Left atrium anatomy
The left atrium is the most posteriorly located of all the heart chambers. Relative to the right atrium, the left atrial chamber is both more superiorly and more posteriorly located. The walls of the left atrium are slightly ticker than the walls of the right atrium.
Left atrium's primary function is to collect the blood from the lungs during systole and modulate the filling of the ventricle during the diastole. The left atrium recieves oxygenated blood from the lungs through the pulmonary veins. Normal inflow of the blood through pulmonary veins is continuous and non-pulsatile, while the output of the heart is pulsatile (due to ventricular contractions). Without the atria, the venous inflow to the heart would be interrupted during the ventricular systole. Four essential characteristics of the atrium enable the uninterrupted venous inflow:
- the veins entering the atria have no inlet valves which interrupt the blood flow during the atrial contractions
- during the atrial systole, the venous blood flows uninterrupted through the atria into the ventricle. This is facilitated by incomplete atrial contractions which do not impede the atrial inflow.
- tied to the above item, the gentle atrial contractions do not create significant back pressure which will impede the blood inflow.
- the timing of the atrial contractions results in the atrium that is relaxed before the start of the ventricular contraction, allowing it to accept the venous inflow without interruption
The pulmonary veins enter the left atrium from the posterior side. Typically, the left atrium has four pulmonary veins joining the LA on each side through individual ostia . The left superior pulmonary vein ( LSPV ) and the left inferior pulmonary vein ( LIPV ) connect to the LA from the left posterior side, while the right superior pulmonary vein ( RSPV ) and the right inferior pulmonary vein ( RIPV ) connect to the LA from the right posterior side. This typical four ostia pattern occurs in around 60% of the patients . Diverging from the typical pattern, there are possible anatomical variations on both the left and the right side. The most common variations are either: the common left trunk ( CLT ) on the left side or the right middle pulmonary vein ( RMPV ) on the right side. However, patients with simultaneous variations on both sides are very rare. The CLT variation is characterized by both left PVs merging in the proximity of the LA body and entering the LA through a single ostium. Configurations where the distance from the ostium to the bifurcation is less than 1cm are called the short CLT and occur in about 10% of the patients. The CLT occurs in about 4 to 8 % of the patients. Both the typical configuration and the short CLT are considered normal. The RMPV configuration manifests with an additional PV joining the right side of the LA body with a separate ostium. This kind of configuration is found more often, in about 13% to 24% of the patients.