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The first accurate measurements of body mass versus metabolic rate in 1932 shows that the metabolic rate R for all organisms follows exactly the 3/4 power-law of the body mass, i.e., R ![]() |
Figure 01 Metabolic Rate |
Figure 02 Small Size MR[view large image] |
encompassing principles in biology. But the law's universality is baffling: Why should so many species, with their variety of body plans, follow the same rules? |
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Then in 1997, a couple of physicist and biologists successfully derive the 3/4 power-law using the concept of fractal. The theory considers the fact that the tissues of large organisms have a supply problem. That is what blood systems in animals and vascular plants are all about: transporting materials to and from tissues. Small organisms don't face the problem to the same extent. A very small organism has such a large surface area compared to its volume that it can get all the oxygen it needs through its body wall. Even if it is multicellular, none of its cells are very far from the outside body wall. But a large organism has a transport problem because most of its cells are far away from the supplies they need. Insects literally pipe air into their tissues in a branching network of tubes called tracheae. Mammals have richly branched air tubes, but they are confined to special organs, the lungs. Fish do a similar thing with gills. Trees use their richly dividing branches to supply their leaves with water and pump sugars back from the leaves to the trunk. The 3/4-power law is derived in part from the assumption that mammalian distribution networks are "fractal like" (Figure 03) and in part from the |
Figure 03 Fractal in Nature |
conjecture that natural selection has tended to maximize metabolic capacity "by maintaining networks that occupy a fixed percentage (6 - 7%) of the volume of the body". |
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There is a more general 1/4 power-law applicable to many physicological variables y as shown in Figures 04, 05, and Table 01. The general form of the power-law is y ![]() |
Figure 04 Life Span |
Figure 05 Brain Mass |
rate. It is obvious that the various physicological variables are determined primarily by the dimensional dependence. The corresponding power-laws are the consequence of the relationship L ![]() |
physiological variables (y) | Dimension | scaling exponent (b) |
---|---|---|
Heart Beat Rate | -1 | -1/4 |
Period of Heart Beat§ | 1 | 1/4 |
Life Span¶ | 1 | 1/4 |
Diameter of Tree Trunks | 3 | 3/4 |
Diameter of Aortas | 3 | 3/4 |
Brain Mass | 3 | 3/4 |
Metabolic Rate | 3 | 3/4 |
Metabolic Rate (latest observations) | 4 | 1 |
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A report in the January 26, 2006 issue of Nature indicates that the 3/4 power law is not observed in plants. The new experiment involved 500 individual plants, across 43 species, from varying environments, and covering six orders of magnitude variation in plant mass. It is found that the slope for a plot with respiration (metabolic rate) against plant mass is close to 1 (see Figure 06). The differences in the intercepts between the indoor and outdoor groups disappears if plant nitrogen mass is used instead of the total plant mass. This new result can be explained by the theory if |
Figure 06 Plant Mass |
the metabolic rate of plants R is proportional to L4 by including the internal dimension as another metabolic site, so that R ![]() ![]() |
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where T is body temperature in kelvin, and the B's are constants to be evaluated by best fit to the observed data as shown in Figure 07 from which yields B0=14.0149, B1=0.5371, B2=0.0294, and B3=4799.0. The quadratic model takes care of the small curvatures at both the upper and lower ends. It is suggested that the quadratic correction could be related to the transition of large vessels with pulsating blood flow and small vessels with smooth blood flow. The correction can be visualized mathematically by taking the derivative (neglecting the temperature term): d(log10R)/d(log10M) = B1 + 2B2log10M which shows that the 3/4 exponential is replaced by B1+2B2log10M. The 3/4 law applies only to |
Figure 07 Power Law Deviation [view large image] |
organisms with mass around 1000 gm. For the rest of the organisms, the 3/4 slope is modified by the mass term. It implies that for some unknown reason, the metabolic rate varies slightly for living organisms with different mass "internal volume" notwithstanding. |