Page 545 - Anatomy and Physiology of Farm Animals, 8th Edition

530 / Anatomy and Physiology of Farm Animals

even lower PO and an even higher PCO , mammalian cardiac muscle to aid with
2
2
depolarization and uniformity in muscle
so gas exchange can continue, even at the
VetBooks.ir last air capillary (Fig. 30‐8). Note that fiber contraction.
Like mammals, the fluid component of
when blood flows past the site of gas
exchange, it is flowing perpendicular to the avian cardiovascular system functions
the direction of airflow within the associ- to carry red blood cells, protein, electro-
ated parabronchus. This arrangement of lytes, nitrogenous wastes, and cells of the
blood flow perpendicular to airflow is immune system. Of note, birds have nucle-
termed a cross‐current exchange model. ated red blood cells with a shorter lifespan
Air exchange can also occur during both (25 to 45 days) than their mammalian
inspiration and expiration, because airflow counterparts. Mitochondria have also
through the parabronchi from caudal to been observed in the red blood cells of
cranial air sacs (Fig. 30‐7) continues during some species, but their function remains
both phases of the respiratory cycle. These unknown. Resting heart rates for adult
factors contribute to a very efficient gas chickens and turkeys are in the range of
exchange, resulting in a greater percentage 200 to 350 beats per minute, with larger
of oxygen extracted from inspired air by birds having lower heart rates and smaller
birds as compared to mammalian lungs. hearts on a body weight basis. A higher
This capacity to extract oxygen from resting metabolic rate and higher normal
inspired air also allows birds to fly at high body temperature (41 to 43 °C) for birds
altitudes where adequate gas exchange contribute to the need to maintain a higher
could not be accomplished by mammalian level of resting cardiac performance. Mean
respiratory systems. blood pressures also tend to be higher in
normal, healthy birds than mammals of a
comparable size, where systolic blood
Cardiovascular System pressure can range from 108 to 220 mmHg
depending on the species. Abnormally
The four‐chambered heart of birds is like high systemic arterial blood pressures
its mammalian counterpart, although are believed to contribute to the devel-
relative to body size it is much larger and opment of dissecting aneurysms or aor-
ejects a proportionally larger stroke vol- tic rupture in rapidly growing turkeys.
ume. Dorsal and lateral aspects of the In this condition, a defect develops in
heart lie in contact with the liver; other the wall of an arterial vessel, often the
surfaces are surrounded by respiratory aorta, and the vessel ruptures at the site
elements and air sacs, and the aorta is of the defect. Massive internal hemor-
found on the right side of the body. The rhage results in sudden death. Increases
right atrium receives the caudal vena in arterial pressures are associated with
cava and a pair of cranial venae cavae. the development of the defect and may
The left atrium receives oxygenated contribute to the ultimate rupture of
blood from the lungs via a single com- the vessel.
mon pulmonary vein. The ventricles and Birds possess two portal systems. The
pulmonic valves are similar to those of hepatic portal system is similar to the
mammalian hearts, although the right mammalian counterpart, whereas the
antrioventricular valve is a simple mus- renal portal system is discussed later with
cular flap lacking chordae tendinae and the urinary system. Veins that are usually
the left atrioventricular valve consists of accessible for venipuncture include the
two cusps. The muscle fibers of the avian jugular vein, basilic vein (sometimes
heart are much smaller than mammalian called the superficial ulnar vein or sim-
cardiac muscle fibers, negating the need ply the “wing vein,” owing to its location
for the extensive invaginations of sarco- on the medial side of the brachium), and
lemma (transverse tubules) found in the medial metatarsal vein.