In the heart, a septum (plural: septa) is a wall that divides the heart into separate chambers. These septa are made of cardiac muscle (myocardium) and are lined on their inner surfaces by endocardium. 

The septa are essential because they keep oxygen-rich blood and oxygen-poor blood from mixing and help direct blood flow through the heart.

• The interatrial septum separates the atria and contains a depression called the fossa ovalis, which is a remnant of a fetal opening that allowed blood to bypass the lungs before birth. 

• The interventricular septum separates the ventricles and is thicker because the ventricles generate high pressure during contraction.

• The atrioventricular septum separates the atria from the ventricles and contains openings that allow blood to flow through the heart.

Because the atrioventricular septum contains multiple openings and valves, it is structurally weaker than other regions of the heart. To provide strength and support, this area is reinforced by dense connective tissue known as the cardiac skeleton. The cardiac skeleton forms four strong rings around the valve openings and serves as an attachment point for the heart valves. In addition to its structural role, the cardiac skeleton also helps regulate the heart’s electrical conduction by separating the electrical activity of the atria from that of the ventricles.

References

1. Gray, H. (1918). Anatomy of the human body (W. H. Lewis, Ed.; 20th ed.). Lea & Febiger.

2. J Gordon Betts, Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J., Womble, M. D., & Young, K. A. (2013). Anatomy & physiology. Openstax College, Rice University. https://openstax.org/details/books/anatomy-and-physiology

Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.

Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

Content paraphrased; adaptations were made.