The human body operates best at a temperature of around 37 degrees Celsius. We can tolerate a change of a few degrees in either direction, but any more than that and things start to go wrong.
Once body temperature drops below 35 degrees Celsius, mild hypothermia kicks in. To conserve heat, the body diverts blood away from the skin and hairs stand on end. The muscles contract and relax involuntarily, burning fuel to generate warmth. The colder the body gets, the more it starts to slow down. Nerve signals become sluggish, speech gets slurred and confusion starts to set in.
If the core temperature drops below 32 degrees Celsius, the situation becomes critical and medical attention is needed. At this point, shivering stops and the person may pass out. Below 30 degrees Celsius, the body loses its ability to warm itself up again, and this is often fatal.
The opposite of hypothermia is hyperthermia. The body has built-in mechanisms to lose heat, but sometimes it’s too warm for them to work properly. If the body can’t get rid of excess heat, core temperature starts to rise.
When sweating isn’t enough to lower body temperature, it can lead to dizziness and nausea. The loss of fluid triggers thirst and headaches. At the same time blood vessels dilate, bringing hot blood to the skin, but as the amount of fluid in the system drops, so too does blood pressure. This can cause dizziness and even fainting.
If the temperature climbs to over 40 degrees Celsius, molecules become misshapen and can no longer do their jobs properly, and cells start to die. Untreated, hyperthermia can lead to multiple organ failure.
Thankfully, the body has a built-in thermostat that normally keeps the temperature constant.