Why can a person feel bad in a sauna or in extreme heat?

At high temperatures, the vessels expand, after which the pressure drops. With a drop in pressure, a person experiences ailments in the form of weakness and headache. This is especially pronounced in hypertensive patients. Therefore, they always talk about the indications of taking saunas. And in the heat, thermoregulation changes throughout the body. Due to changes in the concentration of carbon dioxide, direct rays of the sun can also lead to a loss of coordination and consciousness in general.