Heart can never pump at 100%. It can at the most 80 – 90%. The pumping fraction is the fraction of blood which it receive when it relaxes, that is being pumped out when it contracts. For example, if it receives 100 ml when it relaxes and pumps out 70 ml on contraction, the fraction is 70%. It is almost physically impossible for the heart to pump out the whole amount by squeezing itself.
