It's an axiom of modern life: good nutrition and exercise are crucial for a healthy, and hopefully happy, life.

Eat a balanced diet and you won't become obese. Exercise regularly and you'll reap the benefits, but how, exactly, does exercise make us healthier?

There are many ways, of course, some more specific than others. Here's one pretty concrete reason why exercise is worthwhile: it can lower the risk for heart attack and stroke. Both are arterial diseases. An overabundance of bad cholesterol clogs the arteries with plaque, making blood flow more difficult. If the plaque ruptures, a blood clot can form and cut off blood supply completely. When this happens in an artery supplying the heart or brain, the result is a heart attack or stroke.

How does exercise help? There are some obvious ways. Exercise helps keep down bad, plaque-causing cholesterol and pumps up levels of good, plaque removing cholesterol. Plus, the more you exercise the less likely you are to become overweight or obese, conditions that can lead to clogged arteries.

There are also some more science-y explanations. Regular exercise prompts cells lining the arteries to produce more nitric oxide, which facilitates circulation. Meanwhile, research in mice has found that exercise encourages bone marrow to replace aging arterial lining cells with fresh new ones, which can help heal damaged arteries.

No matter how you look at it, exercise really is beneficial. While it can make you look good on the outside, it can help clean up your insides, too.