Marie Dawlett MD and Alok Kalia MD
The Need for Water
We drink water, or a water-containing beverage, five to ten times a day. We do not have to keep track of our fluid intake.
The thirst-creating mechanism is exquisitely sensitive to an increase in plasma osmolality and as long as there is free access to water, intake will never be less than the need.
So, we rely on thirst to guide water intake. But what about a person who is receiving only intravenous fluids or gavage feeding? In this individual, the thirst mechanism has been bypassed. You, the prescribing physician, must decide how much fluid to administer. Maintenance fluids are necessary to maintain homeostasis when a patient is unable to intake the required water.
So how much water does a person need in a day? To answer this question, we must ask another.
So, the only absolute reason to drink water is to replace losses. The need for water over any period of time is equal to the loss of water over that period of time. Conventionally, water requirement is calculated in daily, or 24-hour, increments.
To be able to calculate daily water requirement, we need to be able to estimate daily water loss in individuals of different ages. For this, we need to know the routes of water loss and be familiar with the physiologic and pathological determinants of the rate of loss from each route.
Photo by Konstantin Tavrov from Photospin