The Nervous System allows us to function by transmitting electrical signals between the brain, the spinal cord and different parts of the body. Of it's many sub-components, the Sympathetic and Parasympathetic networks play critical roles. For a person to stay healthy both of these sub-systems need to step in perfect balance. This balance is Homeostasis, the ability of a living thing to maintain itself in a state of steady internal conditions despite changes in environment, and is fundamental to maintaining life. When this equilibrium is adversely affected problems occur in the body in the form of diseases.
To achieve Homeostasis, the two components of the Nervous system act as counterbalance - Sympathetic system activates fight or flight response, while Parasympathetic system stimulates rest and digestion. Almost all the internal systems are governed by the Parasympathetic nervous system including the Heart (regulation of blood pressure), Lungs (regulation of breathing), Digestion (improved blood flow to the stomach and intestines) and the Blood vessels (through vasodilation or widening of the blood vessels which allows blood to flow smoothly). The Endocrine system, which governs functioning of hormones such as Thyroid and Adrenaline, is controlled by the Hypothalamus, which in turn is governed by Parasympathetic Nervous System.
Parasympathetic nervous system is itself controlled by Vagus nerve (the nerve that emanates from the brain and goes all the way down to the bottom of the spinal cord) which sends out chemical messengers. It is the longest cranial nerve of PSNS. It wanders all over the body to various important organs, reaching the brain, gut, heart, liver, pancreas, gall bladder, kidney, ureter, spleen, lungs, reproductive organs(female), neck, ears and tongue. It is largely responsible for mind-body connection. When vagus nerve is healthy there are fewer problems in the body.
All the breathing practices, such as pranayama, asanas, meditation, singing and chanting are helpful in stimulating Vagus nerve.
By acting directly on the Vagus nerve and strengthening the Parasympathetic nervous system,Yoga enhances our ability to maintain internal equilibrium conditions in any situation. This helps manage stress and sustain conditions for optimal functioning of the body and the mind.
'Samatvam yoga uchyate'(2:48){equanimity is yoga}
Yoga creates balance.