User manual
There are more factors influencing the decrease of the radiation (light) intensity in a
tissue. The thickness of the tissue the radiation (light) has to get through has the greatest
influence on the decrease of the radiation’s (light’s) intensity. Further decrease of intensity is
caused by the radiation (light’s) absorption in venous and arterial blood which passes through
the tissue. The volume of arterial blood can be divided into the volume which is constant in
the arteries, and the so called pulsing volume, which is dependent on the heart muscle’s
pulsations. The absorption of radiation (light) transmitted through the tissue can thus be
divided to a direct component (DC), given by the thickness of the tissue and the amount of the
fixed volume of venous and arterial blood, and to an alternating component (AC), given by
the volume of the pulsating arterial blood, see Fig. 4.2. These pulsations can be depicted via
the so called plethysmographic curve, which in reality depicts the change in intensity of the
transmitted radiation (light) in dependence on the change of the pulsating arterial blood’s
volume.
Fig. 4.2: Transparency of a live tissue with pulsating arterial blood.
As we are interested in the oxygen saturation of the arterial blood, it will be suitable to
only focus on the alternating component of absorption caused by the pulsating arterial blood.
Oxygen transport in blood is realized via two mechanisms. The first one of these is a state
when the oxygen is freely dissolved in the blood plasma. The amount of this oxygen, which is
very small, is directly dependent on the oxygen partial pressure in the alveoli
2
O
A
P and on the
oxygen solubility in blood (at
2
O
A
P = 13.33 kPa, there is about 1.4 % of blood O
2
physically
dissolved). The second mechanism of oxygen transport in blood is based on the oxygen bond
with hemoglobin, which is a protein (chromoprotein) located in the erythrocytes. One
hemoglobin can bind up to four molecules of O
2
. Depending on whether or not the oxygen is
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