ATRIAL DEVELOPMENT AND VENOUS RETURN
Previous chapiter: Cardiovascular development
The heart tube bulges and constricts on the 22nd day of pregnancy. The heart is at its primitive stage of development.
The heart tube folds back on itself
The uneven growth of the cardiac tube causes it to fall back on itself in a very specific direction towards the 23rd day.
The primitive atrium and venous connections pass behind the primitive ventricle and the arterial bulb.
A wall is formed in the center of the primitive atrium
A partition forms in the atrioventricular canal to form two atriums, the right and the left, in the middle of the 4th week.
This separation of the atrium or atrial septum occurs in two segments.
Two parts to the separating wall
A first septum called primum is made of a thin membrane, which comes out towards the center in a crescent form. The opening left by this crescent is called the ostium primum.
When the space closes, another one is created in the center of the membrane; it is the ostium secundum.
Then a second membrane, the septum secundum, covers the first. A half-oval shaped opening is left at its base. This is the foramen ovale.
A temporary passage between the two atriums
During the fetal life, the foramen ovale combined with the ostium secundum forms a communication between the two atriums. This venue between the 2 atriums persists until delivery.
It closes at birth when the baby begins to breathe on its own. The left atrium has a higher pressure than the right one.
The septum secundum is pushed on the primum septum thus sealing the passage between the two orifices.
A way of bypassing the lungs
Fetal CIA is a bypassing venue for pulmonary circulation. As the lungs are not functional, only basic circulation is necessary to ensure its development.
The placenta controls the respiratory function, as all vital exchanges take place.
The left atrium is formed from the primitive pulmonary artery
At the same time as the primary atrium begins to split, the growth of the left atrium is largely due to the absorption of the primary pulmonary vein.
This portion of the atrium is smooth, unlike the primitive atrium, which is rough and wrinkled.
This part of the left atrium is called the left auricle or the left atrium appendage.
A valve between the atrium and the left ventricle
The remaining opening between the atrium and the left ventricle will be the location of the mitral valve.
The return of blood to the heart
The venous network at the primitive stage is symmetrical, or in other words, what goes on the left is also present on the right.
The venous connections, the left and right venous sinuses, each discharge into their respective primitive atrium.
The left venous sinus moves and joins the right one to form a single entry, which receives the inferior vena cava.
The right atrium is formed from the right venous sinus
The junction of the left venous sinus with the one on the right and the incorporation of the latter into the right atrium are responsible for its smooth inner part.
A small, rough, wrinkled part persists and forms the right atrium, or right atrial appendage.
A valve between the atrium and the right ventricle
The area between the atrium and the right ventricle sees the flow move in the same direction as the venous sinus. This area will eventually be the location of the tricuspid valve.
Related chapter: Development of the ventricules and larges arterial vessels