1. Why does urea not contribute to tonicity? 2. Problem 1: solution adds 1 liter

Question

1. Why does urea not contribute to tonicity?

2.

Problem 1: solution adds 1 liter of volume and 300 mOsmoles of Nal Work total body first. Add solute and volume, then calculate the new osmolarity. solution of liter of 300 mosM NaCl to this body described in Starting Conditions. This mosM NaCl to this body described in Starting Since you know that NaCl is a nonpenetrating (NP) solute, carry the new osmolarity across to the ECF and ICF boxes. Next calculate the Solute of the ECF and the ICF. Will any of the solute penetrate to the ICF? Next calculate the Volume of the ECF and the ICF, Use V- s/C to calculate the new ECF and ICF volumes. Total Body ECF ICF Solute mOsmoles) Volume (L) Osmolarity The added solution was isosmotic (300mOsM), and its nonpenetrating concentration was the same as that of the body's (300mOsM NP). What would you say about the tonicity of the added solution? The added solution is calculated ICF) with respect to the ICR(hint: think about the volume in the

Explanation / Answer

1. Tonicity describes the water status (water potential) of a solution in comparison to a cell placed in it. In other words, the concentration of the solutes in the solution determines the tonicity of the solution. However, the case may not be so always, and many a times depends on the penetrability of a solute molecule across the cell membrane. The cell membrane is selectively permeable, making it difficult for ions and charged solutes to cross the barrier without help (active transport). But penetrating solutes like urea and dextrose can diffuse across the cell membrane until the equilibrium is achieved. Thus the concentration of urea (solute) or the water potential do not remain same as during starting conditions. Similarly if a hyperosmolar solution is administered to a patient, the natural tendency of the water is to move outside of the cell. But if the solution is made up of penetrating solutes like urea which can diffuse across the cell membrane and enter the cell, it will increase the intracellular osmolality and prevent the loss of fluids from the cell. Hence urea does not contribute to tonicity.

2. The Starting conditions of the body for this problem are not listed.

So suppose under starting body conditions ECF (x1)= 300 mOsm, ICF (x2)=300 mOsm and volume =2L

=ECF+ICF+300

=300+300+300

= 900 mOsm

=300+300

= 600 mOsm

Since NaCL is a non penetrating solute or an effective osmole, none of the solute will penetrate across the semi permeable cell membrane

WHAT WOULD YOU SAY ABOUT THE TONICITY OF THE SOLUTION? After the previous calculation is done, the volume in the ICF will give an idea if the solution is hypertonic or hypotonic or isotonic. Here the ICF volume is 1L and osmolarity is 300 mOsM, while the NaCL solution is 1 litre and 300 mOsM as well. Therefore for the suggested case, the added solution is isotonic with respect to the ICF.

total body ECF ICF solute (mOsm) x+300 x1+300 x2 Volume (L) (V=S/C) y+1 (x1+300)/(x+300/y+1) x2/(x+300/y+1) Osmolarity (mOsM) x+300 /y+1 x+300 / y+1 x+300 / y+1

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