[Energetics of the human body]

Nature, as we know it, obeys the Laws of thermodynamics. The investigation into the energetics of the human body is an application of these laws to the human biological system. The First Law of thermodynamics, which has been verified many times in experiments on the human body, expresses the constraints of the conservation of energy and the equivalence between work and heat. It considers any energy change as equally possible, not in the least taking into account the irreversibility of a given process. The implications of the Second Law of thermodynamics, on the other hand, have never been examined in detail on the human body. This Law defines the direction in which an energy transformation can occur, as well as the equilibrium conditions of the systems. In this paper, we present the main results of a body of research, aimed at calculating the non-reversible processes of the human body system by means of using the entropy concept as the main operator in applying the Second Law on physical and sometimes even non-physical systems. Determination of body composition was based on Magnetic Resonance Imaging (MRI). In addition, we used direct as well as indirect calorimetric techniques to measure the heat transfers between the human body and its environment, as well as oxygen consumption and carbon dioxide production. These measurements allowed us to compute various energy balances of a human body at rest. Furthermore, we studied also several aspects of energy exchange of the human body during muscular work.