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Spring is coming, and if you like to welcome the crisp March weather with water sports such as fishing and kayaking, remember that lakes, streams and oceans can have freezing temperatures this time of year. Falling into icy water is never part of the plan, but it happens even to the best cold-water adventurers. Exposure to cold water is dangerous and sometimes fatal but not because of hypothermia—low body temperature—as you might expect. In fact, hypothermia takes about half an hour to occur. Most cold-water drownings happen for another reason.

Being in cold water activates two opposing physiological responses at the same time. One is called the cold shock response, which activates the fight-or-flight response, a series of hormonal changes that rev up your body in stressful situations. This cold shock response causes you to gasp and breathe faster, increase your heart rate and pump blood to your muscles so you can escape the icy water quickly.

But mammals (including humans) also experience an opposite response to being in water called the diving reflex. It starts when cold water touches the face, and it prepares the body for a long dive. This response slows your heart rate, helps your body save oxygen and sends blood away from your muscles to your heart and brain.

These contradictory signals cause extreme stress on the heart. While the cold shock response dramatically increases blood pressure and heart rate, the diving reflex sends signals to reduce blood pressure and slow heart rate. These confusing signals can lead to irregular heartbeats (arrhythmias) and drastic changes in blood pressure, which can cause heart attack and lead to drowning. This is especially a risk for people with preexisting heart disease.

Bottom line: Be careful around bodies of cold water, and if you have a heart condition, don’t swim in them. It’s also wise to always swim with a buddy and have someone nearby who knows CPR whenever you are in the water. Following these tips can help you enjoy the weather, not the emergency room, during this spring season.

Emily Johnson, PhD, is an APS member and a former volunteer editor for the I Spy Physiology blog.