Cardiac Path Online
Normal Anatomy - Atrium of the Heart
The Right Atrium
The right atrium of the heart has three characteristic components.
The venous components is the most obvious in the normal heart.
It has smooth wall and receives the superior and inferior caval veins. The coronary sinus also drains into this component.
The appendage is the most constant part of the right atrium and is anatomically distinct.
It is a triangular structure which adjoins the venous component over a wide junction.
The internal surface of the appendage is corrugated by the parallel pectinate muscles, which originate from the terminal crest.
This crest marks the junction with the venous part and corresponds with the obvious external terminal groove.
The vestibule supports the leaflets of the tricuspid valve and characteristically in the right atrium it is surrounded by the pectinate muscles of the appendage.
At the first sight, the septal surface is extensive but this is deceptive.
The true site of the septum is the location of the oval fossa.
This structure marks the position during fetal life where the richly oxygenated blood coming to the baby from the placenta crossed over into the systemic circuit so as to be distributed to the brain.
During intrauterine life, the blood for the growing fetus is oxygenated in the maternal placenta rather than in the fetal lungs.
An essential part of the atrial anatomy is a communication between the right and left sides.
This is found directly adjacent to the mouth of the inferior caval vein, which is the portal of venous entry from the placenta.
The communication in the fetus is called the oval foramen (foramen ovale).
It is designed so that it can close at birth when the lungs take over the role of oxygenation.
It has an obvious rim together with a floor, the flap valve.
The valve is hinged from the rim at its bottom edge, where the rim separates the foramen from the coronary sinus and the vestibule.
This part of the atrial wall is called the sinus septum.
In fetal life, a prominent fibromuscular fold directs the blood from the inferior caval vein into the oval foramen.
This often persists in postnatal life and is the Eustachian valve.
A similar valve during fetal life, marking the junction between venous component and appendage guards the opening of the coronary sinus and if this persists after birth it is called the Thebesian valve.
In normal hearts, the flap valve in the floor of the oval fossa is of sufficient size to overlap the edges of the foramen.
During fetal life its upper margin is free, permitting the oxygenated blood to cross the septum into the left atrium.
At birth, with onset of respiration, the pressure rises in the left atrium and the flap valve is pressed against the rim of the oval fossa, thus closing the communication.
It is often stated that the rims of the fossa itself are also extensive septal structures - the so-called "septum secundum".
(The flap valve itself is derived from the primary atrial septum - the so-called septum primum ).
Sectioning the heart shows that the rim of the fossa for most of its extent is simply formed from infoldings of the walls of the atriums.
In normal development, the upper margins of the flap valve attach by two horns of the left atrial surface of the septum.
With growth, the valve and rim fuse together between these points.
In up to one-third of the population, however, the edges appose rather than fuse, giving a probe patent oval foramen.
This is of no consequence as long as left atrial pressure remains higher than right , but the foramen can permit shunting should right atrial pressure exceed left, as it can do in association with some congenital or acquired diseases producing pulmonary hypertension.
Visit: Congenital Heart Disease
The structure of the left atrium though simpler than that of the right, still has three components.
The venous component is the dominant part receiving one of the four pulmonary veins at each of its corners.
The appendage is small with a characteristic tubular appearance and a narrow junction with the venous component.
The pectinate muscles are confined within the appendage and there is no terminal crest marking the junction with the venous component.
The vestibule of the mitral valve, therefore, is largely continuous with the smooth-walled venous component.
The septal surface of the left atrium is marked by the location of the flap valve, this being most obviously seen where its two horns mark the usual site of fusion with the rim. The anterior wall of the atrial roof is often additionally corrugated at this site.
Visit: Ventricle of the heart
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