The rate of heat loss from the body may fall below the rate of metabolic heat production in hot settings. This time, the body starts to work in the opposite direction. The temperature of the skin and the tissues nearby rises and approaches the deep body temperature first as the body increases blood flow and, consequently, heat transport to the skin. The heart rate may increase to 180 beats per minute under conditions of high heat in order to keep the brain and skin well-blooded. Because there is not enough time between beats to fill the heart with blood, the volumetric efficiency of the heart decreases at higher heart rates, and the blood supply to the skin and, more crucially, the brain also decreases. Heat exhaustion makes the person faint. Dehydration exacerbates the issue. Similar results occur when someone quits working extremely hard for an extended period of time. Since the muscles are relaxed in this scenario, the blood that has saturated the skin has trouble returning to the heart. As a result, there is less blood available to pump to the brain.
Unless the person removes some clothing and lessens their activity level, the next line of defense is sweat glands releasing water and resorting to evaporative cooling. The body can maintain its core temperature at 37 degrees Celsius indefinitely in this evaporative cooling mode, even in environments with higher temperatures (as high as 200 degrees Celsius during military endurance tests), if the person drinks plenty of liquids to replenish his or her water reserves and the ambient air is sufficiently dry to allow sweat to evaporate instead of rolling down the skin. If this precaution is insufficient, the body will have to begin absorbing metabolic heat, causing the deep body temperature to rise. A person can tolerate a temperature rise of 1.4 degrees Celsius without experiencing significant discomfort, but when the temperature rise exceeds 2.8 degrees Celsius, the person may collapse. When the core body temperature climbs beyond 39 degrees Celsius, people feel sluggish and their effectiveness suffers significantly. A core temperature of 41 degrees Celsius or above may damage hypothalamus proteins, resulting in sweating stoppage, increased heat production through shivering, and a heat stroke with irreversible and life-threatening damage. Above 43 degrees Celsius, death is possible.
A surface temperature of 46 degrees Celsius produces skin discomfort. As a result, direct contact with a metal block at this or higher temperatures is painful. However, due to convective resistance at the skin surface and evaporative cooling, a person can stay in a room at 100°C for up to 30 minutes without experiencing any skin damage or pain. We can even put our hands in a 200°C oven for a few minutes without getting burned.
This is actually part-4 of the HUMAN BODY AND THERMAL COMFORT series. part-3 is given in the link below.
Part-3: Cold adaptation in humans.
Part-2: Thermal comfort in buildings.