Bodily Surface and Volume in an Organism

Store Is Closed

Bodily Surface and Volume in an Organism

Jul 25 2011 5:00:00 pm EST

Topics: Evolution, Morphology, Science,

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D’Arcy Wentworth Thompson notes that there is a tendency in all organisms for bodily surface to keep pace with volume.

Multicellular organisms, like unicellular organisms, require a constant supply of nutrients and various gasses. A unicellular organism may absorb the various nutrients and respiratory gases directly from their surrounding environment. However, as the sheer quantity of cells increases in an organism, the distance between the environment and the cells increases, and various organ systems are required to transport nutrients and gasses.

The circulatory system utilizes fractal branching structures to maximize surface area for distribution of these gases and nutrients. Air sacs and alveoli greatly increase the respiratory surface of the lung. The long tract of the intestine is itself lined with small villi to maximize nutrient absorption. These systems each utilize some method to increase surface area to keep pace with volume. Consider the following passage from “On Growth and Form (56):

A simpler phenomenon, and one which is visible throughout the whole field of morphology, is the tendency (referable doubtless in each case to some definite physical cause) for mere bodily surface to keep pace with volume, through some alteration of its form. The development of villi on the lining of the intestine (which increase its surface much as we enlarge the effective surface of a bath-towel), the various valvular folds of the intestinal lining, including the remarkable “spiral valve” of the shark’s gut, the lobulation of the kidney in large animals, the vast increase of respiratory surface in the air-sacs and alveoli of the lung, the development of gills in the larger Crustacea and worms though the general surface of the body suffices for respiration in the smaller species—all these and many more are cases in which a more or less constant ratio tends to be maintained between mass and surface, which ratio would have been more and more departed from with increasing size, had it not been for such alteration of surface-form. A leafy wood, a grassy sward, a piece of sponge, a reef of coral, are all instances of a like phenomenon. In fact, a deal of evolution is involved in keeping due balance between surface and mass as growth goes on.

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