Immersion Effects II

The Dive Reflex

When we immerse ourselves in water, there are bodily responses that occur, termed the dive reflex. These consist of a decrease in heart rate and a decrease in blood flow to the limbs. In aquatic mammals, this reflex is protective in nature. That is, it plays a large role in helping the animal to conserve oxygen while submerged beneath the surface. In some animals, this reflex can allow them to remain underwater for up to several hours before needing to resurface to take a breath. Humans however, do not gain an oxygen conserving benefit from this reflex. In fact, in cold water, oxygen consumption will actually rise as we attempt to produce more heat. Our bodies will burn more of its fuel, and we will get an increase in metabolism. This process requires an increased supply of oxygen.

The reduction of blood flow to the limbs is due to the body's attempt to conserve heat. Water conducts heat to a much greater extent than air, so we lose heat from our bodies much more rapidly when immersed. Even water a few degrees below body temperature will trigger this response. By constricting blood vessesl in the limbs, the body is able to maintain a greater percent of its blood in the head and thorax region. As we describe in the section on "Temperature and its Effects", this area of the body will lose heat at a much slower rate than the long slender limbs. The dive reflex has been shown to become more pronounced with decreasing water temperatures.

Heat concervation and the movement of blood out of the lower limbs due to immersion (See Immersion Effects I) both increase the amount of blood confined to the thorax region. This causes an increase in the volume of blood returning to the heart, and an increase in stroke volume (the volume of blood pumped out with each ventricular contraction). However, the heart attempts to keep the total amount of blood pumped out per minute relatively constant. To offset the stroke volume increase, there will be a reflexive drop in heart rate. Additionally, heart rate is influenced by nerve receptors located on the face. Particularly sensitive are receptors located around the nose and mouth. These detect temperature changes and can cause a reflexive decrease in heart rate. It has been found that simply placing a cold wet cloth on the face will cause heart rate to decrease without immersion.

Finally, holding one's breath has also been found to lower heart rate. In theory, it would seem that heart rate would lower due to increasing levels of carbon dioxide in the blood. This is true to an extent, but the act of holding one's breath also plays a part. One study found that subjects who held their breath, but were artificially maintained with normal oxygen levels, still showed a decrease in heart rate. It was then concluded that this was due to stretch receptors in the lungs that are responsive to stop breathing movements.

While the dive reflex is a normal bodily response to immersion in water, it can be a concern in some situations. The sudden changes that can take place when rapidly immersing one's self in cold water are not tolerated equally by everyone. To slow the rate of these changes, one can:

• Get into the water slowly
• Wet your face before entering
• Wear gear that will provide thermal insulation (ie. wetsuit)
• Keep in shape
• If you have any concerns about your cardiovascular health, consult a doctor before diving.

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