Formation of the Interatrial Septum
The
interatrial
septum forms between the 27th and the 37th day of development and occurs
in conjunction with the development of the ventricular septum as well as
the separation of the truncus arteriosus. The 
sinus venosus remains paired the longest during the heart development.
By the end of the fourth week of development, it consists of a small transverse
portion and the right and left sinus horns. Each horn receives blood from 
three
important veins : the umbilical, the common or cardinal vein and the
vitelline veins. Eventually, the left horn of the sinus venosus becomes
partially obliterated leaving behind the 
oblique
vein of the left atrium and the coronary sinus.
Initially, the sinus venosus 
receives
blood from the yolk sac via the vitelline veins as well as from the
chorionic villi via the umbilical veins. Blood is also received from the
anterior and posterior cardinal veins, which drain the cranial region of
the embryo and body.
The junction between the right side of the sinus venosus and the
atrium folds inward to form the right venous or
sinus valve . In the embryo, this valve is large and directs blood
from the inferior vena cava across the interatrial septum. It also extends
inferiorly to cover the coronary sinus.
interatrial
septum forms between the 27th and the 37th day of development and occurs
in conjunction with the development of the ventricular septum as well as
the separation of the truncus arteriosus. The 
sinus venosus remains paired the longest during the heart development.
By the end of the fourth week of development, it consists of a small transverse
portion and the right and left sinus horns. Each horn receives blood from 
three
important veins : the umbilical, the common or cardinal vein and the
vitelline veins. Eventually, the left horn of the sinus venosus becomes
partially obliterated leaving behind the 
oblique
vein of the left atrium and the coronary sinus.
