根据以下资料，回答5~8题。

Stress makes a person want to stop whatever they are doing, try to relax, and become calm again．Too much stress results in physical, emotional, and mental health problems.

There are numerous physical effects of stress．Stress can affect the heart．It can increase the pulse rate, make the heart miss beats, and can cause high blood pressure．Stress can affect the respiratory system．It can lead to asthma．It can cause a person to breathe too fast, resulting in a loss of important carbon dioxide．Stress can affect the stomach．It can cause stomach aches and problems digesting food．These are only a few examples of the wide range of illnesses and symptoms resulting from stress.

Emotions are also easily affected by stress．People suffering from stress often feel anxious．They may have panic attacks．They may feel tired all the time．When people are under stress, they often overreact to little problems．For example, a normally gentle parent under a lot of stress at work may yell at a child for dropping a glass of juice．Stress can make people angry, moody, or nervous.

Long-term stress can lead to a variety of serious mental illnesses．Depression, an extreme feeling of sadness and hopelessness, can be the result of continued and increasing stress．Alcoholism and other addictions often develop as a result of overuse of alcohol or drugs to try to relieve stress．Eating disorders, such as anorexia, are sometimes caused by stress and are often made worse by stress．If stress is allowed to continue, then one's mental health is put at risk.

It is obvious that stress is a serious problem．It attacks the body．So, reduce stress: stop the world and rest for a while.

Which of the following parts of the body does not have physical problems caused by stress?