Muscles tighten up when the body is under a lot of stress. Tension in the muscles is a body's way of protecting itself from pain and damage when it is stressed.
Respiratory system
The respiratory system oxygenates cells and exhales carbon dioxide. Incoming air passes through the nose, larynx, trachea, and bronchi into the lungs. Red blood cells receive oxygen from bronchioles for circulation.
Cardiovascular system
The cardiovascular system's heart and blood arteries nourish and oxygenate body organs. The body's stress response coordinates these two factors.
Endocrine system
The hypothalamic-pituitary-adrenal (HPA) axis, which drives the endocrine stress response, is activated by the brain when a situation is difficult, dangerous, or unmanageable.
Gastrointestinal system
The gut's hundreds of millions of neurons can act independently and communicate with the brain, allowing for stomach "butterflies" sensations.
Nervous system
The peripheral nervous system includes the autonomic and somatic nervous systems, whereas the central nervous system includes the brain and spinal cord.
See Also
Intermittent Fasting 101 — The Ultimate Beginner's Guide