For elephants, its not just their ears and trunk that make them uniq...( e and diaphragm. Since the venous pres...)

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. '"/>

Contact: Donna Krupa
djkrupa1@aol.com
703-967-2751
American Physiological Society
26-Aug-2002