Perhaps the single greatest benefit of physical fitness is increased cardiovascular function. The heart needs regular physical activity to function properly, and exercise helps the heart to become strong and efficient. People with heart conditions need to check with their doctors before beginning exercise problems, but often patients who have suffered mild heart attacks are told to begin exercise programs to help strengthen their hearts, which have been weakened by infarction. Exercise increases the heart rate, which means more blood is pumped through the heart, increasing the flow of oxygen to the body. When the heart is made stronger through exercise, the resting heart rate decreases, placing less strain on the heart muscle overall. Exercise also increases the production of red blood cells, which also helps oxygen flow through the bloodstream.
There are four basic elements of physical fitness: cardiovascular endurance, muscular strength, muscular endurance, and flexibility. Each can be measurably improved with regular exercise. But keep in mind that exercising to build fitness is not the same thing as working out to improve athletic performance. To be truly fit, you should develop all four elements, not just one or two.
While each element is a part of being fit, the most vital is cardiovascular endurance. Physiologically, cardiovascular endurance is the sustained ability of the heart, blood vessels, and blood to carry oxygen to the cells, the ability of the cells to process oxygen, and the ability of the blood, once again, to carry away waste products. Since every cell in the body requires oxygen to function, there is no more basic element of fitness than this - to see that the heart, lungs, and circulatory system do their job.
Cardiovascular endurance is built up through exercises that enhance the body's ability to deliver even larger amounts of oxygen to working muscles. To achieve this, the exercise must utilize the large muscle groups (such as those in the legs) and, most importantly, it must be sustained. With regular aerobic exercise, your heart will eventually be able to pump more blood and thus deliver more oxygen with greater efficiency. Moreover, your muscles will develop a greater capacity to use this oxygen. This is part of what is called the aerobic "training effect." Because your heart is stronger, it can pump more blood per beat, and as a result your heart rate, both at rest and during exertions, will decrease. Your heart will also acquire the ability to recover from the stress of exercise more quickly.