Atrial and Ventricular Septa - Dividers inside the heart

Dateline: 04/20/97

INTER-ATRIAL AND INTER-VENTRICULAR SEPTA

In earlier articles, you have read about the upper and lower chambers of the heart - the ATRIA and the VENTRICLES. You have also seen that the circulation is divided into two separate streams, one of which has blood returning from other organs back to the heart for oxygenation, and the other from the heart distributing pure blood back to the body. It is essential that the heart chambers are separated from each other by walls to prevent mixing of these two streams of blood.

This week’s discussion is about the walls that partition the two sides - the right and left - of these chambers.

The Inter-Atrial Septum

The two upper chambers of the heart are called the ATRIA. The right atrium receives impure venous blood from the great veins of the body, and directs it into the lungs for purification by addition of oxygen and removal of carbon dioxide. The left atrium on the other hand receives pure blood returning back from the lungs, and sends it to the other parts of the body. To keep these two streams separate, a thin wall exists between the upper chambers of the heart. This is called the Inter-Atrial Septum (IAS).

The inter-atrial septum is a thin structure being barely one or two millimeters thick, and is made up jointly of muscle and fibrous tissue. Viewed from the right atrium, it has a central oval shaped depression called the FOSSA OVALIS. This is the thinnest part of the septum.

The atrial septum develops in the fetal heart in a very interesting manner from the complex interaction between two separate membranes called the septum primum and septum secondum.

The only function of the atrial septum is to separate the blood in the left and right atria. When there is a deficiency or defect in the atrial septum, it is called an atrial septal defect or ASD. This disease results in a mixing of blood from both sides of the heart, and will be discussed in detail in a subsequent article. Depending on which portion of the IAS is deficient, ASDs may be of different types.

Another point of importance about the IAS is its role in recent trans-catheter treatment interventions for a variety of heart diseases. For instance, in a procedure called an ATRIAL SEPTOSTOMY, a catheter is used to perforate the IAS. This is best done in the central fossa ovalis, because it is the thinnest portion of the septum.

The Inter-Ventricular Septum

Just like the atria, the ventricles are partitioned by a wall called the Inter-Ventricular Septum (IVS). The IVS however is a slightly more complex structure than the IAS.

The inter-ventricular septum has a very complex development in the fetal heart, and arises as a contribution from no less than four different structures. The four portions of the IVS are named the Inlet, the Outlet, the Trabecular and the Peri-membranous portion.

The wall between the ventricles is much thicker than the one separating the atria, primarily because of the higher pressures that blood inside the ventricles is subjected to. The greater portion of the IVS is made of heart muscle, which is 12 to 16 mm. thick. In structure, the IVS appears different on the two sides. While the left ventricular side is smooth, the right side has many irregular ridges, called trabeculations. A consistently seen muscular band is called the TRABECULUM SEPTO-MARGINALIS, and is a valuable sign in determining the ventricular morphology using echocardiography tests in cases of birth defects of the heart.

In its upper part, there is a thin portion called the membranous part of the IVS.
The membranous part of the IVS is an interesting structure. It is very closely related to the special conduction system of the heart, which is a collection of modified muscle cells that can carry electrical impulses inside the heart to maintain normal rhythmic contractions. It also has the two AV valves, the mitral and tricuspid, arising partially from it. And, quite surprisingly, a small portion of the membranous septum actually separates the left ventricle from the right ATRIUM !

Unlike the Inter-Atrial septum, which serves only to separate the two sides of the atrium, the Inter-Ventricular Septum has a specialized job to perform. Functionally, it is a part of the left ventricle, and actually contributes to the pumping action of the left ventricle.

Since it’s development is so complex, the inter-ventricular septum is prone to many defects. When a portion of the IVS is deficient or absent, the disease condition is called VENTRICULAR SEPTAL DEFECT (VSD). In keeping with the dictum that "When something can go wrong, it often does" (!) , VSDs are the most common birth defect of the heart ! You can read more about VSDs in a separate article shortly on this site.

By now, I guess, you must be getting a fair idea of how the heart is structured. Next week, we will discuss the Atrio-Ventricular valves that determine the direction of blood flow within the heart.