1) Osmoregulation (regulating water content) - you need to keep a balance between the water you gain (in drink, food, and from respiration) and the water you ppe, sweat and breathe out.
2) Thermoregulation (regulating body temperature) - you need to get rid of excess body heat when you're hot, but retain heat when the environment is cold.
3) Blood glucose regulation - you need to keep the glucose in your blood at a steady level.
A mechanism call negative feed back helps to keep all these things steady:
Changes in the environment trigger a response that counteracts the changes - e.g. a rise in body temperature causes a response that lowers body temperature.
This means that the internal environment tends to stay around a norm, the level at which the cells work best.
This only works within certain limits - if the environment changes to much…