The unique cardiopulmonary system of crocodilian reptiles has interested biologists for hundreds of years. Yet the functional significance of many features of this system remains a mystery. The research proposed herein seeks to understand the functional significance of the dual aortas (left and right) and cardiovascular shunt of crocodilians. Unlike mammals, that have only one aorta that originates in the left ventricle and carries oxygen-rich blood from the heart to the body, crocodilians have two aortas. One aorta originates from the left cardiac ventricle and is analogous with the mammalian aorta. However, crocodilians also have a second aorta that originates from the right cardiac ventricle and terminates at the stomach. The crocodilian shunt refers to a blood flow pattern where oxygen-poor systemic venous blood is directed into this aorta to be sent to the stomach, rather than directed into the pulmonary artery to be sent to the lungs. The research will test two hypotheses regarding this cardiovascular system: (1) that the dual aorta system of crocodilians facilitates digestion of food and (2) that this system functions to maintain appropriate blood pH after exercise. It will consist of measurements of blood flow using ultrasonic technology and measurements of blood gases and pH. A major aim of this research is to contribute to our understanding of the evolution of the vertebrate cardiovascular system.
A leading evolutionary biologist, Theodosius Dobzhansky, astutely said, "Nothing in biology makes sense except in the light of evolution." Evolutionary theory is a profound contribution of biology to knowledge. All aspects of biology have benefited from the use of this theory, including clinically oriented biology. A full understanding of the human cardiovascular system in health and in disease is only possible if we understand the evolutionary history of the system. Because the cardiovascular system is made up of soft tissues and does not preserve well in the fossil record, most insight into the evolution of the cardiovascular system comes from comparing cardiovascular features and functions of different living vertebrates. The early ancestors of mammals almost certainly had a cardiovascular system that was similar to that of a typical reptile, such as is found in lizards and turtles. The crocodilian cardiovascular system is unique because it shares numerous features with that of mammals (e.g., a completely divided cardiac ventricle supported by a coronary circulation) but has many other features that are typically reptilian (e.g., two aortas, the ability to shut down blood flow to the lung and redirect that blood to the body). Thus, the paradoxical assemblage of 'mammalian' and 'reptilian' features found within the crocodilians provides scientists with an exceptional opportunity to gain insight into how and why the mammalian system evolved from the reptilian pattern.