The right atrium is one of the 4 chambers of the heart. The hollow structure is located inside the heart, in the upper right corner above the right ventricle. Deoxygenated blood that enters the heart from the body’s different tissues, via veins, initially passes through the right atrium before going to the right ventricle. The deoxygenated blood is then pumped by the heart to the pulmonary veins present near the lungs. Now, the blood stream gets a new dose of oxygen which then passes into the left atrium and later gets pumped to different tissues of the body.
Right Atrium: Location and Functions
The right atrium, along with the left atrium, forms the upper chambers of the heart called the atria, which in turn function as the chambers that receive the blood that enter the heart. The right atrium is located towards the upper right side of the heart and above the more muscular and much bigger right ventricle. The tricuspid valve is a one-way valve which is present between the right ventricle and the right atrium.
As compared to the ventricles, the right atrium walls are less muscular and thinner. It also features the auricle, a flap-shaped, hollow, and wrinkled appendage that looks like the floppy ears of a dog. The auricle expands outwardly from the front surface so as to increase the right atrium’s internal volume.
Oxygen-deficient blood enter the cardiac right atrium chamber via the 3 main veins, i.e., the inferior and superior vena cava and the coronary sinus. All the blood present in the tissues occurring above the heart such as the neck, head, upper trunk, and arms, etc., are returned to the heart by the superior vena cava. Similarly, blood from the tissues present inferior to the heart such as the lower trunk, legs, and abdomen, etc. are returned to the heart via the inferior vena cava. Blood that is present outside the cardiac region gets accumulated I the coronary sinus and then returned inside the cardiac chambers.
A muscular wall called the inter-atrial septum occurs on the medial or middle border of the right atrium. It delineates the right and left atria and prevents the flow/exchange of blood between the upper two chambers.
When the fetus is in the embryo, the inter-atrial septum features a tiny hole called the foramen ovale which permits the flow of blood from the right atrium to the upper left chamber so as to decrease the passage of blood to the non-functional lungs. This is very important for the circulatory health of the unborn child, which gets its oxygen-rich blood supply from the mother. After the child is born, the lungs become active and the connection between the left and right atria close.
After birth, a tiny tissue flap covers the foramen ovale opening and prevents the blood flow between the upper cardiac chambers. Adults may feature a tiny indent mark, called the fossa ovalis, in the inter-atrial septum at the place where once the foramen ovale used to be present. Sometimes, the foramen ovale may not close. This is an anomaly and the associated condition is known as patent foramen ovale.
The SA or sinoatrial node is also present in the right atrium. It is an essential piece of cardiac tissue which controls the rhythm of the heart and excites the other muscle cells of the heart to contract, thereby functioning as a pacemaker. The brain connects to the SA node via autonomic nerves and facilitate the reduction or increase of the heart beat so as to maintain the blood pressure as well as homeostasis of carbon dioxide and oxygen levels.
In addition to passively receiving blood from the veins, the right atrium also purposefully pumps blood into the lower right cardiac chamber. During the cardiac cycle relaxation stage, the muscles of all the cardiac chambers tend to relax, which then permits an increase of volume of the chambers. Now the returning blood from the right atrium flows into the right ventricle via the tricuspid valve. Nearly 70 percent of the ventricles get filled during this stage. The right atrium then undergoes contraction or systole phase so as to actively pump blood into the lower right chamber and fill it up fully. Then the right ventricle undergoes systole/contraction so as to pump blood into the pulmonary veins/lungs. The tricuspid valve is closed which prevents the backward flow of blood from the right ventricle into the right atrium. This allows the efficient pumping of blood in just one direction.
Enlargement of the right atrium
Enlargement of the right atrium is uncommon and not as prevalent as left atrium enlargement. It is also quite difficult to distinguish on a chest radiograph. Some of the common causes of right atrial enlargement include pulmonary arterial hypertension, elevated right ventricular pressures, cor pulmonale, tricuspid regurgitation, valvular disease, tricuspid stenosis, atrial septal defect, Ebstein’s anomaly, dilated cardiomyopathy, and atrial fibrillation.
- Valvular heart disease: Problems of the heart valve such as tricuspid regurgitation, a condition marked by excessive leaks in the tricuspid valve, can facilitate the backward flow of blood from the right ventricle into the right atrium, eventually causing right atrial enlargement. Similarly, tricuspid stenosis or narrowing of the tricuspid valve can result in excessive buildup of blood in the right atrium and trigger its enlargement.
- Atrial fibrillation: It is a condition marked by irregular heartbeats which affect the normal rhythm of the heart. In health individuals, the atria as well as the ventricles beat together in a coordinated manner, but atrial fibrillation affects and interferes with such synchronicity. Chronic atrial fibrillation can cause the right atrium to enlarge. Untreated cases of right atrial enlargement and/or atrial fibrillation can trigger the formation of blood clots which can then migrate to the brain and cause stroke. Hence, affected people are given blood thinning medications to lower the risk to stroke.
- High blood pressure: Uncontrolled hypertension can cause the heart to work harder to pump blood. As the heart is a muscle, it will enlarge when working against increased pressures. This can then subsequently cause right atrial enlargement.
- Heart failure: Cardiac failure is a condition marked by reduced ability of the heart to effectively pump blood. It can occur due to varied conditions like alcohol or drug abuse, coronary heart disease, etc. Increased weakening of the heart can result in buildup of blood in the left chambers, which can then spill over to the right chambers, thereby causing right atrial enlargement.