The interatrial septum is a wall of tissue located within the heart. Its main purpose is to separate the two upper chambers, the atria. This anatomical partition plays an important role in maintaining normal blood flow. It prevents the mixing of blood with different oxygen levels, contributing to efficient circulation.
Anatomical Location and Structure
The interatrial septum is positioned between the right and left atria. It lies at an angle, reflecting the slightly different positions of the right and left atria within the chest. The septum forms part of the posterior wall of the right atrium.
Structurally, the interatrial septum is primarily a fibromuscular wall. It contains regions of variable thickness; for instance, the central part, known as the fossa ovalis, is thinner than the surrounding muscular rim.
The fossa ovalis is an oval-shaped depression visible on the right atrial side of the septum. This depression is a remnant from fetal development, marking the site of the foramen ovale, an opening present before birth. A prominent margin, called the limbus of the fossa ovalis, surrounds it. Its dimensions can vary.
Role in Blood Flow
The interatrial septum’s primary function in an adult heart is to ensure the separation of oxygenated and deoxygenated blood. Blood returning from the body, low in oxygen, enters the right atrium. Blood returning from the lungs, rich in oxygen, enters the left atrium. The septum acts as a barrier, preventing these two blood streams from mixing.
This separation is important for maintaining the efficiency of the circulatory system. Deoxygenated blood from the right atrium is directed into the right ventricle and then to the lungs to pick up oxygen. Oxygenated blood from the left atrium moves into the left ventricle and is then pumped out to the rest of the body. The integrity of the interatrial septum is therefore important for directing blood into the correct circulatory pathways, supporting proper oxygen delivery to tissues.
Developmental Insights and Common Variations
The interatrial septum forms during the first and second months of fetal development, involving two main structures: the septum primum and the septum secundum. During fetal life, the lungs are not yet functional for gas exchange. To bypass the fetal lungs, a temporary opening, the foramen ovale, exists in the interatrial septum. This opening allows oxygenated blood from the placenta, arriving in the right atrium, to shunt directly to the left atrium and then to the systemic circulation. The septum primum acts as a flap-like valve over the foramen ovale, allowing one-way flow from right to left.
After birth, changes occur in the circulatory system. As the newborn takes its first breath, the lungs expand, leading to a decrease in pressure within the pulmonary circulation. This causes the pressure in the left atrium to become higher than in the right atrium. The increased pressure in the left atrium pushes the septum primum against the septum secundum, functionally closing the foramen ovale. Over time, in about 75% of individuals, these two septa fuse completely, forming the fossa ovalis.
In some individuals, the foramen ovale does not completely seal after birth, resulting in a Patent Foramen Ovale (PFO). “Patent” means open, indicating that the flap-like opening remains unsealed. A PFO is a common variation, found in approximately 25% of the adult population. A PFO is not considered an atrial septal defect (ASD), as it involves an incomplete closure of a normal fetal structure rather than a failure of septal tissue to form. Most PFOs do not cause symptoms or require treatment.