The principle of laplace and scaling of ventricular wall stress and blood pressure in mammals and birds

Maximum left ventricular wall stress is calculated at end-diastolic volume and systemic arterial diastolic blood pressure, according to a thick-walled model for the principle of Laplace. Stress is independent of body mass and averages 13.9 kPa (+/-2.3; 95% confidence interval) in 24 species of mamma ls weighing 0.025-4,000 kg and 15.5 kPa (+/-4.7) in 12 birds weighing 0.014 -110 kg. Birds have higher arterial blood pressures and larger hearts than mammals. Systolic and diastolic arterial blood pressures increase with body mass according to M-0.05 in mammals, and heart mass increases according to M-1.06 in the same species, further supporting the principle. However, blo od pressure in birds is independent of body mass, and heart mass scales iso metrically. End-diastolic stress values, calculated according to Laplace, a re about one-third of peak stresses recorded in isolated mammalian myocardi al preparations.