Ostium primum
Encyclopedia
In the developing heart
, the atria initially communicate with each other by an opening between the free edge of the septum primum and the AV cushions, known as the primary interatrial foramen or ostium primum (interatrial foramen primum), below the free margin of the septum
.
, and as the ostium primum closes, the communication between the atria is preserved with the formation of an opening in the upper part of the septum primum
; this opening is known as the ostium secundum
as it is chronologically the second opening that occurs in the septum prium.
A second entity, the septum secundum
, develops to the right of the septum primum and the opening between the upper and lower limbs of the septum secundum is known as the foramen ovale of the heart
. The part of the septum primum that remains to the left of the septum secundum acts as a one way flow valve due to the greater pressures in the right atrium compared to the left atrium. At birth the neonate begins breathing and the associated decrease in pulmonary vascular resistance leads to a reversal of the pressure differential between the atria. The left atrium now has a greater pressure compared to the right, effectively closing the one way flow valve formed by the two septae. The two septae fuse later in life, to complete the formation of the atrial septum.
Persistence of the ostium secundum is the most common atrial septal defect. Additionally, in a subset of the population, the foramen ovale is not overtly patent, however the two septae have not fused. In normal physiologic circumstances the septum primum acts as a one way valve preventing blood flow as described above, however if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left.
. This is the second most common type of atrial septal defect and is commonly seen in Down's syndrome. Typically this defect will cause a shunt to occur from the left atrium to the right atrium. Children born with this defect may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt. This reversal is called Eisenmenger syndrome.
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, the atria initially communicate with each other by an opening between the free edge of the septum primum and the AV cushions, known as the primary interatrial foramen or ostium primum (interatrial foramen primum), below the free margin of the septum
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
.
Closing of ostium primum
This opening is closed by the union of the septum primum with the septum intermediumSeptum intermedium
Endocardial cushions project into the atrial canal, and, meeting in the middle line, unite to form the septum intermedium which divides the canal into two channels, the future right and left atrioventricular orifices.-External links:*...
, and as the ostium primum closes, the communication between the atria is preserved with the formation of an opening in the upper part of the septum primum
Septum primum
In the developing heart, the cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. The increasingly smaller gap below it is known as the ostium primum...
; this opening is known as the ostium secundum
Ostium secundum
The ostium secundum is a foramen in the septum primum.It should not be confused with the foramen ovale, which is a foramen in the septum secundum.-Clinical significance:...
as it is chronologically the second opening that occurs in the septum prium.
A second entity, the septum secundum
Septum secundum
The septum secundum, semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and ostium secundum....
, develops to the right of the septum primum and the opening between the upper and lower limbs of the septum secundum is known as the foramen ovale of the heart
Foramen ovale (heart)
In the fetal heart, the foramen ovale , also ostium secundum of Born or falx septi, allows blood to enter the left atrium from the right atrium. It is one of two fetal cardiac shunts, the other being the ductus arteriosus...
. The part of the septum primum that remains to the left of the septum secundum acts as a one way flow valve due to the greater pressures in the right atrium compared to the left atrium. At birth the neonate begins breathing and the associated decrease in pulmonary vascular resistance leads to a reversal of the pressure differential between the atria. The left atrium now has a greater pressure compared to the right, effectively closing the one way flow valve formed by the two septae. The two septae fuse later in life, to complete the formation of the atrial septum.
Persistence of the ostium secundum is the most common atrial septal defect. Additionally, in a subset of the population, the foramen ovale is not overtly patent, however the two septae have not fused. In normal physiologic circumstances the septum primum acts as a one way valve preventing blood flow as described above, however if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left.
Clinical significance
Failure of the septum primum to fuse with the atrio-ventricular cushion can lead to an ostium primum atrial septal defectAtrial septal defect
Atrial septal defect is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria...
. This is the second most common type of atrial septal defect and is commonly seen in Down's syndrome. Typically this defect will cause a shunt to occur from the left atrium to the right atrium. Children born with this defect may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt. This reversal is called Eisenmenger syndrome.