Left Atrium.
The left atrium is the upper chamber in the left half of the heart. Previously, the atria were referred to as auricles and the left atrium was called the left auricle. It is separated from the right atrium by the atrial septum or the interatrial septum which has convexity to the left. The wall of the left atrium is slightly thicker than that of the right atrium – at 3mm.
The left atrium receives oxygenated blood from the lungs which is brought by the pulmonary veins (usually four, two from each lung). The left atrium receives this blood by relaxation of its muscles.
Once full, the left atrium contracts and pushes the oxygenated blood into the left ventricle through the bicuspid valve which is situated between the left atrium and the left ventricle.
Picture of Left Atrium
Left Atrial Appendage.
The left atrial appendage is a muscular pouch attached to the left atrium and serves as a reservoir for the left atrium. Clinical significance lies in the fact that in most cases of atrial fibrillation, clots are formed in the atrial appendage which can travel as emboli and get lodged in the brain, kidneys and other organs causing ischemia.
Dimensions of the Left Atrium.
The left atrium dimensions are measured during echocardiography and at end systole, when the left atrium is at its biggest size.
Diameter as measured along the parasternal axis = 28 to 40 mm.
Major axis = 41 to 61 mm.
Area = less than 20 sq cm
Volume of LA = 22 to 58 ml.
Foramen Ovale.
The interatrial septum of the fetus has a hole in it during development. This connects the right atrium to the left atrium and is called the foramen ovale. In the fetus, blood flows from the right atrium into the left atrium bypassing the pulmonary circulation as the lungs do not function in the fetus. After birth, this opening closes and is then referred to as fossa ovale.
Sometimes, in some individuals, this opening does not close and is referred to as the atrial septal defect. As a result of this opening, blood flows from the left atrium to the right atrium resulting in lower output of the heart. This is potentially dangerous and causes cardiac failure. Untreated cases can result in death.
Blood Supply of The Left Atrium.
The blood supply of the wall of the left atrium is mainly from the left circumplex coronary artery. Venous drainage is through the oblique vein.
Functions of the Left Atrium.
- The LA acts as a reservoir to collect oxygenated blood brought from the lungs by the pulmonary veins during ventricular contraction (systole).
- The left atrium acts as a passage for stored blood from the LA to the LV during early diastole.
- LA acts as a pump to push blood from the LA to the LV.
- It also has an endocrine function. Due to the release of ANP (atrial natriuretic peptide hormone) when the atria are dilated, LA (and RA) also regulates heart rate.
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