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Module 1 : Physiology



In Disease States


In disease states, the distribution of body water may differ from the standard model shown on the previous page (Physiology : Fluid Compartments), in a number of ways:


1.Global fluid loss – There is water loss from all fluid compartments  Eg Dehydration.


2.Global fluid gain – There is water gain in all fluid compartments (most often the majority of fluid excess is in the extracellular space)  Eg CKD.


3.Abnormal distribution – This is where fluid losses or gains occur unequally between compartments. This is usually due to changes in hydrostatic or oncotic pressure driving exchange between the extracellular fluid compartments (by Starling’s law).


   Eg Heart failure causes high hydrostatic pressures in the venous system, leading to   extravasation of fluid into the interstitial space, causing oedema.


  Eg Liver failure causes lowered intravascular oncotic pressure (due to reduced   albumin synthesis), again leading to extravasation of fluid causing oedema.


  Test Yourself !  
  Why do we examine most carefully for oedema in the feet and sacrum? Think about it and then click below to reveal the answer.

How did you do?

Excess tissue fluid can move relatively freely through the interstitial space. Because people stand and walk on their feet the fluid most often collects here due to gravity. It is often more noticeable towards the end of the day. In patients who are bedbound this gravity 'dependent' fluid will collect in the sacrum instead.





A further example of abnormal distribution may be the accumulation of fluid in body spaces where there usually is none (or little), for example bowel, peritoneum and pleural space. These areas are collectively known as the ‘Third space’. Although enormous amounts of fluid may collect in these spaces, it is not ‘available’ for exchange with the rest of the fluid compartment model and so effectively acts as a fluid loss. Patients with large third space losses can be dangerously dehydrated, even though they may have low or normal measurable outputs on the fluid balance chart (See Assessment : Fluid balance charts for more info).



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