What would happen on a hot day if your brain did not receive input that your body was starting to heat up

The body will overheat

Explanation:

If the brain of an individual does not receive input that the body was starting to heat up on a hot day, the setpoint temperature of the body would be exceeded and the body will overheat. If the condition persists for a while, the entire systems of the body may shut down due to overheating.

Normal homeostatic response requires that the brain (the control center) receives a message from the skin (the sensor) about a rise in the body's temperature. In turn, the brain will set mechanisms that will bring the body's temperature back to normal in motion, including vasodilation of the blood vessels in the skin to allow more blood into the skin which in turn causes more heat loss to the surrounding. Thus, an individual starts sweating and the evaporation of the sweat causes cooling and a return of the body to the setpoint temperature.