# Gnaiger 1989 Thermochim Acta

Gnaiger E (1989) Physiological calorimetry: heat flux, metabolic flux, entropy and power. Thermochim Acta 151:23-34. |

Gnaiger Erich (1989) Thermochim Acta

*Abstract*: Physiological calorimetry is concerned with the measurement of heat flux in living systems where heat flux is associated with the chemical flux of metabolic reactions. Calorimetry can be related to nonequilibrium thermodynamics if information on both the enthalpy of metabolic reactions and the molar Gibbs energy is available. The molar Gibbs energy of reaction (Gibbs force) is the scalar force conjugated to metabolic flux. The force conjugated to heat flux of an irreversible process is the Gibbs energy/enthalpy ratio. Metabolic power and heat flux of irreversible processes are distinguished as the time rate of Gibbs energy and enthalpy changes, respectively. Power is the product of fluxes and forces, related to the internal entropy production by the absolute temperature. In contrast, *T*·Δ_{r}*S* is the “bound energy” change which equals the heat change of a reversible process in a closed system and is not available for work. Heat flux in general is the sum of the dissipated power and the bound energy change per unit of time. This concept can be extended to vectorial heat flux along a temperature gradient. The temperature difference relative to the temperature of the heat source, traditionally viewed as the “efficiency of a reversible machine”, is in fact the thermal force for heat flux between heat source and sink. The thermal force times heat flux is the thermal power which can be maximally converted into work or can be irreversibly dissipated. A clear distinction between heat flux and power is conceptually revealing, despite the fact that both quantities have the same dimension with units [W per volume, or per mass or per defined system] when describing scalar and discontinuous processes.

* • Bioblast editor:* Gnaiger E

*AT Innsbruck Oroboros*

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*Labels:* **MiParea:** Respiration

**Regulation:** Coupling efficiency;uncoupling

Microcalorimetry