Page 250 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Respiratory Toxicity Chapter | 13 217

VetBooks.ir alveolar epithelium and vascular endothelium by diffusion network of capillaries is said to resemble a miniature
underground parking garage.
along the gas concentration gradient. Carbon dioxide-
enriched air is expelled upon expiration. Total lung capac-
ity refers to the volume of air in inflated lungs. Some Diffusion
volume of air remains in the lungs following maximum
For some gases for which the blood has a large carrying
expiration, known as the residual volume. The volume of
capacity, such as carbon monoxide (CO), the rate of
air moved in and out during breathing is called the tidal
uptake into the blood is limited by the blood gas barrier.
volume (TV), while vital capacity (VC) refers to the vol-
Thus, the gas exchange is described as diffusion limited.
ume of air moving in and out of the lungs during maxi-
For certain other gases, such as nitrous oxide, which do
mum inhalation and expiration. Oxygen delivery to the
not bind to or are taken up by the red blood cells, uptake
blood can be increased by increasing the TV, the respira-
is not limited by diffusion, but by the available blood vol-
tory rate, or the oxygen concentration in the inspired air.
ume provided by alveolar perfusion. These gases are
TV has a fraction in the conducting airways that does not
described as perfusion limited. Oxygen takes the middle
exchange gas, known as the dead volume (West, 2000a).
road so that its uptake is dependent on the blood gas bar-
Anatomic dead space refers to the volume of the conduct-
rier characteristics (diffusion limited), as well as the blood
ing airways to where oxygen becomes diluted (Fowler’s
volume (perfusion limited). If the alveolar wall becomes
method). Physiologic dead space refers to the portions of
abnormally thick, e.g., when collagen is deposited in the
the airways that do not contribute to the exchange of car-
interstitium or with the accumulation of interstitial fluid
bon dioxide (Bohr’s method; West, 2000a). As the breath-
(edema), the oxygen uptake rate across the barrier is
ing pattern becomes shallower and more rapid, the
reduced. Alternatively, and more commonly, a failure to
fraction of the noncontributing dead volume in each
match ventilation with perfusion is the cause of poor gas
breath increases. Although we often assume that all
exchange (West, 2000b). If a lung is heavily perfused
regions of the lung are ventilated equally, positional dif-
with minimal ventilation because of a blocked airway, the
ferences are seen in humans. Such differences are mini-
amount of oxygen that can be exchanged is limited by the
mized when humans are in the supine position. In dogs,
mismatch of poor ventilation with good perfusion.
the anterior main bronchi receive more ventilation than
Alternatively, gas exchange is compromised if the lung is
do the rear ones.
efficiently ventilated, but receives little or no perfusion.
The water solubility of gases influences how deeply
Both conditions are referred to as ventilation perfusion
they penetrate into the airways and terminal lung struc-
mismatches.
tures. Highly water-soluble gases, such as SO 2 , do not
penetrate deeply. Less water-soluble gases, including
NO 2 , ozone, CO, and H 2 S penetrate to the deepest lung Avian Respiration
structures. There are many morphologic, physiologic, and mechani-
Ozone and oxides of nitrogen and sulfur were modeled cal differences between the avian and mammalian respira-
for absorption throughout the respiratory tract (Tsujino tory systems (Brown et al., 1997). In birds, the gas
et al., 2005). All three gases had higher concentration in exchange unit is the parabronchus. The parabronchus has
the airways. For example, ozone was 3 12 times higher no alveoli. The walls of the narrow passageways called
at the 5th generation bronchus. Sensitivity analysis indi- air capillaries serve as the gas exchange surface. Inhaled
cated that TV, respiratory rate, and surface area of the air reaches the parabronchus via the upper air passage-
upper and lower airways significantly affected the results. ways. Oxygen is extracted as it moves along the air
Kinetics of inhaled gaseous substances vary substantially capillaries. After passing through the parabronchus the air
among animals and humans, and such variations are, at is not expelled directly, but moves on to the air sacs.
least partially, the result of anatomical and physiological From there the air is moved back through the para-
differences in their airways. bronchus before it is expelled. The parabronchus does not
expand and contract like mammalian lungs. Instead, air
movement is controlled by expansion and compression of
Perfusion the air sacs. The parabronchus air sac system provides
There are two different types of blood supply to the lung: two opportunities to extract oxygen from inhaled air. In
nutrient vessels that provide nutrition for the lung, and addition, counter flow between blood and air in the para-
pulmonary vessels that specialize in exchanging alveolar bronchus provides an effective oxygen concentration gra-
oxygen onto the hemoglobin to be carried to the target dient over a larger fraction of the gas exchange surface
organs (West, 2000a). The capillaries are just large area. Partially oxygenated blood comes into close proxim-
enough to admit erythrocytes and they are very short. ity to the air with the highest oxygen concentration, while
Blood flows around the alveoli as a sheet, and the air with lower oxygen pressure comes into close