DEAR DR. DONOHUE: Please clear something up for me. Is swimming considered an aerobic exercise? It’s my choice of exercise.

My know-it-all brother says swimming isn’t aerobic exercise because swimmers don’t get their hearts beating fast enough to qualify. I don’t count my heartbeat or take my pulse when I swim. I do know that swimming feels like it gives me a good workout, and my weight has stayed the same ever since I started. — L.P.

ANSWER: Your brother is wrong. Swimming is an excellent aerobic exercise. Have you ever seen an overweight swimmer?

Aerobic exercise is the kind of exercise that benefits the heart and keeps blood pressure normalized. It prevents heart attacks. As a side effect, but as a definite bonus, it keeps weight down. Brisk walking, jogging, biking, rope skipping and swimming are aerobic exercises.

The definition of aerobic exercise is exercise in which large muscles move continuously for a prescribed period of time, and the heart rate reaches a certain target. In swimming, the arms and legs (large muscles) are in constant movement. I don’t know how much time you spend swimming, but 30 minutes qualifies as a long-enough duration to be considered aerobic exercise. Those 30 minutes can be divided into three 10-minute sessions.

Your brother is right about one thing. A swimmer’s heart rate will not reach the same level as the heart rate of an exerciser on dry land doing the equivalent amount of physical exertion. Partly that’s explained by the buoyancy of water, which lessens the pull of gravity on the body. Partly it’s due to the cooling effect of water. And partly it comes from the body’s horizontal position.

If you have calculated what your target heart rate should be by using one of the many formulas given for dry-land exercise, you can deduct 13 beats from that target heart rate for the swimmer’s equivalent. Don’t bother to do this. Keep swimming without counting your heart rate.

DEAR DR. DONOHUE: I check my pulse when I exercise to be sure that I am doing enough exercise and not overdoing it.

A friend tells me a better way to keep track is to take my pulse after exercise. What do I look for? — L.F.

ANSWER: Pulse-taking has become an obsession with exercisers.

After stopping exercise, your pulse should drop 12 beats in a minute’s time from what it was at the very end of your exercise. If your heart was beating 160 times a minute right before you stopped, it should drop to 148 beats a minute after a minute’s rest.

Don’t get hung up on all this. You can judge the intensity and safety of exercise by how you feel.

DEAR DR. DONOHUE: I’ve been talked into donating blood. My hangup is that I have a fitness program that I religiously adhere to. The thought of an extended rest bothers me. How long do you have to spend recuperating after donating blood? — A.A.

ANSWER: You’re not facing an extended recuperation after donating blood. One day is enough. That sounds like too little time, but it’s sufficient. You won’t notice it.

It takes a full month for your blood count to return to what it was before you donated. That slight dip in your blood count isn’t going to affect your exercise performance unless you are into marathon training.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.

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