and treatment may involve medications or, in some cases, a procedure to close the opening.

• Atrial Septal Defect (ASD): An ASD is a congenital condition in which there is a hole in the interatrial septum, which may be located near the fossa ovalis. Unlike a PFO, an ASD involves a larger and more persistent opening, which can lead to abnormal blood flow between the atria. Symptoms of an ASD may include shortness of breath, fatigue, and an increased risk of stroke. ASD may require surgical closure or a catheter-based procedure to correct the defect.

• Endocarditis Risk: Individuals with a PFO or ASD may be at increased risk of developing endocarditis, an infection of the heart lining or valves. The presence of a shunt between the atria may allow bacteria or other pathogens to enter the bloodstream and infect the heart. Preventive measures, such as antibiotic treatment before certain procedures, may be recommended for people with these conditions.

• Assessment in Cardiology: The fossa ovalis, and particularly any abnormalities related to it, may be assessed during cardiovascular examinations. Echocardiograms are commonly used to evaluate the structure of the interatrial septum and detect conditions like PFO or ASD, which can affect cardiac function and increase the risk of thromboembolic events.

The fossa ovalis itself is typically not associated with health problems, but its significance as a remnant of fetal circulation and its potential involvement in conditions like PFO and ASD make it important in cardiovascular health. Proper diagnosis and management of these conditions are essential to prevent complications like stroke and other systemic issues.