Q. Birds and mammals have a four-chambered heart, with two ventricles and two atria, but other modern reptiles have a three-chambered heart, with just one ventricle. Paleontologists debate whether dinosaurs had a typical "reptile-like" heart or a "birdlike" heart. Long-necked sauropod dinosaurs could have had unusual circulatory demands because their head may have been raised far above their heart. The farther the head is above the heart, the greater the systolic pressure needs to be for blood to reach the brain. For example, the long-necked dinosaur Brachiosaurus may have carried its head as much as 6 m (20 feet) above its heart. It is estimated that such an anatomy demanded a systolic blood pressure of 500 mm of mercury for blood to reach the brain! Some paleontologists think this to be evidence that dinosaurs should have had a four-chambered heart that supported a dual circulatory system similar to that of birds and mammals, rather than three-chambered heart of non bird reptiles. Can you explain why?