Heart period is proportional to body length.

The RR interval, i.e. the heart period (T), increases with body mass (M) as: T = T0M0.27. The characteristic time of the arterial system (arterial decay time or decay time of aortic pressure in diastole, tau) equals peripheral resistance times total arterial compliance. Peripheral resistance equals mean systemic pressure over cardiac output. It can be derived that total arterial compliance is proportional to cardiac output times aortic length. Because mean aortic pressure is similar in different mammals the arterial decay time is proportional to the length of the aorta. Since aortic length and body length are related to body mass with the same exponent it follows that the arterial decay time is proportional to body length. It was found that the arterial decay time is related to body mass as tau = tau oM0.29. The heart period and the duration of diastole have similar exponents. We suggest that the heart period is matched to the arterial decay time to provide similar conditions for coronary perfusion. Since the arterial decay time is proportional to body length, heart period is also proportional to body length.