In From Cells to Cities Part 1 Geoffrey West discussed Biological Scaling and therein the nature of scaling, the phenomenon of space-filling and fractals.
Part 1 is prerequisite reading to appreciate the context and continuity of the discussion as well as the nature of Geoffrey West’s scientific argument.
From Cells to Cities Part 2 below Geoffrey West discusses: power laws relating to biological scaling, heart – rates, and your expected maximal life span.
Part 2 – Life Span
Power laws relating to Biological scaling
There is one number that runs through this phenomenon of biological scaling. All the living systems scale through the 1/4 power. Fundamentally the number 4 permeates all these scaling laws. For example, 3/4 the metabolic rate, 1/4 for time scales and for lengths it is also very similar. It’s certainly no accident that we have a 25% saving of energy. That’s what comes out of the theory of these mathematical principles of network design. However ‘4’ in this context is not actually 4! Rather it is 3 +1. 3 signifies that we live in 3 dimensions (up, down and sideways) and the 1 is the reflection of the fractality of systems.
Fractals have a peculiar sense of dimensions. A fully fractal system adds an extra dimension. See the Koch curve. (The Koch snowflake (also known as the Koch curve, Koch star, or Koch island) is a mathematical curve and one of the earliest fractal curves to have been described).
So these give rise to the scaling laws. It provides a theoretical mathematised framework for asking all sorts of other questions such as Why do we age, why do we die..why do we sleep, where does 8 hours of sleep a night come from?
The general average lifespan of mammals is 1.5 billion heartbeats. The mouse has the same number of heartbeats as a blue whale. This comes out of the theoretical framework of biological scaling and is strongly supported by the data that heart-rates decrease in a systematic way according to these 1/4 power laws. And lifespan increases with the 1/4 power scaling law. And when you multiply lifespan by heart-rate, the increase in one namely lifespan is cancelled-out by the decrease in the other. So that lifespan x heart-rate should be the same for everybody which says that even if a mouse lives 2 or 3 years and a whale 150 years they would still have roughly the same number of heartbeats in their life.
At about the middle of the 19th century the average lifespan of a human was somewhere between 35 – 40 years. That corresponds to about 1.5 billion heartbeats. Since the industrial revolution we have been through an extraordinary phase where our lifespan has extended to about twice of what it was 150 years ago. In the western world we now have equivalent to 2.5 billion heart beats. This life extension is a reflection of urbanization, socio-economic and materialism. It begins with introduction of running clean water and sewerage treatment which had a profound effect on longevity. People would go many months and possibly years without cleaning themselves. Since then governments have provided access to health and attention to disease as well as the development of antibiotics.
What is the maximal lifespan you could expect?
What is the system that is keeping us alive? Our metabolic system has built into it dissipating forces. For want of a better word, wear and tear. There is continued damage being done by the flow of blood through the circulatory system. The blood flowing through your capillaries can be quite destructive. It’s like pushing fluid through very thin tubes and there is a great deal of resistance such as scraping between the blood and the walls. So this scraping is really what’s called entropy (2nd law of thermodynamics) which in turn leads to cellular damage. So as already discussed we have the scaling laws theory and you can calculate that the maximal lifespan should scale with 1/4 power scaling and hence gives us a rough estimate of longevity. Essentially the parameters associated with lifespan point to metabolism; not surprisingly because metabolism is that which keeps you alive. However, we also have the physical deterioration of material (wear and tear) which is occurring at the molecular level. And even something outside of that which is the process of repair; that we repair damage. Where does that repair come from? It has its origins in metabolism as well, because you have to supply metabolic energy to clean out damaged cells and regenerate new ones (Autophagy) and avoid premature death.
So you could ask yourself how do you extend lifespan from this picture?
Well, you have to reduce damage and simultaneously increase repair. If you like, it’s analogous to an engineering problem when maintaining complex machinery.
One way to reduce damage is to reduce your metabolic rate. How do you do that? You could eat less. That’s called caloric restriction. Many experiments have been done on mice in particular; extending their lifespan by feeding them less. However it still remains inconclusive because of some ‘controversial’ contrarian results when testing the same on monkeys. Moreover, there still needs to be a lot more research done but certainly the theory demonstrates a lot which is correct about ageing.
Another way of decreasing metabolic rate and just about every other organism can do this; except for us humans is to lower its body temperature. So if you lower the temperature, you lower the metabolic rate because metabolism is derived from chemical reactions. And chemical reaction theory tells you how things slow down when you decrease the temperature. Temperature is a reflection of the interaction among molecules and if you lower the temperature there is less interaction among the molecules and the metabolism goes down even exponentially. So a small change in body temperature produces a large change in our chemical reaction and metabolic rates. However, that is very difficult for us to do. Unlike us, most mammals are cold blooded. There have been experiments on mice which lowered their body temperature and it was shown it did increase their longevity.
So there is lots of supporting evidence that decreasing metabolism and lowering blood temperature increases lifespan. But goodness knows what consequences all this would have. Do we want to live so long yet be couch potatoes? No we don’t. We want to have healthy, lively and passionate lives right to the end. You don’t want to just extend it for the sake of extending it. The other way of increasing lifespan is to increase repair mechanisms. One could imagine genetic intervention to increase repair mechanisms. Interestingly there are animals which have repair mechanisms that we don’t have which attack specific tumors. But one of the unintended consequences of concentrating on genetic intervention to increase repair mechanisms is that we could become tired and exhausted all the time. So again it’s a lifestyle question.