The principles of energetics are fundamental concepts in physiology, just as they are in other natural sciences. At the whole body level, energetics are important in exercise physiology and in maintenance of body weight. At the cellular level, they define the conditions under which biochemical reactions can occur and the possible results of those reactions.
The first law is very simple but also very powerful. The recognition that energy is conserved, that it is neither created nor destroyed, allows the basic principles of arithmetic to be applied to the flow of energy through a system. The amount of energy at the end of a sequence of events must equal the amount at the beginning. The sums must be equal. If they aren't, an error has been made in the analysis.
While the first law deals with quantity, the second law describes the changing quality of energy as it moves through a system. Unlike quantity, the quality of energy tends to change, but only in a certain direction. The second law defines the nature of the change (although it doesn't specify how fast the change will occur).
The figure to the right shows an extremely simple, but fundamental description of the second law. Initially, we have molecules in the left side of a system comprised of two interconnected compartments. Since these molecules possess temperature and heat, they have random, thermal motions. As time passes, initially there is a net movement of molecules from left to right. But soon the number of molecules is approximately the same on both sides, and there is no longer any net movement. A more ordered system has thus collapsed to a less ordered system with time. A more technical, equivalent term for this process is that the entropy of the system increases with time.
Now let' observe how the second law works in a steam engine. High temperature water molecules are initially only on the left side. If a turbine is placed at the opening between the two compartments, the turbine is turned by the net flux of molecules from left to right. Electricity is generated as the system collapses to a state where the temperature is similar on the two sides, which is a more disordered state with higher entropy.
The energetic principles underlying the biochemical reactions in the body are more complex examples built up from the exact principle in this simple example. If during a biochemical reaction there is a net increase in entropy, then the reaction can occur and moreover is capable of powering work or synthesis. Such a reaction is said to have a negative free energy change. At a constant temperature, as in our body, the biggest factor causing a negative free energy change is almost always a net release of heat during the reaction. (But also, a transition to more, smaller molecules likewise adds to the overall increase in entropy of the system. But this is usually small relative the heat factor.)
Let's take the reaction in which glucose combines with oxygen to produce carbon dioxide and water. This reaction can power work and synthesis in our body because there is a net increase in entropy during the reaction due to both the release of heat and the breakdown of glucose to more, smaller molecules.
Above, we were looking at a single reaction. However, the two laws apply equally well to a complex set of coupled reactions. How do we determine the energetic changes for the entire, coupled system? It is a question of simply adding up the energetic changes of all the individual coupled reactions occuring in the system. Thus, viewing the human body as a system, both the first and second law apply: The energy flowing out of your body in the end must be equal to the energy flowing in. There must be a net increase in entropy or disorder during the flow of that energy through the system.
The figure to the right traces the flow of energy through the body. Energy enters as the carbohydrates, fats and proteins in our diet. As it flows through the body, chemical synthesis and work are possible. In the end, energy leaves the body in the disordered form of heat at body temperature. While this is going on, there may or may not be a net change in the amount of energy stored within your body.
Notice that individual changes are possible in which entropy decreases. However, the whole coupled system still proceeds because overall there is a net increase in entropy when all the individual changes are added.
An important and useful physiological measurement is the rate of energy expenditure, which is the rate at which carbohydrates, fats and proteins are broken down. How can we determine this?
The first law immediately suggests the first way, which is the direct measurement. Since over a short period of time there is unlikely to be a significant gain or loss in stored energy, the first law tells us that the rate at which heat is being formed necessarily is the rate of the energy expenditure. Thus, the direct measurement uses the rate at which heat is formed.
In physiology, the units for energy expenditure are often given in kilocalories per unit time. One kilocalorie is the amount of heat energy required to heat one kilogram of water one degree Celsius. It is also equal to a dietary calorie, which is the "calorie" used in labelling the energy content of foods.
Despite the frequent use of the kilocalorie in physiology, the proper energy unit in the international system is the kilojoule, which is equal to 0.24 kilocalories. We really should be using this, but the kilocalorie is entrenched and also helpful intuitively since it is equal to the familiar dietary calorie.
While the direct method follows directly from the laws of energetics, in practice it requires expensive, cumbersome equipment. It is rarely practical.
This is why almost always the indirect method is used. Here the oxygen consumption is measured and used to estimate the energy expenditure. This is a very practical approach, which quickly provides fundamental physiological information.
To see why this approach works, imagine that the body were only using glucose for energy. Each time one glucose molecule is converted to carbon dioxide and water, exactly six oxygen molecules are used. Thus, if only glucose were being used for energy, we could precisely convert oxygen consumption to glucose burnt and thus to energy expenditure. Of course, we are also using fats and protein for energy, and they do use somewhat different amounts of oxygen for each kilocalorie of energy expended. Nonetheless, in practice a single number is sufficiently accurate for most purposes.
The number physiologists use to convert oxygen consumption to kilocalories is 4.8 kcal/l O2. This is an approximation since it assumes a typical mix of carbohydrates, fats and proteins. But it is accurate enough within the context of most physiological measurements.
Another energy unit is the metabolic equivalent (MET). This is useful, since it takes into account the size of the subject. One MET is defined to be 1.0 kcal/hr per kg. A person expends, for example, about 1.0 MET watching TV. In lab, we will determine some MET values for certain activities. Clinical studies looking at the effects of exercise typically express levels of exercise in terms of METs
Suppose you weigh 60 kg (120 lb). How many kilocalories per hour do you use watching TV?
You look in a table of MET values and find juggling listed as 4.0 METs. How many kilocalories do you use when you juggle one hour?
I am looking at the back of an energy bar and it says that it contains 220 calories. Working absolutely from scratch, how could you determine this number?
Suppose you ate the energy bar. What additional information would you need in order to know how long this energy bar would supply all of the energy consumed while you juggled?
For brief periods, ATP can be generated without use of oxygen in mitochondria. But when an activity extends beyond a few minutes, the rate at which ATP is generated is directly proportional to the use of oxygen. Thus, the maximum rate at which oxygen is consumed, which is termed the VO2max, defines the maximum rate of energy expenditure for an activity lasting more than a few minutes. In practical terms, the VO2max is measured as the subject gradually increases his or her energy expenditure to the maximum over about a 15 minute period.
A person's VO2max depends on size, genetics and level of training for endurance exercise. For example, a 70 kg, fit college student might have a maximum oxygen uptake of 3.3 l/min. But for an elite athlete in an endurance sport, such as bicycle racing, the VO2max might be 5.3 l/min or even higher.
But the VO2max in not just measured in endurance sports. It can also be an important clinical measurement. Indeed, a recent clinical study with older patients found that VO2max is one of the best predictors of mortality, especially if cardiovascular disease is present. We will get into this further in lab.
On the webpage for a famous bicycle racer, his VO2max is listed as 80 ml/min per kg. You find your VO2max is 3.1 l/min. Suppose you weigh 70 kg. How much do you need to improve in order to have a shot at winning the Tour de France?
When oxygen consumption is measured to determine the energy expenditure, usually the release carbon dioxide is measured at the same time. The ratio of these two numbers is termed the respiratory quotient (RQ):
If only glucose were being used for energy, the RQ would be 1.0. On the other hand, if only fat were being used, the RQ would be 0.7. The RQ for protein falls somewhere in between. In practice, a mixture of the three types of foods is always being used, so that the RQ usually falls between about 0.8 and 0.9.
Notice that if you assume a value for the RQ, you could use the release of carbon dioxide to determine the oxygen consumption and thus energy expenditure. This provides the principle behind a third means of measuring the energy expenditure, which is the doubly labeled water technique.
The subject is given water in which rare isotopes of both oxygen and hydrogen are included. Both isotopes are not radioactive. Soon the labelled water is intermixed with the rest of the water in the body. Then the rate at which both isotopes disappear from the body is followed.
The rate at which the hydrogen leaves the body shows the rate of loss of water in the urine, since the hydrogen necessary remains part of the water molecule. But the labelled oxygen leaves the body more rapidly because it can also leave the body as carbon dioxide. Thus the difference in rate between the loss of labelled oxygen and hydrogen allows a calculation of the rate at which carbon dioxide is being produced and exhaled from the body. This provides a measurement of energy expenditure that extends over a week or so.
Why can oxygen in water rapidly interchange with that in carbon dioxide? This is not intuitive, but turns out to happen continuously and very rapidly because of the following reversible reaction taking place in the red blood cells:
While this reaction on its own goes to equilibrium in a few seconds, in red blood cells it happens even faster due to an abundant enzyme, carbonic anhydrase. This is perhaps the fastest enzyme in the body. This reaction often comes up in physiology.