MadSci Network: Medicine |
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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