Adv Physiol Educ ADInstruments
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Advan. Physiol. Edu. 26: 134-135, 2002;
1043-4046/02 $5.00
This Article
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kumar, S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Kumar, S.
ADV PHYSIOL EDUC 26:134-135, 2002
© 2002 American Physiological Society

ILLUMINATIONS

An analogy for explaining erythrocyte fragility: concepts made easy

Sanjay Kumar

Department of Physiology, B.P. Koirala Institute of Health Sciences, Dharan, Nepal, (Email: sanjaykr{at}lycos.com)

Undergraduate medical students often find it difficult to understand the phenomenon of osmotic fragility. An osmotic fragility test primarily indicates the surface area-to-volume ratio (SAVR) of red blood cells (RBC). Osmotic fragility of RBCs is defined as the ease with which the cells burst in hypotonic solutions and is expressed in terms of the concentration of the saline solution in which the cells are hemolyzed (Fig. 1).



View larger version (22K):
[in this window]
[in a new window]
  		FIG. 1 The figure shows a normal osmotic fragility curve with right and left shifts in spherocystosis and leptocytosis, respectively. The normal cell swells when placed in a 0.6% saline solution. Normally hemolysis begins at 0.42%, when the red blood cell (RBC) volume increases to ~145% of normal. In a 0.35% saline solution, the cells are fully hemolyzed. RBC volume at this concentration, before it bursts, is about 165% of normal (critical hemolytic volume).

 
To demonstrate this concept, one can use a simple method. Take three small polyethylene bags (4 x 4 in.) to the class. Blow air into one of the bags to fill the bag to the maximum and tie the open end with a thread. Label it "S". Similarly prepare bags one-half and one-quarter filled with air. Label them "N" and "L", respectively. Ask the students to imagine bag N as a normal RBC, bag S as a spherocyte (enlarged RBC), and bag L as a leptocyte (flattened RBC). Draw diagrams for all the bags on the board.

Now, place these bags on the table. Take the S bag and put maximum pressure on it with your palms. In this analogy, the mechanical pressure applied on the bag simulates an osmotic challenge to a hypotonic exposure. The applied mechanical pressure is increasing the pressure inside the bag, which is analogous to the pressure inside an RBC that is exposed to a hypotonic solution. The bag will quickly burst, as it has less SAVR. Repeat the procedure with bags N and L. Bag N will bulge out and resist bursting. This is because of a greater SAVR. Similarly, bag L, because of a higher SAVR, will further resist bursting to pressure compared with bag N.

On the same board, draw an osmotic fragility curve. Point the drawing of bag N to the normal curve. Similarly point the bag S to the right of the curve and bag L to the left of the curve (Fig. 1).

Explain the SAVR in all three states. The osmotic fragility of RBCs reflects their ability to take up a certain amount of water before lysing. This is determined by their SAVR. The ability of the normal RBC to withstand hypotonicity results from its biconcave shape, which allows the cell to increase its volume by ~70% before the surface membrane is stretched. Once this limit is reached, lysis occurs. Emphasize at this point that an RBC cannot accommodate more than its capacity by overstretching.

Now it will be easy for you to explain the abnormal states. A spherocyte (bag S) will burst at higher saline concentrations. Here, SAVR is less, even before the osmotic (mechanical) challenge is given. The RBC’s ability to take in water before stretching the surface membrane is thus more limited than normal, and it is, therefore, particularly susceptible to osmotic lysis. Thus, in this case, the osmotic fragility curve will shift to the right. That is, the lysis will start at an even lower hypotonic challenge. A leptocyte (bag L) has a higher SAVR to begin with. Thus it will accommodate the osmotic challenge of lower saline concentrations and will burst after taking more water. As a result, there will be a shift to the left in the curve. That is, the lysis starts at a higher hypotonic challenge.

This analogy will definitely enhance the students’ understanding of the concept of osmotic fragility, not only in terms of factual recall, comprehension, and analysis, but also in its application and synthesis.





This Article
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kumar, S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Kumar, S.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online