9423297 Jackson An adequate supply of oxygen to heart muscle is essential to normal cardiac function. In mammals, the coronary arteries carry well-oxygenated blood to the heart muscle. Their importance is illustrated by the fact that insufficient oxygenation of heart muscle due to obstruction of the coronary arteries is a leading cause of death in humans. Oxygenation of heart muscle is also important for non- mammalian animals, and may have had a major impact on the evolution of the vertebrate cardiovascular system. In contrast to mammals, the hearts of fish, amphibians and reptiles have no arterial circulation to the heart muscle. Instead, venous blood flowing through the heart chambers supplies the oxygen needs of the spongy cardiac muscle. This poses a problem during exercise, which both increases cardiac oxygen demand and decreases oxygen levels in venous blood returning to the heart. This research will test the hypothesis that selection for maintaining cardiac oxygen supply and performance led to two major innovations in the cardiovascular systems of vertebrates: 1) the evolution of lungs and 2) the persistence in terrestrial reptiles and amphibians of the intracardiac shunt, which permits mixing of oxygenated blood from the lungs with poorly oxygenated blood returning from the tissues. This research will establish the oxygen requirements of the hearts of lower vertebrates and determine the relative importance of lung ventilation, intracardiac shunting and coronary circulation as mechanisms to meet those requirements. If the central hypothesis is correct, it will lead to a new paradigm for the evolution of lungs and help explain the persistence of the complex cardiovascular systems of ectothermic vertebrates. The outcome may alter the traditional view that the cardiac anatomy of modern animals has resulted from selection for innovations that improve nutrient delivery to the systemic circulation, without recognition of the importance of nutrient delivery to the heart itself.
