Biomedical Engineering Reference
In-Depth Information
transiently appearing molecules. The lining of the vessels consists of
endothelial
cells
, which interact with the blood elements in such a way (for example by produc-
ing locally and diffusing into the blood stream certain constituents, if necessary) so
as the blood is in a fluid state under normal physiological conditions. In that sense,
the endothelial lining is considered as the perfect
nonthrombogenic
material. Blood
has a pH
7.40, a relative viscosity 2-4, relative density 1.05.
The
erythrocytes
are the most numerous cells in the blood. The average concen-
tration is 5
D
10
6
ml
1
and occupy on the average 40-45% of the total volume of
blood (this value is also known at the
hematocrit
). Each erythrocyte, at rest, looks
like a biconcave disc with central depressions on both sides, 2 mthick,8min
diameter, 90 m
3
in volume and 140 m
2
in surface area.
3
It is anucleated, con-
sisting of an hyperelastic membrane (able to withstand uniaxial deformation of
more than 100% elastically, but would rupture if its biaxial surface deformation,
i.e., the increase in its surface area exceeded 4-5%) which contains a hemoglobin
solution. In the context of erythrocyte-materials interactions, these cells do not
adhere to surfaces, as for example platelets do. However, under certain stress condi-
tions the erythrocytes may be damaged, their membrane may break with release of
hemoglobin. Erythrocyte particles may then adhere to surfaces via their membrane
glycoproteins. As erythrocytes contain significant amounts of
ADP
,whichisapow-
erful
platelet aggregating
agent, even low-grade hemolysis may induce indirectly
important blood (platelets)-material interactions. Another indirect way of influenc-
ing these interactions is through its flowing behavior. Because of their great numbers
and their deformability, they collide during flow with platelets, leukocytes and pro-
teins, driving them toward the surface. On the other hand, erythrocytes inhibit
the
complement
activation in blood by binding and metabolizing the complement
factor C3b.
The
leukocytes
(white blood cells) are nucleated cells. They are of various types
and their size is from about 6 m to more than 20 m in diameter, their num-
bers in blood are between 5 and 10.10
3
mm
3
. Many leukocytes leave the blood
stream, when certain chemical messengers “inform” them, pass squeezing between
endothelial cells and go to serve their purpose in inflammation, infection or wound
healing situations. There are two classes of white cells: (a) the
granulocytes
or
polymorphonuclear (PMN) ones, having inside them multilobe nuclei and gran-
ules and subdivided in
eosinophils
,
basophils
,and
neutrophils
. Neutrophils form the
largest part of all leukocytes (60-70%). (b)
agranular
white cells which are either
monocytes
(mostly phagocytic) or
lymphocytes
(the second largest group, 20-30%,
mostly involved in immunological reactions). The lifetime of leukocytes is from
hours to days. In terms of leukocyte-materials interactions the mechanisms of their
involvement are not very clear. However, it is known that they interact strongly
3
On average, therefore, 25.10
9
erythrocytes circulate in the human blood stream. The life span
of erythrocytes is approximately 120 days. Every second 3.10
6
erythrocytes are destroyed and
replaced with new ones. Each erythrocyte has 300.10
6
molecules of hemoglobin, the protein that
associates with 4 molecules of oxygen, O
2
. Every time our heart beats, approximately 75 ml of
blood is delivered to our body transferring
500.10
15
molecules of bound O
2
to the tissues!
