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Metabolic rate, blood pressure and flow

Endotherms rely highly on aerobic metabolism and have high rates of oxygen consumption during activity and rest. The oxygen required by the tissues is carried by the blood, and consequently blood flow rates and blood pressures at the heart of warm-blooded endotherms are considerably higher than those of cold-blooded ectotherms. It is possible to measure the minimum blood pressures of dinosaurs by estimating the vertical distance between the heart and the top of the head, because this column of blood must have a pressure at the bottom equal to the hydrostatic pressure derived from the density of blood and gravity. Added to this pressure is that required to move the blood through the circulatory system. It was pointed out in 1976 that, because of their height, many dinosaurs had minimum blood pressures within the endothermic range, and that they must have had four-chambered hearts to separate the high pressure circuit to the body from the low pressure circuit to the lungs. It was not clear whether these dinosaurs had high blood pressure simply to support the blood column or to support the high blood flow rates required by endothermy or both.

Foramen blood flow index, derived from the size of the nutrient foramen of the femurs of mammals, reptiles and dinosaurs

However, recent analysis of the tiny holes in fossil leg bones of dinosaurs provides a gauge for blood flow rate and hence metabolic rate. The holes are called nutrient foramina, and the nutrient artery is the major blood vessel passing through to the interior of the bone, where it branches into tiny vessels of the Haversian canal system. This system is responsible for replacing old bone with new bone, thereby repairing microbreaks that occur naturally during locomotion. Without this repair, microbreaks would build up, leading to stress fractures and ultimately catastrophic bone failure. The size of the nutrient foramen provides an index of blood flow through it, according to the Hagen-Poiseuille equation. The size is also related to the body size of animal, of course, so this effect is removed by analysis of allometry. Blood flow index of the nutrient foramen of the femurs in living mammals increases in direct proportion to the animals’ maximum metabolic rates, as measured during maximum sustained locomotion. Mammalian blood flow index is about 10 times greater than in ectothermic reptiles. Ten species of fossil dinosaurs from five taxonomic groups reveal indices even higher than in mammals, when body size is accounted for, indicating that they were highly active, aerobic animals. Thus high blood flow rate, high blood pressure, a four-chambered heart and sustained aerobic metabolism are all consistent with endothermy.