Aerobic Capacities of Freediving Mammals
Posted By Erik Seedhouse on 31 March 2003
So how does a seal store the oxygen? The answer can be found in the blood and tissues.
A seal's blood has a higher oxygen-carrying capacity than a human's partly because a seal has a greater blood volume and partly because of a higher hematocrit (concentration of hemoglobin). Because there is more blood in a seal there are also more red blood cells (RBCs). The increased number of RBC's increases the amount of hemoglobin, a blood pigment found in RBC's that carries oxygen. However, a seal's RBC is composed of less water than a terrestrial mammal's, so even at the cellular lever, this mammal is designed to carry more oxygen -- this accounts for the higher hematocrit. Of course the amount of RBC's the blood can carry is limited since we know that if there are too many RBC's the blood becomes too viscous for the heart to pump effectively. However, marine mammals overcome this be resorting to accessory methods for storing oxygen at their disposal.
One of these methods is myoglobin, a compound found in muscle tissue that binds oxygen. In fact, the myoglobin is so highly concentrated in seal muscle that when viewed under a microscope it appears almost black! Humans also have myoglobin but sadly -- for freedivers - its storage capability is far inferior to a seals.
Myoglobin concentrations of marine mammals
|
Species |
Myoglobin (g 100 g-1) |
|
Northern fur seal |
3.5 |
|
Sperm whale |
5.0 |
|
Weddell seal |
5.4 |
|
Ribbon seal |
8.1 |
Finally, marine mammals are able to store more oxygen in other tissues of the body than humans do, thus giving them the ability to store more oxygen, most notably, the spleen. The mechanism by which the spleen stores oxygen is similar to those found in humans and was explained in the previous artcicle 'Spotlight on the Spleen'. However, the splenic capacity of marine mammals far exceeds that of a human.
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