One might expect the heart to be stronger after a rest. Yet what is observed here can be considered a general quality of living systems. This principle is one of self-organization, one of the most striking differences between a living system and a nonliving one. A machine is worn out by usage. A living system is worn out by inactivity.
Living systems are able to organize and improved themselves.
S. J. Hajdu and I tried to discover the mechanism that produced the Bowditch staircase. We found that potassium leaks out from the heart fibers into the surrounding fluid when the heart is not beating. When the heart resumes its activity, the potassium is pumped back into the heart fibers. The change in strength of the heartbeats is caused by this change in the distribution of potassium. The movement of the potassium back into the fibers, against the potassium concentration gradient, increases the amount of organization or order in the system.