Barry S. Brown, University Professor of kinesiology in the College of Education and Health Professions, replies:
In general, heat loss is essential to provide thermoregulation and prevent excessive heat build-up in the body to avoid the consequences of heat stroke. However, heat loss in the head is the same as for any exposed body part. The exposed surface area of the body, along with relative humidity and temperature, determines heat loss primarily through evaporation and our head makes up only 10 percent of body surface area. So, at rest, heat loss through the head accounts for only 7-10 percent of total heat dissipation.
The brain is supplied by blood flow through the cerebral and vertebral arteries, which also supply oxygen and heat to the brain. This flow does not change since the oxygen demand to the brain is constant. However, as cardiac output is increased, the blood transported via the cerebral arteries to the brain also is increased. This increase in blood flow to the brain also results in an increase in the percentage of heat loss. As you begin exercise, cerebral blood flow increases due to increased cardiac output and the percentage of heat lost through the head accounts for about 50 percent of total body heat loss. As exercise continues, more oxygen is directed toward muscle and blood flow to this tissue increases. Core temperature has to be maintained and as body heat increases, the skin arterioles expand, or vasodilate, redirecting blood flow to the skin which cools the blood. Hence, total blood flow to the brain is decreased and the percentage of total body heat lost through the head is reduced to about 10 percent. The percent lost through the scalp returns to 7 percent after sweating begins.
One important caveat occurs during hypothermia, or cold exposure. If shivering occurs during hypothermia, 55 percent of the heat loss can occur in the scalp. Therefore, head covering during cold weather is strongly advised.