Our blood is made up of two different components. Plasma comprises
55% of the blood volume and is a yellowish substance composed of
91% water and 9% of other substances (chemicals, proteins, and hormones). The other constituent of our blood is cellular portion.
Cellular portion makes up the other 45% of blood volume and consists of white blood cells, red blood cells, and platelets.
Hematocrit (Hct) is the volume of red cells as compared to the volume of the whole blood sample and is reported:
φ
V rb
=
+ V p
(1)
V rb
where:
φ
- hematocrit; Vrb- volume of red blood cells; Vp – volume of plasma
The procedure of measuring hematocrit involves obtaining a blood sample and then centrifuging the blood. Centrifuge allows for the separation of different blood constituents. Normal ranges for hematocrit are dependent on age.
•
•
•
•
•
•
•
•
Newborns: 55-68%
One (1) week of age: 47-65%
One (1) month of age: 37-49%
Three (3) months of age: 30-36%
One (1) year of age: 29-41%
Ten (10) years of age: 36-40%
Adult men: 42-54%
Adult women: 38-46%
Another method of hematocrit measurement is the Maxwell’s method - determining hematocrit by measuring conductivity. Conductivity is the ability of a solution to conduct electricity.
Electrical current will increase in proportion to the number of ions (or charged particles) found in a solution, to their electrical charge, and mobility, i.e. how easily the ions can move in a solution.
The mobility of an ion in a solution will also depend on how many cells of different sizes and shapes are suspended in the solution.
Both erythrocytes and plasma have specific electrophysical properties. Membrane of an erythrocyte is electrically insulating, mainly due to its content of lipids, so that it appears essentially non-conducting. Resistivity of blood cells is approximately 10
10
Ω m. Plasma is fairly conductive due to its content of electrolytes and charged proteins; with Na+ as a major contributor to plasma conductivity. Accordingly, there exists an inverse relationship between electrical conductance and the hematocrit in blood, when the concentration of charged particles is taken into account.
1

HEMATOCRIT MEASUREMENT.
Concentration of dielectric particles in a conductive solution can be estimated if the shape and size of the particles is known. This fact has been used in commercial blood analyzers to determine the hematocrit.
Hugo Fricke conducted theoretical studies of electric conductivity of a suspension of spheroids
(Fricke, 1924). When applying this method to the conductivity of blood, we obtain what is called the Maxwell – Fricke equation:
ρ
ρ
ρ
1
ρ
1
1
+ 2
=
φ ⋅
ρ
1
ρ
ρ
ρ
1
2
2
1
+ 2
(2) where:
ρ - resistivity of blood
ρ
1
- resistivity of plasma
ρ
2
- resistivity blood cells
φ
- hematocrit
The resistivity of plasma ρ
1 in comparison with the resistivity of blood cells ρ
2
( ρ
1
<< ρ
2
) is very low, the ρ
1
/ ρ
2
value is less than 1, so it can be omitted. The Maxwell equation for the blood is:
φ
=
2
⋅
(
2
⋅
ρ
ρ
-
+
ρ
ρ
1
1
)
(3)
Hematocrit measurement with the Maxwell's method comes down to determining the resistivity of blood and plasma. The electrolyte resistance is determined by means of the Kohlrausch measuring bridge, whose diagram is presented below:
C
A
R
X
L
1
µ A
G
S
R o
L
2
B
R x
- unknown resistance
R
0
- known resistance (82 Ω )
µ A-ammeter
L
1
,L
2
- position of slide contact
S on the scale
G - alternating current generator
Fig. 1. Diagram of the Kohlrausch measuring bridge
2

The bridge is powered with alternating current, otherwise the application of the direct current in the electrolyte would initiate electrolysis. While performing the electrical measurement of blood remember that the intensity of current passing through the measuring cuvette may not exceed the value at which the structure of blood cells can be permanently destroyed.
Measurement of resistance with the Kohlrausch bridge consists in bringing the system to the state of equilibrium by setting the slide contact S in a position in which current does not flow through the ammeter. At that point the potentials of points C and S (fig. 1) are equal. By analogy to the Wheatstone’s bridge, the following dependence is valid in the state of equilibrium:
R
R x
0
=
R
R
2
1
L
=
L
2
1 (4) hence:
R x
= R
0
⋅
L
L
2
1 (5) where: R x
– resistance of the analyzed liquid
R
1
– resistance of the wire of length L
1
R
2
- resistance of the wire of length L
2
R
0
– known resistance (82 Ω )
The dependence between the resistance of liquid R x
and its resistivity can be expressed as:
R x
=
ρ x
⋅
C (6) where: C – constant of the measuring cuvette.
Then, for blood and plasma, the following dependences are valid:
R =
ρ ⋅
C (7a)
R
1
=
ρ
1
⋅
C (7b)
Constant of the measuring cuvette C can be derived by measuring resistance of the standard substance R wz
( 1/50 N solution of KCl ) with known resistivity of
ρ wz
= 4.2
Ω m.
C =
R wz
ρ wz
(8)
Substituting the values of blood and plasma resistivity from formula (7a) and (7b) to formula (3) we may calculate the value of hematocrit.
EXPERIMENTAL PROCEDURE
3

Microhematocrit Method
1.
Plug in the centrifuge and wait for the autotest, which indicates that the centrifuge is ready to start processing.
2.
Fill two capillary tubes with the well-mixed blood up to three-quarters full and seal by inserting them into the sealing putty. The length of the sealing should be 1 to 2 mm.
3.
Insert the capillary tubes into the microhematocrit centrifuge placing them in the seats symmetrically opposing one another.
4.
Screw the rotor cover onto the rotor and close the centrifuge lid.
Caution: Operate the centrifuge in the presence of the assistant. Never operate the centrifuge with the lid open or the rotor missing.
5.
Press the „start/stop” button to start centrifuging.
6.
Wait for 2 beeps, which indicate that centrifuging is completed. If you notice any irregularities in the centrifuge operation, press the START/STOP button. Do not move the centrifuge when it is working; wait until the rotor stops.
7.
To open the centrifuge after it has completely stopped, press the „lid” button.
8.
Open the lid and remove samples.
9.
Turn off the main power switch, unplug the centrifuge.
10.
Read the results using the hematocrit scale. While reading the hematocrit make sure the clay red cell interface is aligned with the 0% line and the bottom of the plasma meniscus is at the 100% line. The reading that corresponds to the top of the red cell column is hematocrit.
11.
Summarize the results in Table 4.
1.
Set the Kohlrausch bridge according to the diagram. Known resistance R
0
=82 Ω is placed in the same casing as the ammeter
2.
Pour the standard salt solution ( KCl) into the cylinder. Find the slide position in which the current does not flow through the ammeter.
3.
Rinse it with the PBS before filling it with another solution.
4.
Find values L
1
and L
2
for blood plasma. Repeat three times. Put the results in Table 2.
5.
Find values L
1
and L
2
for the blood. Repeat three times. Put the results in Table 3.
6.
Calculate the resistance of standard solution R wz
, of blood plasma R
1
and of the blood R according to formula (5) and put the results in the relevant tables.
7.
Calculate the constant C according to formula (8) and put the results in Table 1
8.
Calculate the resistance of plasma ρ
1
and blood ρ according to formula (7a) and (7b) and put the results in Tables 2 and 3.
9.
Calculate the hematocrit according to formula (3) and summarize the results in Table 4.
4

Table 1 Table 2
L
1
[m] L
2
[m] R wz
[ Ω ] ρ wz
[ Ω m] C[1/m
]
L
1
[m]
KCl
L average blood
Table 3 Table 4
L
1
[m] L
2
[m] R[ Ω ] ρ [ Ω m]
L average
L average
The report should include:
1. Diagram of the Kohlrausch measuring bridge.
2. Measurement tables, substitutions, calculation results.
3. Conclusions.
L
2
[m]
I method
φ [%]
R
1
[ Ω ]
φ
ρ
1
[
[%]
Ω
II method m]
5