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To a fair approximation, living organisms may be considered as examples of (b). Approximately, an animal's physical state cycles by the day, leaving the animal nearly unchanged. Animals take in food, water, and oxygen, and, as a result of metabolism, give out breakdown products and heat. Plants take in radiative energy from the sun, which may be regarded as heat, and carbon dioxide and water. They give out oxygen. In this way they grow. Eventually they die, and their remains rot away, turning mostly back into carbon dioxide and water. This can be regarded as a cyclic process. Overall, the sunlight is from a high temperature source, the sun, and its energy is passed to a lower temperature sink, i.e. radiated into space. This is an increase of entropy of the surroundings of the plant. Thus animals and plants obey the second law of thermodynamics, considered in terms of cyclic processes. Simple concepts of efficiency of heat engines are hardly applicable to this problem because they assume closed systems.

To a fair approximation, living organisms may be considered as examples of (b). Approximately, an animal's physical state cycles by the day, leaving the animal nearly unchanged. Animals take in food, water, and oxygen, and, as a result of metabolism, give out breakdown products and heat. Plants take in radiative energy from the sun, which may be regarded as heat, and carbon dioxide and water. They give out oxygen. In this way they grow. Eventually they die, and their remains rot away, turning mostly back into carbon dioxide and water. This can be regarded as a cyclic process. Overall, the sunlight is from a high temperature source, the sun, and its energy is passed to a lower temperature sink, i.e. radiated into space. This is an increase of entropy of the surroundings of the plant. Thus animals and plants obey the second law of thermodynamics, considered in terms of cyclic processes. Simple concepts of efficiency of heat engines are hardly applicable to this problem because they assume closed systems.

从近似的角度来看，生命体可以被认为是(b)的一个例子。近似地，一只动物的身体状态每天循环，使得它几乎没有什么变化。动物吸收食物、水和氧气，经过新陈代谢，输出分解的产物和热量。植物吸收来自太阳的辐射能量，这可以认为是热量，以及二氧化碳和水，然后它们释放氧气。它们就是这样生长的，最终会死亡，尸体腐烂，大部分重新变成二氧化碳和水。这可以看作是一个循环过程。总的来说，阳光来自一个高温的源——太阳，它的能量被传递到一个较低的温度汇，例如向太空辐射。这个过程使得植物周围环境的熵增加。因此从循环过程的角度来看，动物和植物服从热力学第二定律。简单的热机效率概念很难适用于这个问题，因为它们假定系统是封闭的。

从近似的角度来看，生命体可以被认为是(b)的一个例子。近似地，一只动物的身体状态每天循环，使得它几乎没有什么变化。动物吸收食物、水和氧气，经过'''新陈代谢metabolism'''，输出分解的产物和热量。植物吸收来自太阳的辐射能量，这可以认为是热量，以及二氧化碳和水，然后它们释放氧气。它们就是这样生长的，最终会死亡，尸体腐烂，大部分重新变成二氧化碳和水。这可以看作是一个循环过程。总的来说，阳光来自一个高温的源——太阳，它的能量被传递到一个较低的温度汇，例如向太空辐射。这个过程使得植物周围环境的熵增加。因此从循环过程的角度来看，动物和植物服从热力学第二定律。简单的热机效率概念很难适用于这个问题，因为它们假定系统是封闭的。

From the thermodynamic viewpoint that considers (a), passages from one equilibrium state to another, only a roughly approximate picture appears, because living organisms are never in states of thermodynamic equilibrium. Living organisms must often be considered as open systems, because they take in nutrients and give out waste products. Thermodynamics of open systems is currently often considered in terms of passages from one state of thermodynamic equilibrium to another, or in terms of flows in the approximation of local thermodynamic equilibrium. The problem for living organisms may be further simplified by the approximation of assuming a steady state with unchanging flows. General principles of entropy production for such approximations are subject to unsettled current debate or research. Nevertheless, ideas derived from this viewpoint on the second law of thermodynamics are enlightening about living creatures.

From the thermodynamic viewpoint that considers (a), passages from one equilibrium state to another, only a roughly approximate picture appears, because living organisms are never in states of thermodynamic equilibrium. Living organisms must often be considered as open systems, because they take in nutrients and give out waste products. Thermodynamics of open systems is currently often considered in terms of passages from one state of thermodynamic equilibrium to another, or in terms of flows in the approximation of local thermodynamic equilibrium. The problem for living organisms may be further simplified by the approximation of assuming a steady state with unchanging flows. General principles of entropy production for such approximations are subject to unsettled current debate or research. Nevertheless, ideas derived from this viewpoint on the second law of thermodynamics are enlightening about living creatures.

从(a)的热力学观点考虑 ，即考虑从一个平衡状态到另一个平衡状态的路径，只会产生一个粗略近似的图像，因为生命体从来不会处于热力学平衡状态。生物体经常必须被认为是开放的系统，因为它们吸收营养物质并排出废弃物。开放系统热力学目前通常从一个热力学平衡状态到另一个状态的路径的角度来考虑，或者是考虑局部热力学平衡近似下的流。生命体的问题可以通过假定一个流不变的稳态来进一步简化。对于这样的近似，熵产生的一般原理在目前有争论和研究的不确定性。尽管如此，从热力学第二定律的这个角度出发衍生出来的想法对生物是有启发意义的。

从(a)的热力学观点考虑 ，即考虑从一个平衡状态到另一个平衡状态的路径，只会产生一个粗略近似的图像，因为生命体从来不会处于热力学平衡状态。生物体经常必须被认为是开放的系统，因为它们吸收营养物质并排出废弃物。开放系统热力学目前通常从一个热力学平衡状态到另一个状态的路径的角度来考虑，或者是考虑局部热力学平衡近似下的流。生命体的问题可以通过假定一个流不变的稳态来进一步简化。对于这样的近似，熵产生的一般原理在'''目前有争论和研究的不确定性Non-equilibrium thermodynamics|unsettled current debate or research'''。尽管如此，从热力学第二定律的这个角度出发衍生出来的想法对生物是有启发意义的。

==Gravitational systems==

==Gravitational systems==