When you are anxious you feel fearful and tense. In addition you may also have one or more unpleasant physical symptoms. For example, you might have a fast heart rate, a thumping heart (palpitations), feeling sick, shaking (tremor), sweating, dry mouth, chest pain, headaches, fast breathing.
The physical symptoms are partly caused by the brain which sends lots of messages down nerves to various parts of the body when you are anxious. The nerve messages tend to make the heart, lungs, and other parts of the body work faster. In addition, you release stress hormones (such as adrenaline) into the bloodstream when you are anxious. These can also act on the heart, muscles and other parts of the body to cause symptoms.
Anxiety is normal in stressful situations, and can even be helpful. For example, most people will be anxious when threatened by an aggressive person, or before an important race. The burst of adrenaline and nerve impulses which we have in response to stressful situations can encourage a 'fight or flight' response.