What occurs during mitral stenosis?

Restricts blood flow. Mitral stenosis is when the mitral valve fails to open sufficiently. This restricts the blood flow into the left ventricle (the main pumping chamber). This causes the left atrium to dilate and increase pressure in the lungs. This causes shortness of breath, increase BP in lungs, abnormal heart rhythms, and heart failure.
Shortness of breath. In mitral stenosis, blood returning from the lungs is unable to easily pass through the mitral valve to fill the left ventricle. Blood backs up behind the narrowed of mitral valve causing pulmonary congestion. This results in pulmonary hypertension and shortness of breath.