Question

 

If a 65kg female drinks 500 ml of water, what is the percent change in her body fluid osmolarity?

Answer 1

there are few things you should know before doing this calculation. i am going to assume a normal value for calculation here. But remember the normal value always has a range of values. (if the values are given in your question, then use the exact value)

1. percentage of body water in a 65 kg women- approximately 60%
2. normal plasma osmolarity- 290 mOsm/L
3. lets assume 100% of this 500ml water is absorbed by the body and it has an osmolarity of zero.

Calculations

• 60% of 65 kg is 65*60/100= 39 kg of water= 39 L of water
• so you are adding this zero osmolarity 500 ml water to 39 L of 290 mOsm/L total body water.
• then calculate change in osmolarity. ( 39*290/39.5= 286.33 mOsm/L)
• the calculate percentage change in osmolarity. ( osmolarity decreased by 3.67, so percentage change is 3.67*100/290= 1.27%) osmolarity decreased by 1.27%.

but remember in real life, this excess water intake is lost in urine, sweat, respiration etc... so the change will be smaller than this.