Overview of Mitral Regurgitation
Mitral regurgitation is a condition in which disease or injury has caused the heart's mitral valve to become leaky. Normally, the four heart valves function as one-way valves, allowing blood to be pumped forward and preventing blood from regurgitating backwards. When the mitral valve becomes leaky, blood may back up into the lungs, causing shortness of breath. An untreated leaky valve can lead to heart failure.
Normal Blood Flow in the Heart
To better understand what happens in mitral regurgitation, it is worth taking a moment to discuss the normal flow of blood in the body. The heart can be divided into a right side and a left side, with each side having a chamber that receives blood returning to the heart (atrium), and a muscular chamber that pumps blood out of the heart (ventricle).
Blood that has traveled through the body and is now oxygen poor returns to the heart and enters the right atrium. This blood then passes through a one-way valve into the right ventricle. The right ventricle pumps this blood to the lungs, where the blood absorbs oxygen. This oxygen-rich blood returns from the lungs to the heart and enters the left atrium. Blood passes from the left atrium through a one-way valve called the mitral valve and enters the left ventricle.
The left ventricle is the most muscular of the heart's four chambers and can be considered the main pumping chamber of the heart. When the muscular tissue of the left ventricle contracts, blood is pumped through another one-way valve, the aortic valve, into the aorta, the main artery of the body. The aorta delivers blood to other arteries that travel to the head, arms, abdomen, and legs. These arteries supply oxygen-rich blood to the organs and tissues of the body, which require oxygen to function. Oxygen-poor blood then returns from these organs and tissues through the veins back to the right atrium of the heart, and the cycle repeats.
What Happens in Mitral Regurgitation?
The mitral valve functions as a one-way valve, allowing oxygen-rich blood that has returned from the lungs to flow from the left atrium into the left ventricle. This occurs while the left ventricle is in its relaxed, uncontracted phase of the heart cycle. When the left ventricle contracts, the mitral valve closes, preventing blood from being pumped back into the left atrium. This ensures that all blood is pumped across the aortic valve into the aorta. If the mitral valve is not functioning properly, due to injury or disease, blood leaks back into the left atrium (regurgitates) when the left ventricle contracts and, simply put, backs up into the lungs.
Because some of the blood being pumped by the left ventricle flows back (regurgitates) into the left atrium, less blood is pumped into the aorta and, ultimately, throughout the body. The heart compensates for this by increasing the size of the left ventricle to increase the amount of blood it is pumping and to maintain an adequate forward flow of blood throughout the body. Unfortunately, compensation eventually leads to impairment of the left ventricle's ability to contract (decompensate), which leads to further back-up of blood into the lungs.