Some More on Systems

The simple feedback system described in the chapter on the endocrine system has one drawback. It only works in one direction. On a hot day, where instead of losing heat, the room is gaining it, the thermostat can only switch off the boiler. It cannot do anything to correct for excess heat.

During the menstrual cycle, negative feedback is not switched off. It simply becomes inoperable due to the excess estrogen being produced from outside.

The sympathetic and parasympathetic nervous systems, then, are two aspects of one integral system, able to correct for errors in either direction