Lung growth and aging

Form and function

Elastic similarity governs skeletal growthWe have limited ourselves to considering man, only one among the primates. So let us look at other primates. The siamang is a relatively small species, the gorilla a large one. The illustration depicts them at about the same size. One of the striking features is that the diameter of the long bones does not seem to be proportionate to the length of the long bones. The gorilla is much heavier than the siamang. If body weight weight were to increase as length cubed, and cross-sectional area of a bone in arm or leg as length squared, then the static load of the bone per unit of area would not be constant but increase linearly with body size. That would only work if the bone became structurally stronger. However, the skeleton is not only subject to static loads, it should also be able to cope with elastic stresses. Think of the forces that tend to bend and stretch, or create a torque, that occur during running and jumping, getting up from a kneeling or a squatting position, or are exerted during skiing. On that account the gorilla needs a more robust skeleton than a siamang. It turns out that in nature species of the same family are scaled in such a manner that they are capable of dealing with comparable elastic stresses. This can only be achieved if the relationship between the length and the square of diameter are constant across species. One can also apply this to the branching of trees. The derivation of this law of 'elastic similarity' is due to McMahon; it can e.g. be found in 'TA McMahon, JT Bonner - On Size and Life, Scientific American Library, New York, ISBN 0-7167-5000-7'.

Conclusion

Form and function must be adapted to static and elastic loads. The requirements are not the same for neonates, toddlers, children or adults within a species. Hence appropriate dimensional growth is an absolute requirement for optimal function of the individual, and hence for the survival of the species.

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