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Module 1 : Physiology



In Health


In healthy people, the most common forms of water gain and loss are shown below:




In healthy people, factors such as osmolality receptors and normally functioning kidneys ensure that normal fluid homeostasis is achieved and that the fluid balance is ‘neutral’ (ie, the fluid outputs closely match the fluid inputs).


The key to maintaining fluid (and electrolyte) homeostasis is to closely match inputs to outputs. In health, the body does this well.


When we must iatrogenically provide a patient's daily fluid (eg because they are nil by mouth), assuming there are no unusual ‘outputs’ to account for, we can use a mathematical rule-of-thumb to calculate a patient's daily requirements based on their weight. Per kg of body weight, a healthy adult is likely to require aprox 30ml of fluid per day.


However, this rule is only approximate and may not hold true in disease states, where significantly more or less fluid may be required. It is also important to note that this rule of thumb does not apply to estimating paediatric fluid requirements, in which case other estimators are used.



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