Cardiac beat frequency and oxygen supply: a comparative study

The length of diastole in mammals varies between approx 1 s (elephant) and 38 ms (shrew) which makes oxygen supply in high speed cardiac pumps in very small mammals precarious. High capillary density and high blood P50 are reported in mammals with high frequency cardiac cycle. Both are probably insufficient when cardiac frequency is exceedingly high (shrew: 1000 min-1). High respiratory efficiency due to large relative mitochondrial volume per cell (greater than 50%) seems to be preferential solution to maintain sufficient O2-gradient. Similar strategy, i.e. high relative cardiac mitochondrial volume was reported in analogous situation in ice-fish (Chaenocephalus aceratus) where O2 cardiac cell supply is difficult due to the absence of hemoglobin and cardiac myoglobin.