Observations on the external morphology and vasculature of a fetal heart of the bowhead whale, Balaena mysticetus.

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Specialized demands within the aquatic environment for over some 60 million years have shaped unique morphological expressions in the whales, dolphins, and porpoises (Cetacea). Detailed consideration of these features, particularly in the great whales, has often been constrained by difficulties in securing adequate specimens for study. We had the opportunity to examine external heart morphology in a rarely obtained and prepared specimen from the bowhead whale, Balaena mysticetus. The external morphology and in situ relations of a formalin-perfused heart were examined grossly in a near-term bowhead fetus. Latex injections assisted visualization of coronary vasculature. Magnetic resonance imaging was used to clarify heart positioning within the thoracic cavity in two younger (early and mid-gestational) intact fetuses. The heart was globular in form, with a blunt apex and wide base; it was laterally broad relative to height yet considerably compressed between nearly planar atrial (diaphragmatic) and auricular (sternocostal) surfaces. The heart constituted 0.01 of body mass in the near-term fetus. Within the thoracic cavity, the heart tilted forward on its long axis, placing the great basal vessels in the region of the thoracic inlet. The aorta extended forward from mid-base in parallel with the pulmonary trunk, arched sharply to the left, producing in succession the brachiocephalic trunk, left common carotid artery, and the left subclavian artery. Bifurcation of the brachiocephalic trunk yielded the right common carotid and right subclavian arteries. The distal portion of the aortic arch was linked to the pulmonary trunk via the ductus arteriosus. The aorta then swung caudally over the heart base, descending beneath the bodies of the thoracic vertebrae. The ascending aorta featured three bulbous sinuses immediately distal to the three semilunar cusps of the aortic valve. Originating along the distal boundaries of the left and right sinuses were the left and right coronary arteries. The arteries were similar in size and, because each sent contributions along their respective coronary and interventricular grooves, the heart can be described as bilateral relative to arterial supply. Anastomoses were common within and between the two arteries. Venous return from the heart was comprised of the great, middle, and right cardiac veins, all three converging in the coronary sinus. The right cardiac vein also included tributaries that emptied directly into the right atrium. External heart morphology in the fetal bowhead whale examined was distinguished by a laterally broad conformation with significant compression between its cranial and caudal surfaces. Aortic bulb configuration in combination with an expandable aortic arch may support blood service to the heart during diastole. Vascular service to the heart featured a complex vessel network with extensive intraarterial and intravenous anastomoses that enable many alternate blood perfusion pathways and may be adaptive to water-column-pressure fluctuations experienced by a large diving mammal.