Marine vertebrates have many adaptations to their way of life. One of the most interesting is the ability of many species to dive to great depths and stay down for quite extensive periods. Sea lions and humpback whales, for example, often plunge to depths of several hundred yards. Whale-watchers around Juneau know that when a humpback shows its flukes, it is likely to stay down for some time, perhaps 20 minutes or more. Remarkably, sperm whales can dive down an astounding two miles and stay under for well over an hour.
Humans (unassisted) can’t do anything remotely like that. One big problem is having enough oxygen, the “fuel” that is needed for aerobic metabolism (when the fuel limit is exceeded, metabolism is anaerobic, which builds up lactic acid in the muscles, among other things, leading to muscle cramps, as athletes well know). We are a lot smaller than most deep-diving aquatic animals and have correspondingly less capacity to store oxygen. We, and other terrestrial animals, also have smaller volumes of blood relative to our body size and smaller amounts of oxygen-carrying proteins in blood and muscle. Marine divers typically store a lot of oxygen in their muscles, much more than humans can. Myoglobin in the muscles latches onto oxygen much as hemoglobin does in the blood, and the stored oxygen is available for aerobic metabolism, as needed.
A fascinating physiological change happens when an air-breathing vertebrate dives. Although it is often called the “mammalian diving reflex,” it occurs in diving birds too. Humans share the diving reflex with other vertebrates, but it is less well developed than in vertebrates adapted for aquatic life.
As soon as the animals’ face is immersed in cold water, the heart beat slows down. In some aquatic animals, the heart beats at only 10 to 20 percent of its normal rate. Patterns of blood circulation change: although normal blood flow to the essential organs (heart and brain) is maintained, the circulation to other organs and limbs is reduced. Both the slower heart rate and the circulation changes help conserve oxygen.
Thus, the marine divers can both store more oxygen than humans can and also use it more frugally.
There is another problem associated with deep diving by air-breathing animals. When we take a lungful of air, that air is composed of about 78 percent nitrogen, 21 percent oxygen and tiny amounts of other gases. When we dive deep, the great pressure of the water forces gas from the lungs into solution. Oxygen gets used up during a dive, but nitrogen is not used by the body, so it remains in the tissues at high concentrations. Then if we ascend and de-compress too quickly, that gas comes out of solution as bubbles in blood and other tissues. During de-compression, nitrogen bubbles can give us the painful and potentially lethal condition known as “the bends.” That name comes from the common symptom of intense pain in the joints. (It’s a serious potential threat for scuba divers.)
But deep diving marine animals don’t seem to get the bends, or so it has been thought. Why not?
In some cases, the lungs of a deep diver partially, or even almost totally, collapse, so less air is taken down to the depths, and there’s less gas available to be forced into the tissues. But that is not always the case, apparently, leaving me to puzzle over what happens in the divers that do not collapse their lungs.
What happens to nitrogen gas in these aquatic deep-divers? Don’t they get bubbles in their blood vessels or other tissues? According to biologists that study marine mammals, there are indications that whales and sea lions (and perhaps even some extinct marine divers) may indeed suffer some symptoms of decompression sickness or the bends. But the extent of the problem and just how they deal with nitrogen seems to be unknown.
• Mary F. Willson is a retired professor of ecology.