How does exercise help lower the risk of heart disease, according to the passage?

We have all heard it many times: regular exercise is good for you. Yet many people spend most of the day sitting, at a desk, in a classroom or on a bus, and never quite get round to changing their habits. The good news is that it is never too late to begin, and there is no need to start with anything difficult. You can start slowly and find small ways to be more active.

The rewards are real. Along with a sensible diet, regular exercise helps people reach and stay at a healthy weight. It strengthens the heart and improves the flow of blood, which raises the level of oxygen in the body. That in turn lowers the risk of heart disease. Exercise can also help bring down high blood pressure, which is one of the things that make heart disease more likely.

Put together, these effects add up. People who are regularly active tend to feel better, are more likely to prevent or control many diseases, and are likely even to live longer.

None of this requires a gym. A brisk walk, a game in the park or climbing the stairs instead of taking the lift all count, and each small step makes the next one easier.