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Medical-Surgical Nursing · Chapter 2

Where Does the Water Go?

Only about a quarter of extracellular fluid is plasma — the compartment that carries the blood pressure. Move the four forces below (isotonic balance, capillary hydrostatic push, albumin’s oncotic pull, third spacing) and watch which compartment fills or empties. Your job: make the plasma compartment fail three different ways and read the signs each one produces.

Start from a real patient

The four forces

0.0 L
Water and electrolytes lost or retained together — vomiting, diarrhoea, NG suction, haemorrhage on the left; heart, kidney or liver failure and over-resuscitation on the right. 1 kg of weight change = 1 L = 2.2 lb.
Normal
The push of blood against the vessel wall driving fluid out into the interstitium. Raised by heart failure, venous obstruction and over-aggressive infusion.
4.0 g/dL
Normal 3.5 to 5.0 g/dL. Albumin is the pull that keeps fluid in the vessel. Lost in cirrhosis, nephrotic syndrome, malnutrition and burns.
0.0 L
Fluid shifted into a compartment where it does no useful work — peritoneum in ascites, interstitium in burns and sepsis, bowel wall in obstruction, retroperitoneum in pancreatitis.
Model assumptions. A 70 kg adult male: total body water 42 L, two thirds (28 L) intracellular, one third (14 L) extracellular — of which about three quarters is interstitial (10.5 L) and only about one quarter is plasma (3.5 L). Isotonic gains and losses land in the extracellular space and leave cell volume alone, which is why cell volume here only changes when you change sodium (see the sodium widget).

The compartments right now

Intracellular Interstitial Plasma Third space

What the laboratory and the scale say

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