Atria of the Heart
The atria of the heart, which are known singularly as an atrium, are a pair of blood collecting chambers that comprise two of the four chambers of the heart. The main function of the atria is to facilitate circulation by rhythmically contracting and relaxing their walls, pushing the blood that is returned to them via the major venous vessels during ventricular systole into the ventricles.
The atria are divided into the left atrium and the right atrium, which correspond to the left and right sides of the heart. They are situated just above the ventricles of the heart and are separated by the cuspid valves. The atria are built and held up by rigid endocardial muscle fibers that completely encompass the blood as it flows through them.
The left atrium is responsible for collecting the outflow of oxygenated blood that runs back to the heart from the pulmonary veins. It passes this volume of blood on to the left ventricle, as the atrium contracts and the mitral valve opens. This valve is also known as the bicuspid valve because it is formed by two septal cusps. Occasionally, it is also mentioned in literature as the left atrioventricular valve. The blood supply to the left atrium is governed by the left circumflex coronary artery and it is drained by the oblique vein of the left atrium which arises during embryonic development from the left side of the fetal superior vena cava. The main anatomical structure of note is the foramen ovale, which this is an embryological foramen within the atrial walls that allows blood to shunt between the two atria. Normally, around the time of birth this hole in the cardiac walls closes. If the foramen ovale doesn't seal completely an atrial septal defect (ASD) occurs, more specifically, a patent foramen ovale (PFO). This is a congenital heart defect. It is not however generally problematic in child or adulthood. It can be found in 15-35% of all people.
The right atrium receives deoxygenated venous flow from the superior vena cava and inferior vena cava, the coronary sinus as well as the anterior and the smallest cardiac veins. It passes the blood through the tricuspid valve, which has three septae and is also known as the right atrioventricular valve. The main anatomical features of the right atrium include the sinus venarum, which surrounds the openings of the superior and inferior vena cavae as well as the coronary sinus, which in turn are also major structures. The atrial walls are made up of pectinate muscles which form a pouch known as the right auricle. The crista terminalis separates the cardiac and smooth muscle layers of the atrial walls, while the interatrial septum divides the atria.
Atrial septal defects are classed as congenital heart disease and as with most of the disorders in this category, the etiology has yet to be determined. Atrial defects can include a patent foramen ovale, which usually has little clinical significance other than the fact that it stays open after birth. The two major septal defects, known as septum primum and septum secundum are described as follows: a septum primum, which affects the lower part of the atrial septum can be large enough to also involve the atrioventricular valves whereas a defect which is exclusive to the fossa ovalis is known as a septum secundum. A lesser septal defect known as the sinus venosus affects the septal atrial opening of the vena cava. Lastly, when atrial septal defects occur along with mitral stenosis, it is classified as Lutembacher syndrome. Many of these defects only manifest clinical symptoms in adulthood, such as pulmonary hypertension, reversal of blood flow which causes cyanosis and paradoxic embolism.