Simplify the following text for a Grade 8 native English-speaking person.
Original: When the pressure in the left ventricle drops to below the pressure in the left atrium , the mitral valve opens , causing accumulated blood from the atrium to flow into the ventricle .
Simplified: When the pressure in the left ventricle becomes lower than the pressure in the left atrium , the mitral valve opens . The blood then flows from the atrium into the ventricle .