Your heart is the most important muscle in your body

It's about the size of your first, but your heart is the powerhouse of the cardiovascular system. It can pump more than a gallon of blood around your body every minute, 60 minutes an hour, 24 hours a day. That comes to more than 1,400 round trips!

Traveling through the arteries, the blood carries essential nutrients and oxygen to the farthest reaches of your body. After returning to the heart through the veins, the blood is then pumped to the lungs, where carbon dioxide and other waste gases are removed and oxygen is replenished.

All the while, your heart is doing the work. This process goes on when you are awake and when you are asleep. And you couldn't live without it.

Your heart during exercise

If the normal speed of circulation is about 10.5 pints per minute, the rate during exercise is much greater than that. Not only is blood pumped at a faster rate, but its destination is altered during exercise. More blood is routed to the muscles - as much as 12 times the normal volume - and away from the digestive system, which may be visited by a third less blood than usual during this time.

The overweight heart

You already know about the health risks of being overweight. Many of them have to do with the heart. The simple fact is that being overweight and inactive has wide-ranging negative effects on the entire cardiovascular system.

In addition, people who are overweight tend to have less endurance, so it is difficult for them to engage in sustained physical activity. It is a vicious circle, since being active would help them become more fit, but their very lack of fitness is preventing them from being active. The best way out of the vicious circle is to start slowly, with a small amount of low-intensity exercise. Walking is a good choice. Gentle stretching is another.

If you are extremely overweight or have or suspect you have heart problems, do not begin any sort of exercise program, no matter how modest, until you have talked to your doctor about it.

What about all those other muscles?

If the heart is the most important muscle for endurance, then strength and flexibility are the territory of your skeletal muscles. These are the muscles that make it possible for us to move, lift things, and even to stand up. Muscles also give shape to the body. Without muscles, we'd be little more than a bag of bones.

The way to increase muscular strength is make muscles do the work they were designed to do. Working against resistance - pushing or lifting - gives muscles their tone. The more you do it, the stronger you become. Within limits.

Hormones, particularly the hormone testosterone, playa part in muscle development. The more testosterone you have, the more you'll be able to strengthen and build up your muscles. Although women produce some testosterone, men produce a lot more. That's why men look more muscular than women and why working out produces a more sculptured look in men.

Before puberty testosterone is in short supply in both sexes, so preteens should be discouraged from" pumping iron."

Preteens should not work out. It won't give them bulging muscles, but it may damage growing bones and connective tissues.