Medicine and Health related Information

The atrial chamber communicates posteriorly with sinus venosus and anteroinferiorly with the ventricle through the atrio -ventricular canal.
a) A septum arises from the roof of the atrial chamber a little to the left of the opening of sinus venosus. This is the “Septum Primum”. It grows downwards towards the septum intermedium of the atrio-ventricular canal. The lower edge of the septum primum is seperated from the septum intermedium by a gap called “Foramen Primum”. As the septum primum meets and fuses with septum intermedium the upper part of the septum primum breaks down to form “Foramen Secundum”. At this stage the left atrium receives blood only through the right atrium and hence a communication between the atria is essential.
b) A second septum (septum secundum) arises from the roof of the atrial chamber in the space between septum primum and septum spurium. It grows downwards overlaps the foramen secundum. The right and left atria communicates with each other through an valvular opening called “Foramen Ovale”. Through this opening only blood can pass from right atrium to left atrium. Throughout foetal life the interatrial septum is an incomplete partition because lungs are not functioning. Hence blood has to bypass the right ventricle. Blood enters directly from right atrium to left atrium.
After birth when the lungs start functioning the left atrium receives more and more blood from the lungs, then the pressure increases. Septum primum and septum secundum fuse with one another and now interatrial septum is completed. In adult anatomy the “annulus ovalis” represents the lower free edge of septum secundum, while the “fossa ovalis” represents the septum primum.