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 mammals 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, blood pressure in birds is independent of body mass, and heart mass scales isometrically. End-diastolic stress values, calculated according to Laplace, are about one-third of peak stresses recorded in isolated mammalian myocardial preparations.
The effect of oxygen partial pressure (Po(2)) on development and hatching was investigated in aquatic embryos of the myobatrachid frog, Crinia georgiana, in the field and in the laboratory. Eggs from 29 field nests experienced widely variable Po(2) but similar temperatures. Mean Po(2) in different nests ranged between 2.9 and 19.3 kPa (grand mean 12.9 kPa), and mean temperature ranged between 11.9 degrees and 16.8 degrees C (grand mean 13.7 degrees C). There was no detectable effect of Po(2) or temperature on development rate or hatching time in the field, except in one nest at 2.9 kPa where the embryos died, presumably in association with hypoxia. Laboratory eggs were incubated at 15 degrees C at a range of Po(2) between 2 and 25 kPa. Between 5 and 25 kPa, there was almost no effect of Po(2) on development rate to stage 26, but the embryos hatched progressively earlier-at earlier stages and lower gut-free body mass-at lower Po(2). At 2 kPa, development was severely delayed, growth of the embryo slowed, and morphological anomalies appeared. A high tolerance to low Po(2) may be an adaptation to embryonic development in the potentially hypoxic, aquatic environment.
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