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Muscles
The most obvious response to fear is felt in the muscles, as they immediately tense up. This includes the tiny muscles attached to the hairs on your skin, which contract and pull on the hairs, resulting in goosebumps. In addition, involuntary muscles inside organs relax to allow more oxygen to other areas of the body.
Circulatory System
When the fight-or-flight response is activated, many feel heart palpitations as their blood pressure increases. Your body dilates your pupils to take in as much light as possible. As your body builds energy for a potential threat, the veins in your skin constrict to allocate more blood to major muscle groups -- this is what causes the chill some associate with fear, as there is less blood in the skin to keep it warm.
Digestive System
Fear can also be felt in the abdomen, as your body restricts blood flow to the digestive organs. This stops all peristalsis, which can lead to swallowing problems and, in extreme cases, diarrhea. Nausea is often associated with fear --- a result of these changing conditions in the digestive system.
Long Term Effects
The various ways the body feels fear require a great deal of energy, and staying in this prolonged state of hyper awareness can drain the body over time. Those with anxiety disorders or overly stressful lifestyles, for example, are at risk of overusing their sympathetic nervous system and devoting too little time to rebuilding energy stores. This can cause nutritional depletion and contribute to aches and pains, as well as apathy and moodiness.