During regular exercise and we get fever our hearts never get proper rest and oxygen that's why

Heart rate increases during exercise and fever.

During exercise and fever, the body undergoes physiological changes that lead to an increase in heart rate. The heart rate is the number of times the heart beats per minute, and it is an important indicator of the cardiovascular system's functioning. Let's explore why the heart rate increases during exercise and fever.

Exercise:
During exercise, the body's muscles require more oxygen to produce energy. To meet this increased demand, the heart pumps blood at a faster rate to deliver oxygen and nutrients to the muscles. The increased heart rate ensures that an adequate amount of oxygen is supplied to the working muscles and carbon dioxide and waste products are removed efficiently. The heart rate increases gradually during exercise and can reach its maximum level depending on the intensity and duration of the exercise.

Fever:
Fever is a temporary increase in body temperature that occurs in response to an infection or illness. When the body's temperature rises, the heart rate increases as a part of the body's defense mechanism. The increased heart rate helps to distribute heat throughout the body and regulate the body temperature. The heart pumps blood at a faster rate to transfer heat from the core of the body to the skin surface, where it can be dissipated through sweating and radiation. This increased circulation helps to cool down the body and reduce the fever.

Summary:
- During exercise, the heart rate increases to meet the increased demand for oxygen and nutrients by the working muscles.
- Fever leads to an increased heart rate as the body tries to regulate and reduce the elevated body temperature.
- Both exercise and fever cause the heart to pump blood at a faster rate to ensure adequate oxygen supply and heat dissipation, respectively.

Therefore, the correct answer is option C: Exercise and fever.