Interatrial Septum Location and Anatomy in the Heart

The heart is a four-chambered muscular organ that circulates blood throughout the body. Its efficiency relies on an intricate structure of walls, or septa, that separate these chambers. In anatomy, a septum is a partition dividing a cavity into smaller spaces. Within the heart, one of these partitions is the interatrial septum, a wall with a specific purpose for cardiovascular function.

Locating the Interatrial Septum Within the Heart

The interatrial septum is the fibromuscular wall of tissue that separates the right and left atria, the two upper chambers of the heart. This partition is not perfectly vertical; it is positioned obliquely, running from the back and lower part of the heart toward the front and upper part. This angle is a result of the right atrium’s position being slightly anterior and to the right of the left atrium. The septum itself forms a portion of the posterior, or back, wall of the right atrium.

Superior to the much thicker interventricular septum, which separates the lower two chambers, the interatrial septum is a relatively thin structure. Its composition is primarily muscular tissue combined with fibrous elements. This wall is not uniform in thickness, featuring a central, thinner depression that corresponds to a feature known as the fossa ovalis. The septum effectively forms the medial wall for both the left and right atria.

The Role of the Interatrial Septum at its Position

The primary function of the interatrial septum in an adult is to maintain the separation of deoxygenated and oxygenated blood. The right atrium receives deoxygenated blood returning from the body, while the left atrium receives oxygenated blood from the lungs. The septum acts as a barrier, preventing the mixing of these two types of blood for efficient circulation. This separation ensures that only highly oxygenated blood is pumped out to the rest of the body from the left ventricle.

This complete separation, however, is a feature of the postnatal heart. During fetal development, the circulatory system functions differently because the lungs are not yet in use. In the fetal heart, the interatrial septum contains an opening that permits blood to flow from the right atrium directly into the left atrium. This fetal shunt is a normal part of prenatal circulation, allowing blood to bypass the pulmonary circuit.

Distinctive Features of the Interatrial Septum

A distinctive landmark on the interatrial septum, particularly when viewed from the right atrium, is the fossa ovalis. This feature is a shallow, oval-shaped depression that marks the former location of the foramen ovale. The fossa ovalis is surrounded by a muscular border known as the limbus fossae ovalis.

The foramen ovale is an opening formed during development by two overlapping septa that create a flap-like valve. After birth, with the first breath, pressure changes in the heart cause this flap to press against the septum, effectively closing the opening. Over time, this flap fuses with the septal wall, leaving behind the fossa ovalis.

Health Considerations Related to the Interatrial Septum’s Site

Defects in the structure of the interatrial septum can lead to health issues. An atrial septal defect (ASD) is a hole in the septum that persists after birth. This opening allows oxygenated blood from the higher-pressure left atrium to flow into the right atrium, mixing with deoxygenated blood. This abnormal flow increases the volume of blood the right side of the heart has to handle, leading to enlargement and strain over time.

Another condition is a patent foramen ovale (PFO), which occurs when the fetal opening fails to close completely after birth. Unlike an ASD, which is a structural absence of tissue, a PFO is more of a tunnel-like passageway between the septal flaps that can open when pressure in the right atrium exceeds that in the left. While many people with a PFO experience no symptoms, the opening can allow small blood clots to pass from the right to the left side of the heart and travel to the brain.