Mammals have a four-chambered heart, with two ventricles and two atria. So do birds, alligators, and crocodiles, but other living reptiles have a three-chambered heart, with just one ventricle. Paleontologists debate whether dinosaurs had a three-chambered or four-chambered heart. Long-necked sauropod dinosaurs could have unusual circulatory demands because their head may have been raised far above the heart. The further 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 demands a systolic pressure of 500 mm of mercury for blood to reach the brain! Some paleontologists consider this evidence that dinosaurs had a four-chambered heart that supported a dual circulatory system similar to that of birds and mammals, rather than the three-chambered heart of most non-bird reptiles. Can you explain why?