This pertains to how living organisms function. In psychology context, this often relates to bodily responses such as heart rate increase when scared.
The ability of an organism or system to maintain stable internal conditions despite changes in external conditions.
These are chemical messengers used by neurons (nerve cells) to transmit signals across a synapse (the gap between two neurons).
A network of glands in your body that make hormones which help cells talk with each other. They're responsible for almost every cell, organ and function in our bodies.