Ed Rivers, a 3rd year medical student, was alone in the hospital ER one night. It was unusually quiet that night, and the resident was getting some much needed sleep. A patient, Mrs. X, was brought in showing signs of serious dehydration. Ed tried to give here water, but she vomited this back up. Feeling he must try something, and not wanting to wake the resident, Ed administered 1 liter of sterile distilled water IV. Assume for simplicity that the red blood cells contain only solutes to which the rbc membrane is impermeable, and that the rbcs and plasma are in osmotic equilibrium when the patient is brought in. The osmolarity of the rbc is 300 mOsm/L.
1. Predict the direction (increase, decrease, no change) you would expect Ed’s infusion to have produced in these parameters, and explain your predictions in terms of what you know from lecture and lab.
a. Mrs. X’s plasma osmolarity after the infusion Prediction: Why?
b. Mrs. X’s rbc volume after the infusion equilibrates with blood? Prediction:Why?
c. The osmolarity of the rbcs and plasma after equilibration? 
2. The volume of Mrs. X’s plasma was 3 liters before Ed administered the IV. Assumethat there was a complete mixing of the administered water with her plasma but no mixing with her interstitial fluid. Calculate Mrs. X’s plasma osmolarity after the infusion mixed with her plasma but before any water entered the rbcs.
Hint: What was the total # of mOsm in her plasma before the infusion? Once the plasma volume is increased by 1 L, the total # of mOsm remains the same, but the volume has changed. Now what is the new osmolarity (mOsm/L?) Does this fit your prediction, above?
Mrs. X got much worse after the Ed’s treatment, so the resident was called. She drew some blood from the patient and centrifuged it. The supernatant above the packed rbcs was pink. The resident then decided to infuse a sucrose solution into the patient (the rbc membrane is impermeable to sucrose).
3. Why was the plasma pink?
4. Had Ed given Mrs. X a sucrose solution instead of water, he might have helped her. What concentration of sucrose solution should he have administered to leave her rbc volume unaffected?