It is a matter of common experience that our level of stress is related with our breathing.
The following quote is from the American Institute of Stress:
"For many of us, relaxation means zoning out in front of the TV at the end of a stressful day. But this does little to reduce the damaging effects of stress. To effectively combat stress, we need to activate the body’s natural relaxation response. The “Relaxation Response” was discovered and coined by AIS Founding Trustee and Fellow, Dr. Herbert Benson. The relaxation response is a physical state of deep rest that changes the physical and emotional responses to stress (e.g., decreases in heart rate, blood pressure, rate of breathing, and muscle tension)."