Page 377 - Anatomy and Physiology of Farm Animals, 8th Edition

362 / Anatomy and Physiology of Farm Animals

forward and outward movement of the mines the resistance to airflow. Much of
this smooth muscle has β ‐adrenergic
ribcage by the contraction of appropriate
VetBooks.ir thoracic muscles. These are skeletal mus- receptors that produce smooth muscle
2
relaxation when stimulated. Sympathetic
cles innervated by somatic motor nerves.
After inspiration, the pressure in the pleu- stimulation during exercise reduces resist-
ral cavity remains at its lowest point until ance to airflow and promotes alveolar
expiration begins and the thoracic cavity ventilation.
begins to return to its original volume. Airway smooth muscle constricts in
Expiration in a resting animal is a pas- response to stimulation by histamine or
sive process that does not require muscle leukotrienes. These substances are
contraction. Relaxation of the muscles released during allergic reactions
contracted during inspiration permits the from mast cells around airways. The
intrinsic elastic properties of the lungs and resulting decrease in airway diameter
the thoracic wall to recoil to their original increases the resistance to airflow, and a
volume. The start of expiration includes a more forceful skeletal muscle contrac-
rise in alveolar pressure so that it is greater tion is needed to move a given volume
than atmospheric pressure, and air is of air. This greatly increases the work of
forced out of the lungs. breathing. Heaves (also known as recur-
Forced expiration is an active process rent airway obstruction) is a condition
that forces more air from the lungs than in horses that is characterized by
would occur during a normal passive expi- labored or difficult breathing, and one
ration. Forced expiration requires contrac- potential cause is chronic exposure to
tion of abdominal muscles to force viscera environmental allergens, which results
against the diaphragm and contraction of in chronic obstructive airway disease.
other muscles to pull the ribs caudad. Both The chronic need for increased skeletal
of these actions reduce the size of the tho- muscle force may induce visible hyper-
racic cavity and permit recoil of the lungs trophy of the abdominal muscles (a
to a smaller volume than typical for resting heave line).
expiration. This causes a further increase Surface tension, a property of fluids,
in intrapulmonic pressure and forces more results from the cohesive forces that tend
air out than would occur with passive expi- to pull or hold the molecules of a fluid
ration. Even with the most forceful volun- together. An example is the tendency of a
tary expiration, a residual volume of air layer of fluid on a flat surface to come
still remains in the lungs. Vital capacity is together and form a bubble rather than
the maximal amount of air that can be remaining as a thin, flat layer. A thin layer
expired after a maximal inspiration. The of fluid lines the microscopic alveoli. The
feeling of having “the wind knocked out anatomical arrangement of the alveoli with
of you” is really the loss of residual vol- a thin layer of fluid present would normally
ume in the lungs. The lack of a residual cause the walls of the alveoli to be drawn
volume in the lung results in collapse of near each other and eventually collapse;
the alveoli and significant inspiratory however, the alveolar fluid contains a pul-
effort to re‐expand the lung tissue on monary surfactant, a combination of sub-
inspiration. stances that reduces surface tension. The
Both the upper airways and the air pas- reduction in surface tension promotes
sages in the lungs resist airflow through alveolar stability and makes alveolar expan-
them. Similar to a blood vessel, the resist- sion during inspiration easier. Production
ance of an air passage to airflow is inversely of pulmonary surfactant does not begin
proportional to its diameter. Most of the until late in gestation. Neonates born
air passages in the lungs have smooth mus- prematurely may have insufficient
cle in the walls, and the constriction or amounts of surfactant, which results in
relaxation of this smooth muscle deter- labored breathing.