The increase of thyroid hormone speeds up your metabolism, providing quick extra energy. More endorphins decrease your sensitivity to pain. Digestive activity shuts down in order to divert all available energy to muscles. Extra cholesterol provides energy in place of the digestive system. Increased heart rate pumps more blood and, therefore, more oxygen and nutrients to the muscles.
Blood thickens to reduce bleeding, carry more oxygen, and fight infection.
Unfortunately, modern life turns these responses into health threats. Adrenals become depleted, unable to produce more endorphins, so pain levels from migraine, backaches, and other conditions increase. High blood pressure becomes chronic, and all that cholesterol dumped into thickened blood gums up the works. Eating on the run, which means dumping food into an unwilling stomach, can cause heartburn, constipation, or diarrhea. Sperm count, ovulation rates, and libido all decrease, causing marital discord.