The nervous system has two parts the sympathetic and parasympathetic they both supply the same organs but cause opposite effects, this is because the neuro transmitters are different.
The sympathetic are called the "fight or flight" system, they prepare the body for emergencies, it shuts down the blood supply to the muscles and increases blood pressure, heart rate and breathing which tends to allows us to cope with stressful situations.
The parasympathetic nervous system maintains and refreshes energy it directs blood to the digestive tract and makes us actively digest food. it also maintains Blood pressure heart rate and breathing rate.
Breathing is an involuntary process (we don't think about doing it) like many involuntary processes it is controlled by the medulla oblongata which is situated in the brain and its nerves are part of the autonomic nervous system.
The respiratory centre transmits regular impulses to the diaphragm intercostal muscles to cause inhalation and sends signals to the respiratory centre to cause exhalation, the negative feedback system prevents damage to the lungs.
Ventilation is also under voluntary control from the cortex, it allows you to hold your breath for a few seconds but if you hold your breath to long carbon dioxide builds up in the blood to a dangerous level, it forces us to exhale and gasp for breath.
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