A SMALL GROUP EXERCISE

The twelve small group exercises in Medical Physiology are integral components of the course and are highly rated.  The following is the first of these exercises:

Problem Set for Small Groups

Topic:  Body Fluid Compartments

Note:   the focus of the small group discussion will be on questions 4 and 5; answers to questions 1-3 will be given at the end of the session.

Independent Study Questions

1.     Two ml of a solution containing 6.3 gm% of Evans Blue are injected intravenously into a patient.  After a steady state was attained, the plasma concentration of Evans Blue was 0.03 mg/ml; the hematocrit was 40%.  Calculate the patient's plasma volume, red cell volume, and whole blood volume.

2.     A quantity of red blood cells is tagged with 59Fe.  Two ml of these packed red blood cells are then injected into a patient with each ml of packed red blood cells containing 4 X 106 counts per min.  At steady state, 1 X 103 counts per min per ml of whole blood were obtained and the patient’s hematocrit was 40%.  Calculate the patient's RBC volume, plasma volume, and whole blood volume.

3.     Calculate the plasma osmolarity following the ingestion of 1L of water.  Assume a 70 kg person, a time zero plasma osmolarity of 280 mosmoles/L, an initial plasma volume of 5% body weight, and a total body water of 60% body weight.

4.     A patient has an ECF volume of 14L, an ICF volume of 28L, a plasma volume of 3.5L, a whole blood volume of 6.3L, and a plasma osmolarity of 270 mosmoles/L.  A resident calculated that an injection of 280 mosmoles of NaCl in 100 ml of water (i.e., a 2800 mosmoles/L solution of NaCl) would restore the patient's plasma osmolarity to a normal value of 290 mosmoles/L.

       a) What would the patient's plasma osmolarity be following infusion of the 100 ml?

       b) Would there be a net movement of water between the ICF and ECF following the infusion?  How much and in what direction?

       c) Calculate the same series of events and explain the differences that would occur if 100 ml of water containing 280 mosmoles of urea were injected.

       Some points to assist you in approaching this problem:

       a)  The total number of osmoles in each body fluid space should be calculated from the osmolar concentration times the volume of that space.

       b)  Remember there is always an osmotic equilibrium between body fluid spaces; water moves very rapidly between the plasma compartment and the ISF and between the ISF and the ICF.  Therefore, the osmolar concentration in plasma (Pos) = ISFos = ICFos, i.e., you should assume a steady-state must exist since any difference would be very transient.

       c)  You should assume that NaCl is restricted to the ECF whereas urea will equilibrate between the ICF and the ECF (in the previous problem, water will also equilibrate between the ECF and ICF so that the entire body fluid volume must be used in that problem).

5.     As illustrated in the following figure, the plasma concentration of Evans Blue decreases following a vascular hemorrhage (the colloid osmotic pressure and the hematocrit also decrease).  Explain.

 Some points to assist you in approaching this problem:

       a) Explain the small decay in the plasma concentration of Evans Blue both before and after the hemorrhage.

       b)  An important concept to think about relates to the initial impact of blood loss on the plasma concentration of electrolytes such as Na or Cl, of non-electrolytes such as urea or the total osmolar concentration or other physiological variables such as the hematocrit.  Should these “concentrations” change as a result of hemorrhage?  Along these lines, if I have a liter of saline and I remove 500 ml what is the Na concentration in the remaining 500 ml?  If you are clear about your answer, you can then proceed to address the apparent inconsistency that the Evans Blue concentration, i.e. reflecting the protein concentration in plasma, decreases following a hemorrhage.  The following questions should help you address that point.

       c)  What happens to arterial blood pressure?

       d) What happens to capillary blood pressure?

       e)  What happens to sympathetic nerve tone?

       f)  What happens to capillary blood pressure in non-critical organs as a consequence of changes in sympathetic nerve tone?

       g)  What is the net effect of these changes on the balance of Starling forces in these capillaries?

       h)  What will be the impact of these changes on net fluid movement into or out of the capillary?

       i)   If there are changes in fluid movement, what will be the consequences on ISF volume and composition?

       j)   If there are changes in fluid movement, what will be the consequences on vascular volume and composition, including protein concentration and hematocrit?  Again would the loss of blood, per se, alter the plasma profile of any of these variables?

       k)  If there are changes in fluid movement, what will be the consequences on ICF volume and composition?

       l)   Based on the above considerations, what is the most appropriate fluid to give a patient that is hemorrhaging?

       m) Finally there are conditions during which more water than salt is lost and fluid will shift from the ICF into the ECF; you should be able to discuss the effects of sweating and prolonged dehydration due to insensible water loss through the lungs and skin on fluid movement.