The 2nd Thermodynamics Law was developed & integrated by a bunch of great scientists such as Carnot, Clausius, Thomson(Lord Kelvin)...etc(You can check their papers to learn better than just hearing from me, for my English is poor). Simply saying to look at the 2nd law in Biochemical viewpoint is emphasized as the idea of ENTROPY (randomness) and in Mechanical Engineering viewpoint is emphasized as the idea of Heat Engine Efficiency. It affirmed that the heat will spontaneously go to the place of low temperature from high temperature and fill into the surrounding environment to form a final but homogeneous thermo-equillibrium-status. The 2nd law is regarded as an essence of our nature (it pointed out the proceeding direction of time flow) therefore it predicted a final thermo-status with extremely high temperature and maximum randomness of the destiny of our Universe which also called HEAT DEATH. This idea violated and defied the existence of Heaven of Christianity.
Again, any ordered structure has the tendency to fall apart according to the 2nd law (to increase the total entropy of system). We need to consume energy to resist this kind of nature and maintain the homeostatic structures. It doesn't matter what kind of the way that you adopted to input the energy into your body (hot shower, eating food...) to do works (the work in our body is Symbolic metabolism). Once you stop ingesting energy into your body you will be dead soon. But not all energy that you gained from food or whatever methods can 100% convert to useful works, it has more or less to convert to heat and soon release into our surrounding environment therefore the total entropy of system (we + environment) is still increased. Think Entropy as the "Waste Heat" will be more easier for you guys to understand the 2nd Thermodynamics Law.
