Snorkeling Elephants
What the overwhelming majority of the animal's numerous fans are unaware of is that the elephant is the only mammal whose pleural space, the potential space between the lung and the chest wall, is obliterated by connective issue.
What is the connection between these two unique elephantine attributes? Some recent studies have some scientists believing that elephants have an aquatic ancestry and these physical and behavioral characteristics are the result of evolution. This theory has not been proven.
The Presentation
There is, however, a physiological reason for the lack of space around an elephant's lungs that may support the evolutionary hypothesis of the animal's origin. Explaining why nature works the way is does is one of America's most renowned respiratory physiologists.
John B. West, of the University of California, San Diego's Department of Medicine. Dr. West is the author of "Why Doesn't the Elephant Have A Pleural Space," and "Snorkel Breathing in the Elephant Explains the Unique Anatomy of the Pleura." He will be discussing his research at the upcoming meeting of "The Power of Comparative Physiology: Evolution, Integration and Application" an American Physiological Society (APS) intersociety meeting being held August 24-28, 2002, at the Town & Country Hotel, San Diego, CA. For further information about the conference and the presentations, go to: http://www.the-aps.org/meetings/aps/san_diego/home.htm
In his presentation Dr. West will highlight the following:
· The anatomy of the elephant: Dr. West has attended three elephant autopsies and confirms that the animal's two pleural layers are firmly adherent although it is possible to separate them with some difficulty by pushing a finger or blunt instrument through the soft connective tissue joining the two layers. No satisfactory explanation has been advanced for the peculiar anatomy of the elephant pleural space.
· Snorkeling behavior in the elephant: Snorkeling at any substantial depth creates very large pressures around the animal's lung. In spite of these very large pressures around the lungs, alveolar pressure is essentially atmospheric, because the lung is connected to the atmosphere by the open proboscis, or trunk. The result is that the systemic vascular pressures are very high, whereas the pressure inside the thoracic cavity remains low. This inequality of pressures has dramatic effects at the interface between the lung and the rest of the body that is in the pleura. Other mammals such as humans cannot survive anything like this degree of ''negative pressure breathing'' (one reason why it is impossible to buy a snorkel longer than about 30 centimeters).
· Vulnerability of the pleura during snorkeling: During snorkeling, all of the elephant's body tissues including the head, neck, chest wall, abdomen and limbs are exposed to a high pressure because of immersion in the water. The only exception is the lung, because it is connected to the air by a tube. Consequently, a region just outside the lung has a large pressure differential. This is the reason why the pleura is so vulnerable. The structure apparently at greatest risk is the parietal pleura, which lines the thoracic cage and diaphragm. Since the venous pressure in the systemic system must be very high in the snorkeling elephant (150 mmHg if the tissue is 2 meters below the surface), the microvessels of the parietal pleura must have an even higher pressure inside them.
The problem is highlighted by the small blood vessels in the elephant's parietal pleura. In a normal mammal, such as a sheep, there is a single layer of mesothelial cells, and the microvessels are very close to the pleural surface. The transudate or fluid from these microvessels enters lacunae and is then discharged into the pleural space, thus providing the lubricant for the two pleural surfaces so that they can slide over each other.
It is easy to see that this anatomic arrangement would be impossible in the snorkeling elephant. The microvessels of the parietal pleura are supplied from the systemic circulation, in which the venous pressure exceeds 150 mmHg Therefore, the pressure inside the pleural microvessels must exceed that. However, the pressure in the pleural space (if one exists) will be very close to alveolar pressure (that is, atmospheric), only differing from this by the elastic recoil of the lung. In other words, the microvessels of the parietal pleura will have a transmural pressure approaching 150 mmHg. Clearly, they would either rupture or the great imbalance would cause an excess flow of fluid.
· How evolution answered the problem: Nature's answer was to replace the delicate pleura with dense connective tissue. The visceral pleura is also at risk and thickened. A layer of loose connective tissue allows some sliding of the two pleural surfaces. With this anatomic arrangement, the normal production of pleural fluid to lubricate the surfaces no longer exists. However, in the elephant, the layer of loose connective tissue between the two dense connective tissue plates is very extensible; therefore, some sliding of the two pleural surfaces across each other can occur.
Conclusions
As a result of his work, Dr. West has made a number of observations. Among them is the fact that the primary evolutionary response to the vulnerability of the pleural membranes is to replace these with plates of dense connective tissue. The obliteration of the pleural space by loose connective tissue, which is the most obvious anatomic peculiarity appears to be a secondary response brought about because the normal mechanism for providing lubricating fluid for the pleural membranes no longer exists, and there is apparently some advantage in allowing the membranes to slide over each other.
He also notes that the mechanical problem facing the elephant's diaphragm during snorkeling. This is because the pressure difference across the diaphragm is about 150 mmHg, and this has to be sustained for many minutes while the animal is walking under the surface of a river or lake. This is a far greater trans-diaphragmatic pressure than can be sustained by the human diaphragm.
There are relative pressure changes that occur when the elephant raises water in the trunk for drinking or washing. For example, if f the water is raised through 200 cm, the alveolar pressure must be 200 cm H2O below atmospheric pressure, and the relative pressures are essentially identical to those during snorkeling. Since the elephant can raise water in its trunk with its mouth open, it is known that the low pressure is not developed by the buccal (near its cheek) muscles. It could be argued that this behavior, which is frequent in the elephant, is an additional evolutionary pressure for the anatomic changes in the pleural space. However, raising water in the trunk only takes a few seconds, whereas snorkeling lasts for many minutes and is therefore a much greater potential problem. As indicated earlier, the snorkeling behavior may have developed when the animal lived in water, and this would provide a very strong evolutionary pressure.
Finally, Dr. West notes that many evolutionary biologists believe that the elephant's ancestors were aquatic, i.e., lived in the water, and the trunk may have developed at that time. There is evidence that the elephant's closest living relatives are the manatee and dngong.
The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions by which animals live, and thus ultimately underlie human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.
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