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When you drink water, how is it distributed around the body?

by on June 4, 2014
 

Once water is absorbed through the GI tract, it enters the bloodstream as plasma volume and is carried throughout the body. There is no specific distribution (eg the body does not route water specifically to an organ), but the flow is just determined by the circulatory dynamics. From there, water follows a general distribution guided by the 60-40-20 rule. Typically your body’s water weight is 60% of your mass. From there, 2/3 (or 40% total body mass) is intracellular water. Of the remaining 1/3 (or 20% total mass) that is extracellular, about 3/4 (15% of total) is contained in the interstitium around your cells while the last 1/4 (5%) is contained in the vascular system.

When you take in fluid, where it goes within those compartments depends very heavily on the osmotic characteristics of the fluid. This comes into play a lot with intravenous fluid administration in terms of deciding the right choice of fluid to give. Giving isotonic water with dextrose results in a even distribution of 2/3 going to the intracellular space and 1/3 staying in the extracellular space, as the dextrose is metabolized and the water is evenly distributed by volume space. However, giving normal saline (0.9% NaCl) results in most of it staying in the extracellular space because the salt creates an osmotic pressure keeping the fluid from entering the cells. In patients who have pathologies resulting in increased loss of vascular fluid through extravasation (eg bad edema from heart failure) we’ll sometimes give an albumin solution as that is thought to favor keeping fluid within the vascular compartment.