Page 376 - Anatomy and Physiology of Farm Animals, 8th Edition

The Respiratory System / 361

which normal gas exchange cannot occur. pleural pressures is defined as the transpul-
monary pressure, and is a measure of the
It is important to recognize that the
VetBooks.ir observed respiratory rate of an animal may elastic forces in the lungs that tend to col-
lapse at expiration. When there is no air
not accurately reflect actual alveolar venti-
lation. Similar to the animal that is panting, movement in or out of the lungs, the alveolar
an animal that is ‘hyperventilating’ is in pressure is said to be equal to atmospheric
fact exhibiting rapid, shallow respirations pressure (0 cm H O). During inspiration,
2
that are not effectively exchanging gas expansion of the thoracic cavity pulls out-
within the alveoli. ward on the lungs, making pleural pressure
As described earlier, there is no physical and alveolar pressure more negative, draw-
connection between the visceral and parietal ing air into the lungs and increasing lung
pleural surfaces (except at the hilus), and the volume (Fig. 19‐12). The opposite is true of
closed pleural cavity between them is a expiration and the rise in alveolar pressure
potential space filled with a small amount of helps to expel air from the lungs. Expansion
fluid. Pleural pressure is the pressure of the increases the volume of the air passages
fluid in the pleural cavity, which is always in the lungs but reduces the pressure in
slightly more negative than alveolar pres- them. The inverse relationship between
sure (Fig. 19‐12). Alveolar pressure is the volume and pressure of a gas is described
pressure of the air inside the alveoli and it is by Boyle’s law.
these pressures that exert a pulling force Enlargement of the thoracic cavity is
along the pleura to keep the lungs expanded. accomplished by contraction and flatten-
The difference between the alveolar and ing of the dome‐shaped diaphragm and a

Lung volume
0.50
Volume change (liters) 0.25

0.0

Alveolar pressure
+ 2

0

Pressure (cm H 2 O) –2 Transpulmonary pressure

–4

–6

Pleural pressure
–8

Inspiration Expiration

Figure 19-12. Changes in lung volume, alveolar pressure, pleural pressure, and transpulmonary pres-
sure during normal breathing. Source: Guyton and Hall, 2006. Reproduced with permission of Elsevier.