ENHS Nature Trails November 99 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Depths of joy: To dive like a seal

Humans have such interesting fantasies. Some people wish they could fly like a bird. Others wish they could be a swift as a cheetah. Still others want to swim like a fish. I have another fantasy - to dive like a seal.
Seals and other marine mammals are fascinating creatures. They "soar" through he water like birds on a thermal. They can hold their breath for several hours and dive to incredible depths. The deepest recorded dive is 1539 meters recorded for the northern elephant seal. [That's over 5000 feet deep in English units, almost a mile. ed.] The pressure at that depth is an astounding 150 atmospheres (150 times the pressure at sea level). How can seals and whales tolerate deep dives and rapid ascents?
Diving is ecstatically addictive - the more you do it, the more you want to do it again. And again. And again. But there is an inherent and serious risk associated with diving, the medical syndrome variously known as the bends, diver's disease, caisson disease and aeroembolism. It occurs when a diver returns to the surface after a prolonged dive at depths below 20 meters. The bends is caused by the appearance in tissues and in the blood of gas bubbles which form because the blood holds less dissolved gas at lower pressures. The diver goes to a depth, say 20 meters, where the atmospheric pressure is higher and more gases are absorbed into the bloodstream. Upon returning to the surface, the diver faces a reduction in pressure which forces those dissolved gases to come out of the blood as bubbles and it is those bubbles that cause the bends. This phenomenon is the same as that occurring when bubbles appear in a bottle of soda water when the cap is opened. In both cases gas comes out of solution as the pressure of the solution is decreased. In the case of the soda water, the dissolved gas is carbon dioxide. In humans it is nitrogen bubbles that cause the bends.
Bubbles often form in joints where they are quite painful (hence the name of "the bends"). If they form in the bloodstream they block small blood vessels, reducing the flow of oxygen to critical tissues. This is particularly dangerous in the central nervous system, and can result in death if not properly treated. The only treatment for the bends is to increase the pressure quickly so as to force the nitrogen gas bubbles to dissolve back into the blood. This is accomplished either by returning the diver to the original depth and slowing down the re- ascent, or by placing the diver into a pressure chamber and slowly reducing the pressure back to normal.
Humans get the bends after a prolonged stay at a depth of 20 meters. How can marine mammals tolerate dives to 1500+ meters without getting the bends?
One reason is that marine mammals, unlike humans, exhale before a dive and thus have little air to be forced into tissues and the blood during a dive. Recent work on the lungs of a Weddell seal has shown that they are completely collapsed at a depth of about 25 meters so that no air can enter the blood. Another reason is that the volume of the lungs is small compared to that of the upper respiratory airways. During a dive, the air remaining in the lungs of a whale is forced into the upper airways which is rigidly reinforced to prevent collapse at diving depths. With no air in the lungs, the is no possibility of nitrogen entering the bloodstream. A third reason is that the blood flow to the lungs is dramatically reduced during a dive; thus, even if the lungs contained air before it collapsed, little nitrogen enters the blood.
Another explanation for the amazing diving ability of marine mammals is their high oxygen storage capacity in the blood. Humans take a deep breath when they dive to increase the amount of oxygen in their body. However, seals and whales, who can stay under water for up to 3 hours without surfacing, exhale before they dive. Where is their oxygen source? Recent studies have demonstrated that seals have twice the blood volume of humans per kilogram of body weight. Moreover, seal blood has a much higher oxygen carrying capacity than human blood, double that of humans. Another source of oxygen in seals is hemoglobin in their muscles. Seals have up to 8 times the levels of muscle hemoglobin than humans. The various sources of oxygen contribute to seals having 20 fold more oxygen than humans.
Yet another adaptation that allows marine mammals to dive so deep and so long is their ability to re-direct blood flow and reduce metabolism. They are able to selectively direct blood flow only to essential organ systems (brain and muscles) while simultaneously reducing blood flow to other tissues. This plus a reduction in overall metabolism enables them to slow the utilization of their oxygen stores and increase their ability to stay under water.
Despite our current level of understanding about the physiology of diving in marine mammals, we still do not have a complete explanation for this amazing ability. Recent work using telemetry tags to track individual elephant seals has shown that most elephant seals, rather than being surface dwellers that occasionally dive, spend most of their time at depth and only periodically return to the surface to breathe. One elephant seal was recorded as diving to 900 meters for over 2 hours and then returning for only 2 minutes to the surface before diving again for another 2.5 hrs. Another seal went for 40 days with no surface period longer than 6 minutes. Ah, to be a seal!!

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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