The pressure-volume relationship in the ascending aorta ("windkessel") of the hooded seal was determined and the morphology of its vasa vasorum described in some detail. We found that the ascending aorta has a high compliance and can easily accommodate the entire stroke volume when the peripheral vascular resistance becomes much increased and maintain perfusion pressure during the much extended diastole and thereby reduce cardiac stroke work during diving. We also found that the 3-5 mm thick wall of the ascending aorta had a very elaborate vasa vasorum interna with a hitherto undescribed vascular structure that penetrates the entire vascular wall. If similar structures with similar importance for the nutrition of the wall of the vessel are found in man, important implications for the understanding of pathological conditions, such as aneurisms, may be indicated.