The parasympathetic systems assists with “rest and digest” in the body. This system conserves energy by slowing the heart rate.
The sympathetic system is part of the autonomic nervous system. It’s primary function is the fight or flight response. When this response is activated by a stressor (say being chased by a cougar) the pupils dilate, sweating increases, blood pressure and heart rate increase to prepare the body to run (flight) or to fight back (fight.)
These systems are important to understand for yoga because in modern society our sympathetic nervous system is overworked, being late for work, social conflict or road rage could illicit an unexpected “fight or flight” response and we have no way to distinguish between a true and a socially accepted as true-fear.
Yoga calms the mind, slows the heart rate and creates a calming effect via the parasympathetic nervous system. The body can now heal from the damage caused by the excess adrenaline released during stress. Focusing on your breath, forgetting negative thoughts from your busy day and focusing on what you are doing when you practice can greatly benefit the body and the mind.