The piston has a cross-sectional area, a, and the force pressure is force per volume area.

我们以活塞系统为例，来说明膨胀功。

麻省理工公开课 - 热力学与动力学课程节选

You just change volume to pressure and basically you're looking at enthalpy under a constant -- anything that's done at a constant volume path with energy, there's the same thing happening under constant pressure path for enthalpy.

可以看到这就是把体积换成了压强,一般我们都是在一种恒定状态下,考虑焓的，任何在恒容条件下,能伴随能量变化的东西，也在恒压条件下伴随焓同样地变化,所以你可以经常。

麻省理工公开课 - 热力学与动力学课程节选

As it does that it changes the volume of the heart and gives the - creates the pressure that moves blood around your body, so it has that muscular system.

这使心脏的体积发生变化,产生了足以使血液流遍全身的压力,这就是心脏中有肌肉系统的原因

耶鲁公开课 - 生物医学工程探索课程节选