Interatrial septum
Encyclopedia
The interatrial septum is the wall of tissue that separates the right
and left atria
of the heart
.
. Formation of the septum occurs in several stages. The first is the development of the septum primum
, 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 left and right atria; the opening that remains is called the ostium primum
. 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
begins to form in the septum primum
. The ostium secundum
allows continued shunting of blood from the right atrium to the left.
To the right of the septum primum, the septum secundum
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
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
s are nonfunctional in fetal life, pressure in the pulmonary circulation
is greater than that of the systemic circulation
. 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, 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.
Right atrium
The right atrium is one of four chambers in the hearts of mammals and archosaurs...
and left atria
Left atrium
The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and pumps it into the left ventricle, via the mitral valve.-Foramen ovale:...
of the heart
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...
.
Development
The interatrial septum forms during the first and second months of fetal developmentFetal development
Prenatal or antenatal development is the process in which a human embryo or fetus gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.After fertilization the embryogenesis starts...
. Formation of the septum occurs in several stages. The first is the development 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...
, 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 left and right atria; the opening that remains is called the ostium primum
Ostium primum
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 , below the free margin of the septum.-Closing of ostium primum:This opening is closed by...
. 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
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:...
begins to form in 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...
. 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:...
allows continued shunting of blood from the right atrium to the left.
To the right of the septum primum, 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....
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
Foramen ovale
There are multiple structures in the human body with the name foramen ovale :* In the fetal heart, the foramen ovale is a shunt from the right atrium to left atrium....
. 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
Pressure gradient
In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...
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
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s are nonfunctional in fetal life, pressure in the pulmonary circulation
Pulmonary circulation
Pulmonary circulation is the half portion of the cardiovascular system which carries Oxygen-depleted Blood away from the heart, to the Lungs, and returns oxygenated blood back to the heart. Encyclopedic description and discovery of the pulmonary circulation is widely attributed to Doctor Ibn...
is greater than that of the systemic circulation
Systemic circulation
Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart. This physiologic theory of circulation was first described by William Harvey...
. 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, 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.
Disorders
- Atrial septal defectAtrial septal defectAtrial 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...
is a relatively common heart malformation that occurs when the interatrial septum fails to develop properly.
External links
- Diagram
- Animation, from Indiana UniversityIndiana UniversityIndiana University is a multi-campus public university system in the state of Indiana, United States. Indiana University has a combined student body of more than 100,000 students, including approximately 42,000 students enrolled at the Indiana University Bloomington campus and approximately 37,000...
