The primitive atrium is a stage in the embryonic development of the human
heart
The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...
. It grows rapidly and partially encircles the
bulbus cordis; the groove against which the bulbus cordis lies is the first indication of a division into right and left
atria.
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
primary interatrial foramen, 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 re-established 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
The pulmonary veins are the veins that transfer oxygenated blood from the lungs to the heart. The largest pulmonary veins are the four ''main pulmonary veins'', two from each lung that drain into the left atrium of the heart. The pulmonary ve ...
; each pair unites to form a single vessel, and these in turn join in a common trunk which opens into the
left atrium
The atrium (; : atria) is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves.
...
.
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.
References
External links
Overview at mcgill.ca
Embryology of cardiovascular system
{{Portal bar, Anatomy