When you work out, you might be concerned only with losing weight or getting in shape, but almost every cell in your body responds when you lift weights, attend an aerobics class or hop on the stationary bike for a climate-controlled bike ride. When your heart rate starts rising, you know your blood is pumping, but you probably didn't know -- and certainly cannot feel -- your blood vessels getting larger as you exercise.

Why They Expand

The point when the intensity of your workout exceeds your body's oxygen reserves is when you start breathing heavier. It's your body's way of signaling that the whole system -- your brain, lungs and muscle tissue -- needs more oxygen. This is when small blood vessels, also known as capillaries, expand. It allows them to carry more oxygen to your muscles and other various body parts, as well as carting off lactic acid and carbon dioxide.

They Increase, Too

Exercising has a general growth affect on blood vessels. Not only does it cause them to increase in size, it also has the effect of increasing the number of blood vessels you have. In his book "Renewal: The Anti-Aging Revolution," Timothy J. Smith writes that regular aerobic exercise that increases the size of your capillaries also conditions your vascular system to anticipate extra loads. To meet the anticipated demand, your body will expand the number of blood vessels by up to 50 percent in muscles that are frequently exercised.

It's a Good Thing

More is better when it comes to increasing the number and size of blood vessels from working out. The result of more blood vessels is improved cardiovascular efficiency for supplying oxygen throughout your body, benefitting your entire system. Your muscles are able to function, grow and repair themselves, and your heart health will improve, too. In an article on exercise for "Scientific American," Coco Ballantyne tells readers that exercise has a "fine-tuning" effect on the system -- the autonomic nervous system -- in your body that controls the contracting and expanding of your capillaries. The outcome is lower blood pressure, lowered resting heart rate and a reduced risk of heart disease.

Use 'Em or Lose 'Em

Causing your blood vessels to get larger is all in your hands -- you decide whether or not you'll work out on a regular basis, which is optimal for maintaining an efficient vascular system. But just like muscle, your capillaries won't maintain their enlarged size, nor will the number of blood vessels remain increased for long, if you don't continue to exercise. If you don't keep up your level of physical activity, your body won't use all the capillaries that have developed. It will then start "downsizing," shutting down the extra blood vessels that aren't being used.

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