Page 543 - Anatomy and Physiology of Farm Animals, 8th Edition

528 / Anatomy and Physiology of Farm Animals

avian larynx is a protector of the airway and to have eight or nine named air sacs. These
include cervical, clavicular, cranial, and
is not the organ of phonation in birds.
VetBooks.ir tracheal rings that, unlike the C‐shaped caudal thoracic and abdominal air sacs;
The avian trachea is constructed of
some of these are paired.
rings of the mammalian trachea, are com-
plete cartilaginous circles that are joined
by annular ligaments, themselves so short Ventilation and Gas Exchange
that adjacent rings may overlap. The tra-
chea bifurcates into the two primary Similar to domestic mammals, air move-
bronchi within the body cavity. The distal ment into and out of the respiratory tract of
end of the trachea and the first portion of birds requires skeletal muscle contraction.
the bronchi are modified into a uniquely However, as noted above, avian lungs are
avian organ, the syrinx. The syrinx is the not contained in a closed thoracic cavity
avian organ of phonation. Male ducks and and change little in size during ventilation.
swans possess an additional dilation off Gas exchange by diffusion does occur
the left side of the syrinx, believed to be a within the avian lung, but air moves through
resonator. the lungs as a result of pressure differences
The lungs of birds are unlobed and lie between the air passages within the lungs
adjacent to and deeply imprinted by the and the avian air sacs.
thoracic vertebrae. The primary bronchus Like domestic mammals, birds have
passes through the lung and at its caudal skeletal muscles that, based on their func-
border is continuous with the abdominal tion, are considered to be either inspira-
air sac (Fig. 30‐7). Within the lung, it gives tory or expiratory muscles. Contraction
off many small groups of secondary bron- of inspiratory muscles in birds results in
chi, which themselves give rise to hun- a cranial and ventral movement of the
dreds of parabronchi. Most of these are sternum and a cranial movement of ribs
arranged in tightly parallel loops, and of the thoracic cage. These movements
others form more irregular loops. These enlarge the body cavity in which the air
parabronchi are the site of gas exchange. sacs are located. The enlargement results
The secondary bronchi are also continu- in a lowering of atmospheric pressure
ous with other air sacs of the body cavity within both the cranial and caudal air sacs,
(Fig. 30‐7). and the resulting pressure gradient pro-
Air sacs are blind‐ended expansions of motes the movement of outside air into the
the respiratory tree, the walls of which are air sacs via the trachea and upper airways
composed of connective tissue covered (Fig. 30‐7). Contraction of expiratory
externally by serosa. They are lined by a muscles moves the sternum and ribs in the
simple squamous epithelium and are opposite direction to reduce the size of the
relatively poorly vascularized. In many body cavity, which increases atmospheric
locations, the air sacs surround and are pressure within the air sacs and promotes
attached to the organs within the body an outward movement of air. Unlike domes-
cavity. Some air sacs have diverticuli that tic mammals, in which expiration at rest is
extend into the medullary cavity of bones; typically a passive event associated with
bones thus modified are said to be pneu- relaxation of inspiratory muscles, a normal
matic bones. Domestic fowl, in which respiratory cycle in resting birds involves
flight is somewhat limited, have fewer the appropriately timed cyclic contraction
pneumatic bones than species with good and relaxation of both inspiratory and
flight capabilities. Fractures of pneumatic expiratory muscles. However, during both
bones can lead to air sac infection. inspiration and expiration, air also flows
Depending on whether or not certain through the lungs in a caudal to cranial
compartments are considered separate direction via parabronchi. The unidirec-
sacs, most domestic birds are considered tional airflow through the lungs during