View source for Primary interatrial foramen

{{Infobox embryology
| Name          = Primary interatrial foramen
| Latin         = foramen primum
| Image         = Gray465.png
| Caption       = The [[heart development|developing human heart]], at day 30. The ostium primum joins the [[primitive atrium]] and lies beneath the septum primum (top middle) and becomes progressively smaller as the septum grows in size.
| Image2        =
| Caption2      =
| System        = [[Cardiovascular system]]
| CarnegieStage =
| Days          =
| Precursor     =
| GivesRiseTo   =
| Location = Developing [[heart]]
}}
In the developing [[heart]], the [[Atrium (heart)|atria]] are initially open to each other, with the opening known as the '''primary interatrial foramen''' or '''ostium primum''' (or '''interatrial foramen primum'''). The foramen lies beneath the edge of [[septum primum]] and the [[endocardial cushions]]. It progressively decreases in size as the septum grows downwards, and disappears with the formation of the [[atrial septum]].

==Structure==
The foramen lies beneath the edge of [[septum primum]] and the [[endocardial cushions]]. It progressively decreases in size as the septum grows downwards, and disappears with the formation of the [[atrial septum]].

==Closure==
{{main|Interatrial septum#Development}}

The septum primum, a {{wt|en|septum}} which grows down to separate the [[primitive atrium]] into the [[left atrium]] and [[right atrium]], grows in size over the course of [[heart development]]. The primary interatrial foramen is the gap between the septum primum and the [[septum intermedium]], which gets progressively smaller until it closes.

==Clinical significance==
Failure of the septum primum to fuse with the endocardial cushion can lead to an ostium primum [[atrial septal defect]].<ref>{{Cite GPnotebook|-402259967|ostium primum atrial septal defect}}</ref> This is the second most common type of atrial septal defect<ref name="mayoclinic.org">[http://www.mayoclinic.org/atrial-septal-defect/types.html Diagram of Ostium Secundum Atrial Septal Defect at Mayo Clinic]</ref> and is commonly seen in [[Down syndrome]]. Typically, this defect will cause a shunt to occur from the left atrium to the right atrium.  Children born with this condition 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]].

==References==
{{Gray's}}
{{Reflist}}

==External links==
* {{EmbryologyTemple|Heart98/heart97a/sld036}}
* [http://staff.um.edu.mt/acus1/Heart-b.htm Overview and diagram at um.edu.mt]
* {{LoyolaMedEd|GrossAnatomy/thorax0/Heart_Development/Atria.html}}

{{Development of circulatory system}}
{{Portal bar|Anatomy}}
{{Authority control}}

[[Category:Embryology of cardiovascular system]]