Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.2.1 Use of breath monitoring 1081
The airways consist of a series of tubes that become narrower and shorter as they extend
deeper into the lung. The first segment, the trachea, divides into two main bronchi,
which in turn divide into lobar and segmental bronchi, which turn into terminal bronchioles.
All these segments collectively constitute the anatomical dead space with a volume of about
150 mL for an adult male. The function of this conducting portion of the lung is to transport
the inspired air into the gas exchange regions. The terminal bronchioles divide within a distance
of 5 mm into respiratory bronchioles and alveolar ducts, which are completely lined
with alveoli. This region of the lung is the respiratory zone where the gas exchange takes
place. The volume of this zone (about 2,500 mL) makes up most of the lung volume.
15.2.1.3.1 Ventilation
Air is drawn into the lungs by contractions of the diaphragm and the intercostal muscles,
which raise the rib cage, and flows to the terminal bronchioles by bulk flow. After that point,
the velocity of the inspired air diminishes as the cross sectional area of the airways increases
dramatically. Thereafter, ventilation is carried out by molecular diffusion, which results in
rapid exchange of gases.
Similarly to the airways, the pulmonary blood vessels form a series of branching tubes
from the pulmonary artery to the capillaries and back to the pulmonary veins. The diameter
of a capillary segment is about 10 μm, just large enough for a red blood cell. The capillaries
form a dense network in the walls of the alveoli, with the individual capillary segments so
short that the blood forms an almost continuous sheet around the alveoli, providing ideal
conditions for gas exchange. In about one second, each red blood cell transverses two or
three alveoli in the capillary network, achieving complete equilibration of oxygen and carbon
dioxide between alveolar gas and capillary blood.
For an adult male at rest, a typical breathing frequency is 15 breaths per minute and a
typical exhaled air volume (tidal volume), at rest, is about 500 mL. The total volume of air
exhaled in 1 min is, therefore, about 7,500 mL. This is the minute volume total ventilation.
The inhaled volume during the same time is slightly greater, since more oxygen is taken up
than carbon dioxide released. Not all the inhaled air reaches the alveoli; the anatomical dead
space (volume of conducting airways where no gas exchange takes place) contains approximately
150 mL of each breath. Therefore, the volume of fresh inhaled air that reaches the alveoli
in 1 min is 5,250 mL [(500-150) mL/breath * 15 breaths/min = 5,250 mL/min]. 8 This
is the alveolar ventilation and it represents the volume of fresh air available for gas exchange
in 1 min. The alveolar ventilation cannot be measured directly, but it can be calculated
by measuring the minute volume and subtracting the anatomical dead space. The
former is easily measured by collecting all the expired air in a bag. The volume of the anatomical
dead space can be assumed to be 150 mL, with minimal variation for all adults, and
the dead space ventilation can be calculated given a respiratory frequency. Subtraction of
the dead space ventilation from the total ventilation (or minute volume) results in the alveolar
ventilation. Pulmonary ventilation may be affected by chronic diseases such as asthma
or pulmonary fibrosis, or even by transient hyperventilation or hypoventilation.
15.2.1.3.2 Partition coefficients
Whereas the rate of transfer of a solvent vapor between alveolar air and capillary blood is
determined by its diffusivity, the equilibrium between these matrices is determined by the
blood/air partition coefficient (λ). This is the ratio of the concentration of the vapor in blood
and air at 37 o C, at equilibrium. Partition coefficients are commonly determined in vitro, and
occasionally in vivo. 2,9,10 Although considered constant at a particular temperature, partition
coefficients may be affected by the composition of the blood. The blood/air partition coeffi-