MadSci Network: Medicine
Query:

Re: What is the viscosity of blood?

Date: Sat Mar 20 11:56:50 1999
Posted By: Nicole Davis, Grad student, Department of Genetics, Harvard Medical School
Area of science: Medicine
ID: 920476394.Me
Message:

Hi Kevin!

Thanks for your questions!  You have touched on a very exciting area of 
active research.

Blood is essential for life.  The main function of the circulating blood is 
to carry oxygen and other nutrients through the body and to remove carbon 
dioxide and waste products.  However, blood also transports other 
substances, such as hormones, white blood cells, and platelets, to sites 
within the body where they are needed.  Thus, blood is also important for 
hemostasis (the prevention of blood loss when the vascular system is 
damaged) as well as immune defense.  

Blood is a suspension of red blood cells (erythrocytes), white blood cells 
(leukocytes) and platelets in a complex solution (called plasma) of gases, 
salts, proteins, carbohydrates, and lipids.  The viscosity, or internal 
friction, of blood increases as the percentage of cells in the blood 
increases: more cells mean more friction, which means a greater viscosity. 
 The percentage of the blood volume occupied by red blood cells is called 
the hematocrit.  With a normal hematocrit of about 40 (that is, 
approximately 40% of the blood volume is red blood cells and the remainder 
plasma), the viscosity of whole blood (cells plus plasma) is about 3 times 
that of water.  On the other hand, the viscosity of plasma alone is about 
1.5 times that of water.  Although the concentrations and types of proteins 
in the plasma can affect its viscosity, this has little if any effect on 
the overall viscosity of whole blood.  When the hematocrit rises to 60 or 
70, which it often does in patients with polycythemia, or abnormally high 
red blood cell counts, the blood viscosity can become as high as 10 times 
that of water.  Alternatively, when the hematocrit falls drastically, as it 
does in patients with anemia (a decreased number of red cells in the 
blood), blood viscosity can approach that of plasma alone.  If you are 
interested in the raw numbers, the coefficient of viscosity for water is 
0.001 Newton-second per meter squared (N-s/m2) at 20 degrees Celsius.  The 
coefficient of viscosity for whole blood at 37 degrees Celsius is 0.0027 
N-s/m2. 

Ideally, a blood substitute would closely mimic the properties of real 
blood while offering the advantages of immediate and wide availability, 
safety from blood-borne diseases, and ease of storage.  These substitutes 
may, in theory, exhibit fluid characteristics that are similar to whole 
blood, but they should, above all, assume the functional properties of real 
blood.  While blood does many things, the blood substitutes that are 
currently being designed can do only one thing: transport oxygen and carbon 
dioxide.  For this reason, these blood substitutes are usually described as 
oxygen carriers.  Currently, there are two types of oxygen carriers that 
are being developed: one is based on hemoglobin, and the other on 
perfluorochemicals.  Hemoglobin-based oxygen carriers utilize the body's 
endogenous oxygen-carrying protein: hemoglobin is a protein within red 
blood cells that is responsible for carrying oxygen from the lungs to the 
various tissues in the body.  On the other hand, perfluorochemicals are 
synthetic materials in which oxygen can be readily dissolved.  If these 
products can be successfully implemented, their major utility lies not in 
the replacement of whole blood, but rather in short-term replenishment for 
patients with acute blood loss (e.g. trauma victims, surgical patients, 
etc.).      

I hope this information is helpful!  Please feel free to email me with any 
further questions.  I have listed some useful resources below.

Nikki
nmdavis@fas.harvard.edu

References

I.  General texts for information about the properties of blood:

Guyton, A. C. and Hall, J. E. Textbook of Medical Physiology, ninth 
edition. 1996

Berne, R. M. and Levy, M. N. Principles of Physiology, second edition. 1996

II. Oxygen carriers:

There is a lot of information regarding blood substitutes on the Web.  I 
would suggest searching under the terms 'blood substitute' or 'artificial 
blood'.  Here are two of the sites that I found most helpful:

http://www.sciam.com/askexpert/medicine/medicine20.html

http://www.people.virginia.edu/~rjh9u/blood.html




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