In “Hot-blooded crocodiles?” (Tuesday, April 19, 2005), Mr PZ Myers writes:
Crocodiles are beasts with an odd mix of features: they are ectothermic (meaning that they derive their body heat from their environment) reptiles, like lizards and snakes, but unlike those smaller critters, they have a fairly sophisticated, high performance cardiovascular system: they have a true four-chambered heart, just like us mammals and birds, and they also have a diaphragmaticus, a muscle analogous to our diaphragm that is used to inflate the lungs. At the same time, their hearts are relatively small—heart mass is roughly 0.15% of body mass, compared to 0.4%-0.7% of body mass for mammals—and generates relatively low systemic blood pressure. It’s weird. It’s like they have this fancy, sophisticated engine in low-tech chassis, that the animal never revs up to its full potential. How did it get in there, and why do crocodiles have such fancy hearts?Mr PZ Myers makes the interesting comment that:
I was always taught that the most important function of the separate pulmonary and systemic circulation was to prevent mixing of oxygenated and deoxygenated blood. Seymour et al. mention another very important reason, though: it also allows the two pumps to generate different blood pressures. In us mammals, our systemic pressure is relatively high (120/80 mm Hg, on average), but our pulmonary pressure is much, much lower—more like 20-40 mm Hg. Pumping blood at systemic pressures through the thin, delicate membranes of the lung would not be a good thing. More plasma would be forced out into the spaces of the lung, drowning us.But then we have the fascinating and important question: what does this knowledge depend on? One would have to have a prior grasp of pressure, fluids, pumps, circulation, methods of measurement and experimental methods. There is a great deal of scientific and mathematical knowledge upon which the above paragraphs depend. Hierarchy and context rule.