Anatomy of the sinus node, av node, and his bundle of the heart of the sperm whale (Physeter macrocephalus), with a note on the absence of an os cordis

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Abstract
Background: Atrioventricular (AV) conduction time in large whales is only slightly greater than in smaller mammals even though their hearts are enormously larger. Little is known of the detailed histology or cytology of the conduction system of large whales. Such knowledge could be useful in defining the nature of cardiac rhythm and conduction of the whale as well as smaller mammals including humans. Methods: We studied hearts from seven sperm whales. After fixation in formaldehyde and later dissection, specimens were prepared for histological examination. Results: Cell size, histological organization, and innervation of the sperm whale's sinus node, AV node, and His bundle are similar to most mammalian hearts, except the sinus node is substantially larger. There is no central fibrous body between the atrial and ventricular septa, and the whale has no os cordis. Only the upper quarter of the interventricular septum is fully formed; below that there is only a thin layer of fatty connective tissue between the two ventricles. Conclusions: Given our morphological findings, we believe that the whale's comparatively short AV conduction time may be best explained by the sinus node and AV node functioning as coupled relaxation oscillators. Absence of an os cordis or central fibrous body or strong attachment between the two ventricles may pose both electrophysiological and hemodynamic hazards when the whale is no longer in its normally buoyant aquatic environment.