Biomedical Engineering Reference
In-Depth Information
plasma are the globulins, which account for approximately 35% of the plasma protein con-
centration. Antibodies (immunoglobulins) and transport globulins are the major constitu-
ent of this portion of blood. Antibodies play a role in immunology and transport globulins
can bind to hormones, metallic ions, and steroids, among others, to transport these mole-
cules throughout the body. The remaining plasma protein composition is made up of
fibrinogen (approximately 3%) and all of the other proteins. Fibrinogen is the precursor to
fibrin and forms a mesh during clot formation, as discussed previously. The other plasma
proteins have a wide range of functions but include insulin, thyroid-stimulating hormone,
coagulation proteins, and many others.
The liver is the primary organ that synthesizes plasma proteins, including albumin,
fibrinogen, and some of the hormones. Antibodies are produced by white blood cells as
discussed earlier. The other components of plasma are typically synthesized in cells, trans-
ported between cells, or ingested through the digestive system. The relative concentration
of these components is regulated tightly, but can change relatively fast depending on
many factors. For instance, after eating a candy bar, it is probable that your blood glucose
level increases significantly. However, under normal conditions, the body regulates this
concentration closely, and the blood glucose level should return back to its normal level
rather quickly.
Plasma does not have a function per se because it is an amalgam of many different bio-
logically active components. In general, the water portion of plasma is used to reduce the
viscosity of the cellular component of blood. This in effect reduces the resistance to flow
and even can be considered to allow for blood flow to occur (i.e., cellular matter itself
would not “flow”). A major function of the remaining plasma components is to maintain
equilibrium with the interstitial space, which aids in homeostasis. Sugars are used as the
nutrient source for cells. Cholesterol can be used within the cell membrane to increase its
rigidity so the cell can withstand forces better. Proteins have specific functions and each
protein may have a different task. Therefore, plasma in effect has a very critical function
for the human body. Serum is a special component of plasma, in which the protein fibrino-
gen has been removed. This is most likely due to clot formation, but can be made in the
lab to study various properties of plasma.
5.4 BLOOD RHEOLOGY
Rheology is the study of flowing materials combined with the study of the physical
properties of the particular material. In this section, we will discuss a more accurate way
to model the viscosity of blood. As we have seen in previous sections, blood is a special
non-Newtonian fluid that is also composed of at least two phases. Blood is a combination
of cells, proteins, and electrolytes, among other compounds, all suspended in a fluid
medium. Plasma (the portion of blood not including cells) behaves as a Newtonian fluid
and has a viscosity of approximately 1.2 cP. Whole blood, however, does not have a con-
stant viscosity. The viscosity of whole blood varies with shear rate, hematocrit, tempera-
ture, and disease conditions (
Figure 5.6
), and this is predominantly due to the presence of
cells and other compounds within the fluid. There have been many investigations into the
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