Aristotle helped define the standards of fitness 2,500 years ago when he taught that a thing that suits its purpose well is fit. Fortunately for us, the cardiovascular system, lungs, skeleton, muscles, endocrine system and all the other amazing components of the body function for our purpose: to live well.

Exercising aids fitness in numerous ways, each involving one or more of those systems.

Increased physical activity causes the heart to work harder than at rest. That increases blood flow, floods tissues with fresh oxygen and removes cellular waste products.

Exercise causes the lungs to draw in extra oxygen to bathe the tissues and help power the heart. Exhalation removes carbon dioxide, a waste product of certain biochemical reactions.

Regular, moderate exercise helps raise HDL (High-Density Lipoprotein) cholesterol (the 'good' type). It helps regulate blood sugar levels and converts stored fat into sugars that are used to provide energy. That process also prevents obesity.

The other benefits of a regular fitness program are more obvious and usually among the more direct goals of most people who make the effort: increased muscle mass, toned legs, buttocks, arms, stomach and healthier looking skin. Along the way, the individual receives the added value of greater strength, improved balance, higher endurance and (often) a better frame of mind.

Different types of routines will emphasize one area more than another. Aerobic routines help the cardiovascular and pulmonary systems, weight lifting focuses on building muscle tone and mass; yoga and Pilates helps balance, flexibility and muscular control. But each of these, and several more, help more than just the intended focus group. The body is an integrated system and improving one area almost always has beneficial consequences for others.

All those benefits, at least to a moderate degree, can be had for minimal daily effort. Moderate intensity activity for 30 minutes per day, at least five days per week, will go a long way toward optimizing fitness.

A brisk walk, taking the stairs up one or two flights, a short daily jog, jumping rope and many other simple activities can be carried out with no special equipment or training.

More intense activity, done properly, can raise that level even further. A vigorous tennis game, a few laps in the swimming pool, an hour on the treadmill or exercise bike, or any of a dozen others, can raise your fitness to a peak with only a moderate investment of time and money.

For the truly committed there are, of course, a thousand and one classes at the gym, and every conceivable kind of home fitness equipment to fit a variety of budgets. A daily routine using free weights, followed by a good jog around the park will keep all systems functioning well.

And, as Aristotle taught all those centuries ago, to function well is to live well.