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.
.jpg)
