The number of times your heart contracts during each minute is your heart rate. Actually, both the right and left sides of the heart (the ventricles) contract simultaneously, but these two contractions count as one beat. The left ventricle of the heart fills with blood from the lungs during its rest period between beats. When the heart beats, it pumps that blood, out to the muscles. The right ventricle fills with blood returning from the muscles during the rest period and then pushes that blood out to the lungs.
