Human thermoregulation

As in other mammals, human thermoregulation is an important aspect of homeostasis. In thermoregulation, body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles. Humans have been able to adapt to a great diversity of climates, including hot humid and hot arid. High temperatures pose serious stress for the human body, placing it in great danger of injury or even death. For humans, adaptation to varying climatic conditions includes both physiological mechanisms resulting from evolution and behavioural mechanisms resulting from conscious cultural adaptations.
There are four avenues of heat loss: convection, conduction, radiation, and evaporation. If skin temperature is greater than that of the surroundings, the body can lose heat by radiation and conduction. But, if the temperature of the surroundings is greater than that of the skin, the body actually gains heat by radiation and conduction. In such conditions, the most efficient means by which the body can rid itself of heat is by evaporation. So, when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise. During sports activities, evaporation becomes the main avenue of heat loss. Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.
Humans cannot survive prolonged exposure to a wet-bulb temperature above 35 °C (95 °F). Such a temperature used to be thought not to occur on Earth's surface but has been recorded in some parts of the Indus Valley and Persian Gulf. Occurrence of conditions too hot and humid for human life is expected to increase in the future due to global warming.
The core temperature of a human is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system, and more specifically by the anterior hypothalamic nucleus and the adjacent preoptic area regions of the hypothalamus. As core temperature varies from the set point, endocrine production initiates control mechanisms to increase or decrease energy production/dissipation as needed to return the temperature toward the set point (see figure).
In general, humans appear physiologically well adapted to hot dry conditions. However, effective thermoregulation is reduced in hot, humid environments such as the Red Sea and Persian Gulf (where moderately hot summer temperatures are accompanied by unusually high vapor pressures), tropical environments, and deep mines where the atmosphere can be water-saturated. In hot-humid conditions, clothing can impede efficient evaporation. In such environments, it helps to wear light clothing such as cotton, that is pervious to sweat but impervious to radiant heat from the sun. This minimizes the gaining of radiant heat, while allowing as much evaporation to occur as the environment will allow. Clothing such as plastic fabrics that are impermeable to sweat and thus do not facilitate heat loss through evaporation can actually contribute to heat stress.
The more physically fit a person is, the greater their ability to adjust to temperature variation. This includes adapting for heat (keeping cool) and for cold (keeping warm).
Age can be a factor in a person's ability to adapt to temperature variations. Studies have shown that younger people adapt more efficiently to contact with cold surfaces than elderly people. Notably, a good level of fitness allowed the elderly people to cope better and offset somewhat the drop off to their ability to thermoregulate due to old age.
A high body mass has been found to help with thermoregulation in regard to adapting for hot environments. This is considered on the basis that the levels of body fat were within healthy ranges i.e. the person's muscle-to-fat ratio was healthy. However, extra body fat has been shown to offer some benefit in terms of keeping warm, especially during immersion in cold water. For this reason long distance outdoor swimmers often have a generous layer of body fat. This is not necessarily always the case though, and high levels of physical fitness can allow thinner swimmers to also perform effectively in cold water environments.
Adjusting the human body temperature downward has been used therapeutically, in particular, as a method of stabilizing a body following trauma. It has been suggested that adjusting the adenosine A1 receptor of the hypothalamus may allow humans to enter a hibernation-like state of reduced body temperature, which could be useful for applications such as long-duration space flight.
The thermoregulatory sweat test (TST) can be used to diagnose certain conditions that cause abnormal temperature regulation and defects in sweat production in the body. To perform the test, the patient is placed in a chamber that slowly rises in temperature. Before the chamber is heated, the patient is coated with a special kind of indicator powder that will change in color when sweat is produced. This powder, when changing color, will be useful in visualizing which skin is sweating versus not sweating. Results of the patient's sweat pattern will be documented by digital photography, and abnormal TST patterns can indicate if there is dysfunction in the autonomic nervous system. Certain differentials can be made depending on the type of sweat pattern found from the TST (along with history and clinical presentation) including hyperhidrosis, small fiber and autonomic neuropathies, multiple system atrophy, Parkinson disease with autonomic dysfunction, and pure autonomic failure.