7.7 Membranous Interventricular Septum
The membranous interventricular septum is a thin wall between the heart's lower chambers, crucial for separating oxygenated and deoxygenated blood.
Membranous Interventricular Septum is a small, thin, fibrous portion of the interventricular septum located near the cardiac base, forming part of the boundary between the left and right ventricles. It is characterized by its membranous (fibrous) composition rather than muscular tissue, making it the thinnest and least muscular segment within the interventricular septum. This septal component plays a crucial role in the structural integrity of the heart by closing the interventricular communication in the region adjacent to the atrioventricular septum, contributing to the division of the ventricular cavities.
Structure and Composition
The membranous interventricular septum is primarily composed of dense fibrous connective tissue rather than myocardium. It forms a small, flat, and thin sheet that is situated near the central fibrous body of the heart. Unlike the muscular interventricular septum, which is thick and muscular, the membranous septum lacks contractile fibers and is non-muscular.
The septum can be subdivided into two portions:
Membranous Septal Atrial Portion
This portion forms the atrioventricular component of the membranous septum. It lies superiorly and is closely related to the septal leaflet of the tricuspid valve and the atrial chambers. It contributes to the fibrous continuity between the aortic and atrioventricular valves.
Membranous Septal Ventricular Portion
Located inferiorly, this portion separates the left ventricular outflow tract from the right ventricle. It lies adjacent to the septal leaflet of the tricuspid valve and is involved in the fibrous skeleton of the heart, particularly the central fibrous body.
Anatomical Position and Relations
The membranous interventricular septum occupies a central position in the base of the heart, lying just inferior to the right and non-coronary cusps of the aortic valve. It is situated at the junction where the atrial and ventricular septa meet, forming part of the fibrous skeleton.
Relation to Aortic Root
The membranous septum is closely related to the aortic root, specifically the interleaflet triangle between the right and non-coronary cusps of the aortic valve. This relationship is important in surgical approaches to the aortic valve and in the pathology of certain congenital heart defects.
Relation to Tricuspid Valve
The septal leaflet of the tricuspid valve attaches near the membranous septum, contributing to the atrioventricular fibrous continuity. This proximity defines the atrioventricular septum and is significant in the function and structure of the tricuspid valve apparatus.
Relation to Fibrous Skeleton
The membranous septum is an integral part of the cardiac fibrous skeleton, serving as a central fibrous body that supports valve attachments and electrically insulates the atria from the ventricles, except at the atrioventricular node.
Clinical Significance
Due to its thin and fibrous nature, the membranous interventricular septum is a common site for ventricular septal defects (VSDs), especially perimembranous VSDs, which are the most frequent type of congenital septal defects. These defects occur when there is incomplete formation or tissue deficiency in this region, allowing blood to flow abnormally between the left and right ventricles.
The proximity of the membranous septum to the conduction system of the heart, particularly the bundle of His which penetrates nearby, makes surgical repair of defects in this area delicate to avoid iatrogenic heart block.
Membranous-Muscular Septal Junction
The membranous interventricular septum is continuous inferiorly with the muscular interventricular septum. The junction between the membranous and muscular septa is well-defined, marking the transition from fibrous to muscular tissue in the interventricular septum. This junction is important for anatomical orientation and for understanding the spread of electrical impulses through the septum.
Spatial Orientation and Shape
The membranous septum is generally triangular or quadrilateral in shape when viewed from the right ventricle. It occupies a small area compared to the muscular septum but is strategically positioned at the base of the ventricular septum.
Its spatial map can be visualized as a thin fibrous plane wedged between the left ventricular outflow tract anteriorly and the right ventricular inflow tract posteriorly.
Summary Table of Key Features
| Feature | Description |
|---|---|
| Composition | Dense fibrous connective tissue (non-muscular) |
| Location | Near cardiac base, between left and right ventricles |
| Relation to Valves | Adjacent to aortic valve (right and non-coronary cusps) and tricuspid valve (septal leaflet) |
| Function | Separates ventricles; part of fibrous skeleton; electrical insulation |
| Clinical Relevance | Common site for perimembranous ventricular septal defects; proximity to conduction system |
| Shape and Size | Small, thin, triangular or quadrilateral fibrous sheet |
This formula expresses the shunt volume in ventricular septal defects, where oxygen contents (O₂) in systemic (S), pulmonary (P), and right (R) circulations are used to calculate the magnitude of abnormal blood flow through the septal defect.
The membranous interventricular septum is a vital anatomical structure bridging the fibrous skeleton of the heart and serving as a key landmark for both anatomical and clinical considerations in cardiac function and pathology.