Handbook of Solvents - George Wypych - ChemTech - Ventech!

1082 Myrto Petreas
cients of some industrial solvents were shown to increase by up to 60% after meals, 4 presumably
due to an increase in lipid content of the blood.
Depending on the affinity of a gas or vapor for blood, the transfer from the alveoli to
the capillaries may be diffusion-limited or perfusion-limited. The difference can be illustrated
by examining two solvents, styrene and methyl chloroform. Because of the high solubility
of styrene (λ=52) 11 large amounts of it can be taken up by the blood, and the transfer
is only diffusion-limited. On the other hand, methyl chloroform is not very soluble (λ=1.4) 12
and its partial pressure rises rapidly to that of the alveolar air, at which point no net transfer
takes place. The amount of methyl chloroform taken up by the blood will depend exclusively
on the amount of available blood flow and not on the diffusion properties of the
gas-blood interface. This kind of transfer is perfusion-limited.
It is the concentration in the mixed venous blood, however, that better reflects the concentration
at the target organ. It has been shown that: 13
λ
P / Pv =
λ+
V / Q
where:
P partial pressure in the alveoli and in the arterial blood leaving that region of the lung
Pv partial pressure in the venous blood coming to the lung
λ blood/air partition coefficient
V alveolar ventilation
Q blood flow of the lung region under consideration
The greater the blood/air partition coefficient, the closer the arterial concentration will
be to the venous concentration.
The concentration in the alveolar air will reflect both arterial and venous blood if the
blood/air partition coefficient of the vapor is greater than 5, 14 a criterion that applies to most
industrial solvents. In a normal subject at rest, the ventilation-to-perfusion ratio (V/Q)
ranges from 0.7 to 1.0, with an average of about 0.9. 15 Variations in the V/Q ratio exist in
various parts of the lung because not all alveoli are ventilated and perfused in ideal proportions.
The V/Q ratio may vary from 0.5 at the lung base to 3.0 at the apex, 16 and it becomes
more homogeneous with physical exertion. 17 When the partition coefficient exceeds 10,
however, even large degrees of ventilation/perfusion imbalance have very little effect on
the relationship between alveolar and mixed venous solvent partial pressures. 14
15.2.1.3.3 Gas exchange
When a person is exposed to a volatile organic solvent through inhalation, the solvent vapor
diffuses very rapidly through the alveolar membranes, the connective tissues and the capillary
endothelium and into the red blood cells or plasma. With respiratory gases the whole
process takes less than 0.3 seconds. 15 This results in almost instantaneous equilibration between
the concentration in alveolar air and in blood and, therefore, the ratio of the solvent
concentration in pulmonary blood to that in alveolar air should be approximately equal to
the partition coefficient. As the exposure continues, the solvent concentration in the arterial
blood exceeds that in the mixed venous blood. 18 The partial pressures in alveolar air, arterial
blood, venous blood and body tissues reach equilibrium at steady state. When the exposure
stops, any unmetabolized solvent vapors are removed from the systemic circulation through
pulmonary clearance. During that period the concentration in the arterial blood is lower than
in the mixed venous blood 18 and the solvent concentration in alveolar air will depend on the
pulmonary ventilation, the blood flow, the solubility in blood and the concentration in the