The mitral heart valve separates the left atrium from the left ventricle and allows oxygenated blood to flow between the two chambers. Oxygenated blood flowing from the lungs collects in the left atrium. The mitral heart valve remains closed until pressure in the left atrium and left ventricle changes. It then opens, allowing oxygenated blood to flow into the left ventricle until the ventricle is filled. The mitral heart valve then closes as the left ventricle – which is the pumping chamber of the heart – contracts. This prevents the oxygenated blood from flowing back in to the lungs.
As the left ventricle contracts, the oxygenated blood leaves the heart and crosses the aortic heart valve.