Diving mammals have an elastic aortic bulb thought to help maintain arterial pressure during the extended intervals between heartbeats during dives, and have high blood volume, combined with large storage capacity in veins and retes of the thorax and head in seals and dolphins. Chronic physiological adaptations of blood include elevated hematocrit, hemoglobin, and myoglobin levels which enable greater oxygen storage and delivery to essential organs during a dive. Oxygen use is minimised during the diving reflex by energy-efficient swimming or gliding behaviour, and regulation of metabolism, heart rate, and peripheral vasoconstriction.