A "relief valve" can only be accessed when the pool is empty, and no swimming pool should ever be left empty. The only exception is when absolutely necessary due to repair or resurfacing, and that should be completed within the shortest possible time.
Any pool that is empty, and that has a relief valve, should have the valve removed for the entire time, regardless of the weather forecast.
A "hydrostatic" relief valve opens and closes when needed as it attempts to equalize the pool water pressure with the ground water pressure. Human intervention is not involved because the pool is still filled. Hydrostatic valves are great in theory, but seldom work after a few years.
The mitral valve is in the left heart between the atrium and the ventricle.
Mitral valve or biscupid valve
This valve has different names. It is the LAV valve or Mitral Valve or Bicuspid valve.
left atrioventricular valve and left mitral valve
The bicuspid valve.
The bicuspid or mitral valve and the aortic valve are on the left side of the heart. The mitral valve separates the left atrium and left ventricle, and the aortic valve is between the left atrium and the aorta.
Bicuspid valve. It is situated between the left atrium and the left ventricle. It permits blood to flow one way only, from the left atrium into the left ventricle.
right atrium - right ventricle = TRICUSPID VALVE; right ventricle - lungs = PULMONARY SEMILUNAR VALVE; lungs - left atrium = not really a valve, more a perforation in the left pericardium; left atrium - left ventricle = MITRAL VALVE; left ventricle - aorta = AORTIC SEMILUNAR VALVE.