Question

As a dialysis patient, Michelle has a 4-h dialysis treatment three times a week. When she arrives at the dialysis clinic, her weight, temperature, and blood pressure are taken and blood tests are done to determine the level of electrolytes and urea in her blood. In the dialysis center, tubes to the dialyzer are connected to the catheter she has had implanted. Blood is then pumped out of her body, through the dialyzer where it is filtered, and returned to her body. As Michelle's blood flows through the dialyzer, electrolytes from the dialysate move into her blood, and waste products in her blood move into the dialysate, which is continually renewed. To achieve normal serum electrolyte levels, dialysate fluid contains sodium, chloride, and magnesium levels that are equal to serum concentrations. These electrolytes are removed from the blood only if their concentrations are higher than normal. Typically, in dialysis patients, the potassium ion level is higher than normal. Therefore, initial dialysis may start with a low concentration of potassium ion in the dialysate. During dialysis, excess fluid is removed by osmosis. A 4-h dialysis session requires at least 120 L of dialysis fluid. During dialysis, the electrolytes in the dialysate are adjusted until the electrolytes have the same levels as normal serum. Initially the dialysate solution prepared for Michelle's pre-dialysis blood tests shows that the electrolyte levels in her blood are as follows:

HCO3− 24 mEq/L, K+ 6.0 mEq/L, Na+148 mEq/L, Ca2+ 3.0 mEq/L, Mg2+ 1.0 mEq/L, Cl− 111.0 mEq/L.

A dialysis solution is prepared for Michelle that contains the following:

HCO3−  35.0 mEq/L , K+  3.0 mEq/L , Na+  120.0 mEq/L , Ca2+  5.5 mEq/L , Mg2+  3.0 mEq/L , Cl−  96.5, glucose 5.0%(m/v).

Question: What is the osmolarity of the dialysate fluid?

Express your answer using two decimal places.

Answer 1

Ans. Osmolarity is the number of osmoles of solute per liter of solution.

Osmole is the number of moles of a solute that contributes to osmolarity of the solution.

Step I. Milliequivalent to moles-

            # meq of an ion = (mmol of ion / i )              ; [where, i = charge on ion]

            # 1 mol = 1000 mmol

Step II. Moles of Glucose:

Given, [Glucose] = 5% (w/v)

That is, 100.0 mL of solution has 5.0 gram of glucose.

Mass of glucose in 1.0 L (= 1000.0 mL) = (5.0 g/ 100.0 mL) x 1000.0 mL = 50.0 g

Moles of glucose in 1.0 L = Mass / Molar mass

                                                50.0 g / (180.0 g/mol)

                                                = 0.27778 mol

III. Calculating Osmolarity of Dialysate solution:

The osmolarity of dialysate solution is calculated following steps I and II in excel.

Result: 1 L of dialysate fluid has total 0.51 moles that contribute to osmolarity.

Therefore, osmolarity of dialysate fluid = 0.51