The interatrial septum is the wall of tissue that separates the right and left atria of the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

Structure

The interatrial septum is a that lies between the

left atrium The atrium ( la, ātrium, , entry hall) is one of 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 valves. There are two atr ...

and

right atrium The atrium ( la, ātrium, , entry hall) is one of 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 valves. There are two atr ...

of the

human heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to ...

. The interatrial septum lies at angle of 65 degrees from right posterior to left anterior because right atrium is located at the right side of the body while left atrium is located at the left side of the body.

Development

The interatrial septum forms during the first and second months of

fetal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...

. Formation of the septum occurs in several stages. The first is the development of the

septum primum During heart development of a human embryo, the single primitive atrium becomes divided into right and left by a , the septum primum. The septum primum () grows downward into the single atrium. Development The gap below it is known as the ostium ...

, a crescent-shaped piece of tissue forming the initial divider between the right and left atria. Because of its crescent shape, the septum primum does not fully occlude the space between the left and right atria; the opening that remains is called the

ostium primum In the developing heart, the 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 endocardi ...

. During fetal development, this opening allows blood to be shunted from the right atrium to the left. As the septum primum grows, the ostium primum progressively narrows. Before the ostium primum is completely occluded, a second opening called the

ostium secundum The foramen secundum, or ostium secundum is a in the septum primum, a precursor to the interatrial septum of the human heart. It is not the same as the foramen ovale, which is an opening in the septum secundum. Development The foramen secund ...

begins to form in the

. The

allows continued shunting of blood from the right atrium to the left. To the right of the septum primum, the

septum secundum The septum secundum is a muscular flap that is important in heart development. It is semilunar in shape, and grows downward from the upper wall of the atrium immediately to the right of the septum primum and ostium secundum. It is important in the ...

begins to form. This thick, muscular structure initially takes on the same crescent shape as the septum primum, except that it originates anteriorly, whereas the septum primum originates posteriorly. As the septum secundum grows, it leaves a small opening called the foramen ovale. The foramen ovale is continuous with the ostium secundum, again providing for continued shunting of blood. The ostium secundum progressively enlarges and the size of the septum primum diminishes. Eventually, the septum primum is nothing more than a small flap that covers the foramen ovale on its left side. This flap of tissue is called the valve of the foramen ovale. It opens and closes in response to

pressure gradient In atmospheric science, the pressure gradient (typically of Earth's atmosphere, air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particu ...

s between the left and right atria. When the pressure is greater in the right atrium, the valve opens; when the pressure is greater in the left atrium, the valve closes. Because the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s are nonfunctional in fetal life, pressure in the

pulmonary circulation The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. ...

is greater than that of the

systemic circulation The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

. Consequently, the right atrium is generally under higher pressures than the left atrium, and the valve of the foramen ovale is normally open.

At birth

At birth, there is a reversal in the pressure gradient between the atria, resulting in functional closure of the valve of the foramen ovale. Permanent anatomical closure of the foramen ovale occurs with time in normal infants. Inappropriate closure of the foramen ovale results in

patent foramen ovale Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale; however, when this d ...

Clinical significance

Atrial septal defect Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale; however, when this d ...

is a relatively common heart malformation that occurs when the interatrial septum fails to develop properly. Persistence of the ostium secundum is the most common

atrial septal defect Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale; however, when this d ...

.Diagram of Ostium Secundum Atrial Septal Defect at Mayo Clinic
/ref> Additionally, in a subset of the population, the foramen ovale is not overtly patent but the two septa have not fused. In normal physiologic circumstances, the septum primum acts as a one-way valve preventing blood flow as described above; but, if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left. Failure of the septum primum to fuse with the endocardial cushion can lead to an ostium primum

. This is the second most common type of atrial septal defect and is commonly seen in

Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual dis ...

. 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 Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It is distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number.Updated by Linda J ...

of the right side of the heart will lead to a reversal of this shunt. This reversal is called

Eisenmenger syndrome Eisenmenger syndrome or Eisenmenger's syndrome is defined as the process in which a long-standing left-to-right cardiac shunt caused by a congenital heart defect (typically by a ventricular septal defect, atrial septal defect, or less commonly, pat ...

. Lipomatous atrial septal hypertrophy (LASH) is the fat deposition in the infoldings of the interatrial septum adjacent to the true atrial septum. It is shaped like a “dumbbell” because the deposition is located at the above and the below the fossa ovalis, sparing the fossa itself. The incidence of LASH increases with older age and obesity. On CT scan, it shows homogeneous, dumbbell mass which is non-enhancing at the interatrial septum. MRI shows extension of the mass into interventricular septum and ventricular wall.

References

* ''Gray's Anatomy: The Anatomical Basis of Clinical Practice, 39th ed.'' (2005). * "Septum, interatrial." ''Stedman's Medical Dictionary, 27th ed.'' (2000).

External links

Diagram

from

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