- Primitive atrium
Infobox Embryology
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Caption = Heart of humanembryo of about fourteen days.
Caption2 = Interior of dorsal half of heart from a humanembryo of about thirty days.
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GivesRiseTo =right atrium ,left atrium
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The primitive atrium is a term used to describe a stage in the embryonic development of the humanheart . It grows rapidly and partially encircles thebulbus cordis ; the groove against which the bulbus cordis lies is the first indication of a division into right and leftatria .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.For a time the atria communicate with each other by an opening, the
ostium primum of Born , below the free margin of the septum.This opening is closed by the union of the septum primum with the septum intermedium, and the communication between the atria is reëstablished through an opening which is developed in the upper part of the septum primum; this opening is known as the
foramen ovale (ostium secundum of Born) and persists until birth.A second septum, the
septum secundum , semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and foramen ovale.Shortly after birth it fuses with the primary septum, and by this means the foramen ovale is closed, but sometimes the fusion is incomplete and the upper part of the foramen remains patent. The
limbus fossæ ovalis denotes the free margin of the septum secundum.Issuing from each lung is a pair of
pulmonary veins ; each pair unites to form a single vessel, and these in turn join in a common trunk which opens into theleft atrium .Subsequently the common trunk and the two vessels forming it expand and form the vestibule or greater part of the atrium, the expansion reaching as far as the openings of the four vessels, so that in the adult all four veins open separately into the left atrium.
External links
* [http://sprojects.mmi.mcgill.ca/embryology/cvs/early_chambers.html Overview at mcgill.ca]
* [http://www.med.umich.edu/lrc/coursepages/M1/embryology/embryo/13cardiovascular_system.htm Description and diagram at umich.edu]
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