Page 378 - Anatomy and Physiology of Farm Animals, 8th Edition

The Respiratory System / 363

In normal animals a soft rustling alveolar air consists of the endothelial cell
of pulmonary capillaries, a type I squamous
VetBooks.ir sound associated with air movements epithelial cell lining the alveoli, and a fused
may be heard with a stethoscope.
basement membrane contributed by both
Abnormal lungs, for example lungs with
abnormal amounts of fluid in the air- cells (Fig. 19‐13). Gases readily move back
ways, may produce exaggerated sounds and forth across this very thin and fragile
termed rales. Pleuritis (pleurisy), or structure. Any abnormality that thickens
inflammation of the pleura, may pro- this barrier (e.g., pulmonary edema with
duce a rasping sound because of the an accumulation of extracellular fluid in
roughened surfaces rubbing together. the alveolar wall) can greatly reduce the
Hydrothorax (fluid in the pleural cavity) efficiency of exchange. Oxygen exchange
may also result from pleuritis. is usually affected first because its
solubility is much less than that of carbon
Gas Exchange dioxide.
Exchange begins as soon as blood enters
a pulmonary capillary from pulmonary
The oxygen and carbon dioxide concentra- arterial vessels and continues until equilib-
tions in air can be described in two ways: rium between alveolar air and plasma is
partial pressures and percentages. Room reached. Figure 19‐14 shows typical values
air is approximately 21% oxygen and 0.3% for partial pressures of oxygen and carbon
carbon dioxide. The primary component dioxide in alveolar air and a pulmonary
of room air is the inert gas nitrogen (about capillary. Plasma entering pulmonary cap-
78%). The partial pressure of an individual illaries from the pulmonary arteries con-
gas in a mixture of gases is the product of tains the highest concentration of carbon
the percentage of the individual gas in the dioxide and the lowest of oxygen. Because
mixture and the total barometric or of the continuous gas exchange in the pul-
atmospheric pressure. Thus, at sea level, monary capillaries, alveolar air contains
where atmospheric pressure is 760 mmHg, less oxygen and more carbon dioxide than
the partial pressure of oxygen in room air inspired air (PO of 160 mmHg and PCO
is approximately 160 mmHg. of 0.23 mmHg for inspired air). 2
2
The partial pressure of an individual gas
To be most efficient, the rate of pulmo-
in a mixture is one factor that determines nary artery blood flow into an area of the
the amount of the gas that will dissolve in a lung must be balanced with the rate of air
liquid (such as blood plasma). The partial movement in and out of the alveoli in the
pressure of a gas in a mixture can be viewed same area. To appreciate the importance of
as the driving force that moves molecules this balancing, consider an extreme case in
of an individual gas from the air into the which one lung is collapsed so that air move-
liquid when the liquid is exposed to a gas ment is impossible, but the collapsed lung
mixture. Thus, because partial pressures receives the same amount of blood flow as
depend on both total atmospheric pressure the inflated normal lung. The lack of airflow
and the percentages of individual gases, in the collapsed lung means that any alveolar
both of these factors also determine the air is stagnant and contains high levels of
amount of an individual gas that can be carbon dioxide and low levels of oxygen.
dissolved in a liquid. The unit of measure- Gas exchange in the collapsed lung does not
ment of the amount of a gas dissolved in a occur, and levels of oxygen and carbon diox-
liquid is in millimeters of mercury.
ide are the same as those entering. The blood
Gas Exchange in the Lungs. Gas exiting the collapsed lung mixes with an
exchange between the blood and alveolar equal volume of blood from the intact lung,
air in the lungs occurs across the walls of so that blood re‐entering the heart from the
alveoli. At its thinnest point, the alveolar pulmonary circulation is deficient in oxygen
wall barrier between blood plasma and and has an excess of carbon dioxide.