The great veins and Atria

The great veins, which include the superior and inferior vena cavas, the coronary sinus, and the four pulmonary veins, can either connect normally or abnormally to the atria. In patients with atrial isomerism, these connections are frequently abnormal, and their normal arrangement is therefore of particular importance.

The systemic veins

There are three great veins which connect to the right atrium. These are the superior and inferior vena cavas and the coronary sinus. They all join the atrium within the confines of the sinus venosus, that area of the right atrium which is bounded by the atrial septum and the crista terminalis. Externally, the location of the crista terminalis is marked by the terminal groove, while the position of the atrial septum marked by Waterston's or Sondergaard's groove, which lies between the connections of the vena cavas to the right atrium and the right pulmonary veins to the left atrium.

The orifice of the coronary sinus and it's valve, the Thebesian valve occupy the most posteroinferior corner of the right atrium. A commissure is formed between the Thebesian valve and Eustachian valve of the inferior vena cava, which extends as a fibrous strand known as the tendon of Todaro to insert into the central fibrous body. The atrioventricular node and bundle of His are located within the triangle of Koch, which is formed by the origin of the septal leaflet of the tricuspid valve, the Thebesian valve, and the Eustachian valve along with its anterior extension, the tendon of Todaro.

A left superior vena cava is present during early fetal life but usually becomes absorbed and attenuated during development. Although usually small in comparison to the vena cavas, the coronary sinus can be large when it drains a persistent left superior vena cava. A persistent superior vena cava runs between the left atrial appendage and the left pulmonary veins, and enters the right atrium underneath the oblique ligament of the left atrium.

The pulmonary veins

Each of the four pulmonary veins normally connect to the left atrium in a different corner of the posterior atrial wall, so that together they enclose a substantial area. Either the oblique ligament of the left atrium or a persistent left superior vena cava runs between the left pulmonary veins and the left atrial appendage. Normally, the right pulmonary veins are separated from the vena cavas by Waterston's or Sondergaard's groove.

The atria

Each atrium contains four main components, the sinus venous, the muscular portion, the septal surface, and the atrial portion of the atrioventricular septum:

1. The sinus venous (or venous portion) is the area of connection of the great veins to the atria. For the right atrium, this is the area of connection of the vena cavas and the coronary sinus, and for the left atrium, it is the area of incorporation of the pulmonary veins.
2. The muscular portion consists of the atrial appendages and the pectinate muscles. The muscular portion of the atrium serves to identify the morphologic right from the left atrium.
3. The septal surface connects the two atria. It is composed of the septum secundum on the right atrial side, and of the septum primum on the left atrial side. The septal surface of each atrium comprises only a very small portion of the total size of the atrium.
4. The atrioventricular septum consists of the atrial portion of the atrioventricular septum. The septal leaflet of the tricuspid valve joins the atrioventricular septum on the right atrial side, while the anterior mitral valve leaflet joins the atrioventricular septum on the left atrial side.

The right atrium

The right atrium is distinguished by the right atrial appendage, which resembles a thumb. The right atrium is derived from four components: The sinus venosus, the muscular portion, the septal surface, and the atrioventricular septum.

The sinus venosus includes the orifices of the superior and inferior vena cavas along with the coronary sinus and the smooth venous tissue in between these ostia. There is a shallow sulcus between the superior vena cava and the right atrium, which is known as the sulcus terminalis. This sulcus marks the termination of the sinus venosus component of the right atrium and a beginning of the muscular right atrial appendage. The sulcus terminalis is important because this is where the SA node resides. The pacemaker is called a sinoatrial node because it is located at the sinus venosus-atrial junction.
The muscular portion is the primitive portion of the atrium, and consists of the pectinate muscles, tenia sagittalis and the crista terminalis, the latter which serves to delineate the sinus venosus from the muscular atrium.
The septal surface contains the septum secundum, the inferior edge of which is the limbus, and the fossa ovalis. The superior portion of the septum secundum is a muscular structure that lies directly behind the aortic root.
The atrioventricular septum includes the septal leaflet of the tricuspid valve and the atrial portion of the atrioventricular septum.

The left atrium

The left atrium is distinguished by the left atrial appendage, which resembles the fifth digit, and like the right atrium is also composed for four components: The sinus venous is the major portion of the left atrium and contains the incorporation of the main pulmonary veins into each of its four corners; the muscular portion consists of the left atrial appendage and its pectinate muscles. The pectinate muscles are confined to the appendage, extending only a limited distance toward the atrioventricular septum; the septal surface contains the septum primum, which is the flap valve of the foramen ovale; and the atrioventricular septum consists of the left atrial portion of the atrioventricular septum and the attached anterior leaflet of the left ventricular valve.

Atrial arrangements

It should therefor almost always be possible to distinguish the right from the left atrium. Because of variable venoatrial and atrioventricular connections, in addition to the variability in the septal surfaces of the atria, the major distinguishing feature that identifies the right from left atrium is the morphology of the atrial appendages. Virtually all hearts have two atria, and each atrium almost invariably has an appendage. There are four types of atrial arrangements. The first two of these being the usual and the mirror-image arrangements, in which each has a right atrial appendage one side and a left one the other side. The remaining two arrangements are isomeric, those having right or left atrial appendages on both sides. Almost always the rest of the organs in the body will be arranged according to the atrial appendages. The most useful clinical confirmation of atrial isomerism is obtained by assessing the bronchial arrangement and the presence of a asplenia or polysplenia. A second feature of distinguishing the right from left atrium is the connection of the suprahepatic segment of the inferior vena cava to the right atrium, although this feature does not directly identify the morphology of the remaining atrium, which may be a left or right one.