Whether it's being outdoors in the cold weather or diving into cold water, your body responds to the change in temperature in many ways, all of them aimed at conserving heat at the core of the body. One of those responses is a slower heart rate, which can be dangerous if it slows too much. A heart rate that is too slow cannot supply the body's organs, muscles and tissue with a healthy, continuous flow of oxygen-rich blood, which in turn can result in serious health complications.
Thermoregulation
Thermoregulation is the way the body tries to maintain a healthy and consistent internal temperature regardless of the temperature outside. In hot weather, for example, the skin sweats to help cool the body and keep the core of the body, the area around the heart and internal organs, from overheating. In cold weather or cold water, however, the body seeks to warm the core. One result of that inner warming is that blood vessels in the fingers, toes and the surface of the skin constrict to reduce blood flow in the extremities so that blood flow in the core can be increased.
The Heart Rate Slows
With a higher-than-normal amount of blood flowing back to the heart, the upper chambers of the heart, known as the atria, become distended and fill with a greater volume of blood they normally contain. The heart responds reflexively by slowing its rate of beating, reducing the amount of blood that is pumped away from the core and to the extremities.
Effects of Slow Heart Rate
When the heart is suddenly pumping a higher volume of blood, it sends a signal to the kidneys to reduce fluid levels in the body. The kidneys maintain healthy fluid levels throughout the body, including blood volume. As a result of the cold slowing your heart rate, your urine production is likely to increase. But one of the most serious complications following a slowed heart rate is an irregular heart rhythm. A slowing of the rate of contraction and relaxation of the heart muscle can lead to an uncoordinated rhythm of that activity, meaning the upper and lower chambers have episodes during which they stop working in an efficient, coordinated manner to help pump blood throughout the body.
Awareness
You may not be aware when your heart rate slows in the cold, but you should be able to spot the most obvious signs. If your skin is growing very cold to the touch and pale, and you're shivering, your body is heading toward hypothermia and you should take precautions, such as getting out of the cold or at least adding layers to help warm you up all over. By the time your body starts shivering, it has already reached the first level of hypothermia and your heart rate has already started to slow.
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