Human body is the place of irreversibilities related to entropy increase, due to internal operations which is always positive according to the second principle of thermodynamics. Exergy analysis, based on first and second principles, quantifies these irreversibilities and enlightens differently the biological processes. Exergy computation requires the knowledge of body composition, which has been obtained by Magnetic Resonance Imaging. Direct and indirect calorimetric measurements enable to determine the energy transfers between the body and its surroundings, as well as the metabolism. A model of the human body at rest, whith thirteen components and three temperature levels has been developed. Results show that energy losses (convection, sweating) are high when the corresponding exergy contributions are zero or very small. Moreover the values of the exergetic and energetic metabolisms are very close. Finally the exergy yield of a man at rest is zero.