Allometric relations of cardiovascular function in birds.

The avian cardiovascular system is interesting because birds have larger hearts and lower heart rates than mammals of the same size. This study defines how cardiovascular function is related to body size in nine species of birds encompassing a 1,000-fold size range. Heart mass (HM, g) was related to body mass (M, kg) by the following equation: HM = 8.76 M0.92, and the heart rate (HR, beats X min-1) to body mass by HR = 178.5 M-0.282. Cardiac output (Q, ml X min-1) was related to body mass as Q = 290.7 M0.69. Birds also had a greater cardiac output for a given body mass than mammals. Cardiac output was directly proportional to the rate of O2 consumption (VO2, ml X min-1): Q = 17.5 VO2(1.04), with birds having a greater cardiac output for a given VO2 than mammals. (Variables are considered directly proportional if the exponent does not differ significantly from one). As in mammals, stroke volume (SV, ml) was also directly proportional to body mass: SV = 1.72 M0.97, but for a given body size stroke volume was larger in birds than in mammals. However, as bird hearts are disproportionately large in comparison with body size, when stroke volume is expressed per gram of heart (SV = 0.177 HM1.05) birds have a significantly lower stroke volume-to-heart weight ratio than mammals.(ABSTRACT TRUNCATED AT 250 WORDS)