Deep Diving Physiology in Dolphins

Earth Day is dedicated to deepening understanding of the earth, and the creatures that live on it. By understanding the various ways life has adapted to different conditions, we gain an appreciation for life in all its forms and increase our resolve to preserve habitats for all forms of life. One unique adaption has allowed mammals, most commonly land residents, to live in the sea.

 Orcas (killer whales) are in the dolphin family, Delphinidae.

­­Dolphins are marine mammals and as such are perfectly adapted, both anatomically and physiologically, to life in the sea. Before we go any further, let’s make sure we understand each other by defining a few terms. A marine organism is one that lives in the sea. According to New Webster’s Medical Dictionary, anatomy is “the structure or study of the body”, and physiology is “the science of dealing with the normal functions of living organisms or their organs”. To put it more simply, anatomy and physiology collectively mean how a creature is “put together” and how it “works”.

Since dolphins and whales are mammals, they must breathe air. Since they live in the sea, they must swim and dive. Most of the dolphin’s adaptations to the marine environment are related to allowing them to swim and dive for extended periods, yet 

^ typical dolphin anatomy, click for larger image

breathe using lungs like those of humans instead of using gills like those of fish.

The marine environment is much different than that of the land, so to understand how dolphins have adapted to sea life we must first understand some of the unique problems that they face. One of the major problems involves the relationship between gas exchange and pressure. As one descends into the ocean depths, the pressure exerted on the body by the surrounding water increases. This increased pressure on the lungs causes the gases in the lungs, namely oxygen and nitrogen, to leave the lungs and enter the bloodstream. If much time is spent at depth, a large amount of nitrogen may be dissolved into the blood. Upon resurfacing, the pressure decreases and the dissolved nitrogen may expand enough to form bubbles in the blood and other tissues. This is similar to the formation of carbon dioxide bubbles inside of a bottle of soda pop when the cap is removed, and the pressure inside the bottle is suddenly decreased. The accumulation of such nitrogen bubbles in the joints causes a painful disease known as “the bends” in humans. Since dolphins dive frequently and for extended periods, why don’t they get the bends? Let’s consider another problem that diving mammals have and since the adaptions for bother are inter-related, we will find how nature has provided for the dolphin.

When humans take a breath-hold dive, they can stay underwater for 3 to 5 minutes, depending mainly on how much oxygen they have in their lungs. The lungs of a dolphin, relative to its body size, are no larger than our own. How then can a dolphin remain submerged for such long periods?

Among the numerous adaptations of the dolphin for life in the sea, the most important ones for oxygen conservation are: 1) lung collapse, 2) circulatory redistribution, 3) modified oxygen – storage mechanism, and 4) resistance of the tissues to lactic acid build-up.

The lungs of the dolphin are made to collapse under pressure. Some of the ribs are even jointed to assist in this collapse. When the pressure is great enough, the lungs collapse to the point where exchange ceases. With no gas exchange, excess nitrogen cannot enter the bloodstream and the bends cannot occur.

To reduce the use of oxygen during a dive, the circulatory system has become adapted to supply oxygen only to tissues that need it most, namely the heart and the brain. When a dolphin dives, the heart slows way down and blood flow is restricted to the heart and the brain. The muscles and other organs receive only a little blood.

The method by which oxygen is stored is quite different with the dolphins, too. “Hemoglobin”, a pigment in our blood, carries oxygen from our lungs to the tissues, with a small amount being 


 

stored in "myoglobin”, another pigment in our muscles. Dolphins, however, have more hemoglobin than we do and much more 
myoglobin, so the muscles can 
get plenty of oxygen during periods of reduced blood flow.

When muscles operate with oxygen for a while, they build up a waste product called “lactic acid”. Everyone knows that burning sensation you get in your muscles from working out too hard. This is caused by working the muscle so hard that the stored oxygen is depleted and lactic acid accumulates. Due to the circulatory redistribution described above, the muscles must operate for prolonged periods with little blood circulation. Their muscles have evolved with a biochemical resistance to such side effects as burning and cramping due to over-accumulation of lactic acid.

Respiratory specializations such as those listed above have enabled the dolphin, and air-breather, to spend very little time at the surface breathing, and spend the greater majority of its time swimming in the ocean’s dept