Basic needs of Surviving

To reiterate, the main elements required for survival are FOOD, FIRE, SHELTER, and WATER. Use  PLAN (Protection, Location, Acquisition, and Navigation) to help you prioritise your survival needs. If you don't have shelter in the desert, having 2 liters of water is of little use to you in the longer term.
  It takes a healthy person quite a long time to die of starvation, for the body can use up its stored resources, but exposure to wind, rain and cold can be fatal even in temperate climates and death comes in only minutes in the icy waters of the poles. Food is rarely the first priority. Even in those places where it is difficult to find there are usually other problems to face first. Shelter will often be the prime necessity in extremes of climate or temperature. Not just in the frozen polar regions  or the baking deserts, but for walkers trapped by mist on a hillside. The need for fire is closely linked.
    Water is something that most people in the modern world take for granted. They are so used to turning on a tap that until an extreme drought causes water rationing they scarcely think about it.  Yet the survivor at sea, or after a flood, thought surrounded by water, may be desperate for drinkable water. And thee are many places where, unless it rains, no obvious water is available. The other survival necessities are dealt with later in my blog post, But water is universally important.

JUST HOW LONG CAN THE HUMAN BODY GO WITHOUT THE ESSENTIALS??
In general the human body can survive for 3 minutes without air, 3 days without water, and 3 weeks without food. There are always exceptions to this and there are examples of people pushing these boundaries and surviving for longer periods. It is amazing what the human body can endure but such a survivor's health can suffer in the long term due to such trauma. For example a person surviving for more than 9 days without water will undoubtedly suffer kidney damage or failure.