Changes

===Atrial Septation===

===Atrial Septation===

Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the Septum Intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals. The septum is shifted to the right allowing the left side of the heart to become bigger than the right

Septation of the tubular heart begins in the atrioventricular canal. Four masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other, the dorsal (inferior) and the ventral (superior) cushions grow more then the left and right cushihons and eventually fuse. The fused endocardial cushions form the Septum Intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals. The myocardium secretes factors which cause and epithelial to mesenchymal change in the endocardium. These mesenchymal cells go on to form the bicuspid and tricuspid atrioventricular valves.

As the endocardial cushions proliferate the initial atrial septum forms in the common atrial chamber. The Septum Primum emerges from the roof of the common atrium and extends down through the chamber, eventaully fusing with the Septum Intermedium to create a seperate left and right atria. Programmed cell death occurs and a large hole is created in the septum primun allowing blood to flow between the left and right. A muscular sheet known as the Septum Secundum then grows on the right of the  Septum Primum. This occludes the majority of the hole in the Septum Primum, only the small Foramen Ovale is left. The window in the Septum Primum is slightly offset from the window in the septum Secundum and this creates a valve like effect so that blood can only flow in one direction between the atriae.

As the endocardial cushions proliferate the initial atrial septum forms in the common atrial chamber. The Septum Primum emerges from the roof of the common atrium and extends down through the chamber, eventaully fusing with the Septum Intermedium to create a seperate left and right atria. Programmed cell death occurs and a large hole is created in the septum primun allowing blood to flow between the left and right. A muscular sheet known as the Septum Secundum then grows on the right of the  Septum Primum. This occludes the majority of the hole in the Septum Primum, only the small Foramen Ovale is left. The window in the Septum Primum is slightly offset from the window in the septum Secundum and this creates a valve like effect so that blood can only flow in one direction between the atriae.