Respiratory water loss/kg follows the same pattern. MR/kg is highest in newborns and lowest in adults. The MR does not rise linearly with increase in body weight. The amount of air breathed depends on oxygen need and carbon dioxide production, which in turn depend on the metabolic rate (MR).Unless the ambient relative humidity is 100%, this air needs to be humidified, and the water used for humidification is breathed out. Respiratory water loss is a function of the amount of air that is moved in and out of the lungs, or breathed, per unit time.Note: water loss in normal stool is inconsequential. What is the other major insensible route of water loss? Check out the cards below. This is water that is used to humidify inspired air and is then breathed out as water vapor. Urine, of course is a "sensible" loss - it can be seen, felt, and measured! You've lost it, but you don't know that you've lost it (and, of course, you do not know how much you have lost.) "Insensible" losses can neither be perceived nor measured directly. If you've lost it, you know you've lost it! "Sensible" loss is loss that can be perceived by the senses and can be measured. Water loss from the body is said to be either "sensible" or "insensible". Photo by Konstantin Tavrov from Photospin 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. To be able to calculate daily water requirement, we need to be able to estimate daily water loss in individuals of different ages. Conventionally, water requirement is calculated in daily, or 24-hour, increments. The need for water over any period of time is equal to the loss of water over that period of time. The only absolute reason to drink water is to replace losses.
So how much water does a person need in a day? To answer this question, we must ask another. Maintenance fluids are necessary to maintain homeostasis when a patient is unable to intake the required water and should be prescribed, ordered, dosed, and delivered like any other drug. You, the prescribing physician, must decide how much fluid to administer. But what about a person who is receiving only intravenous fluids or gavage feeding? In this individual, the thirst mechanism has been bypassed. So, we rely on thirst to guide water intake.
#SODIUM NORMAL RANGE FREE#
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. We do not have to keep track of our fluid intake. We drink water, or a water-containing beverage, five to ten times a day. Marie Dawlett MD and Amy Gonzalez MD The Need for Water A Chapter in Core Concepts of Pediatrics, 2nd Edition Fluid and Electrolyte Therapy
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