Agriculture Reference
In-Depth Information
with three-valvule valves. The venae cavae (cranial and
caudal) open into the right atrium, whereas the pulmonary
veins open into the left atrium. Uneven muscle fi bers with
distinct design can be identifi ed inside of the atria and the
ventricles. All prominences inside of the heart are covered
by a delicate membrane called the endocardium. The latter
will continue inside of the vessels starting from, and
ending at, the heart, as endarteries and endoveins, respec-
tively. Around the origin of the aorta, two cardiac bones
can be detected.
of the endocardium. The veins of the limbs and ventral
abdomen are provided with valvules, which orient the
blood fl ow in one direction only, toward the cranial and
the caudal venae cavae.
The Lymphatic System
The lymphatic system consists of lymph nodes and lymph
vessels. The lymph nodes receive the lymph via afferent
lymphatic vessels, and lymph runs away from the lymph
nodes via efferent lymphatic vessels. The lymph is that
fl uid, clear and transparent, which is collected from the
body tissues, and fl ows in the lymphatic vessels passing
through lymph nodes.
The lymph nodes are located under the skin, deep
between muscles, in the head and in the natural cavities
of the body (the thoracic and abdomino-pelvic cavities).
The lymph nodes in the natural cavities receive lymph
from the internal organs. The lymph reaches fi nally the
venous system through the cranial vena cava.
There are lymphatic structures (tissues) scattered
throughout the body, such as the tonsils, whose function is
to guard against the passage of infection deep into the respi-
ratory, digestive, and/or reproductive systems. They are
found in the pharynx, larynx, intestine, prepuce, vagina, etc.
Some organs do not have lymph vessels, such as the
bone marrow, the articular cartilages, the alveoli of the
lungs, the renal cortex and medulla, the placenta, the
splenic pulp, the epidermis, the central nervous system,
and the meninges.
The lymph nodes and the other lymphatic structures are
involved in the immune system (see the Immune System).
The Arteries and the Veins
The blood is a red-colored circulating fl uid in the arterial
and venous trees, whose physical, chemical, and biological
characteristics can be found in corresponding topics.
The heart's function is complex and can be demon-
strated by the systemic circulation and the functional cir-
culation. The systemic circulation starts in the left ventricle,
which pumps the oxygenated blood into the whole body
through the aorta, arteries, arterioles, and capillaries up to
the fi nest structures. From the tissues, through capillaries,
venules, and veins, the blood with carbon dioxide empties
into the right atrium through the cranial and caudal venae
cavae. From here, the blood is pumped into the right ven-
tricle. The functional circulation starts from the right ven-
tricle, which pumps the venous blood into the lungs, via
the pulmonary trunk, pulmonary arteries, and arterioles. At
the level of pulmonary alveoli, the blood yields the carbon
dioxide and accumulates the oxygen through the phenom-
enon of hematosis. From the lungs, the oxygenated blood
passes via pulmonary venules and veins into the left
atrium, and from there it is pumped into the left ventricle,
and the cycle continues. The contraction of the ventricles
and the atria is called “systole” and the relaxation “dias-
tole.” The complete cycle of cardiac contraction and relax-
ation is called the “cardiac cycle.” During the diastole, the
corresponding chamber (atrium or ventricle) is fi lled with
blood, while during the systole, the chambers contract and
expel blood.
The venous blood from the gastrointestinal tract and the
pancreas is collected in the portal vein. This vein starts
from capillaries, and ends by capillaries, in the liver. The
blood from the portal vein is processed in the liver in
species- specifi c nutrients. From the liver, the blood travels
by way of the caudal vena cava to empty into the right
atrium.
The walls of the arteries up to arterioles and the veins
(except the venules) consist of an outer layer, a middle
layer, and an internal layer. The outer is fi brous, the middle
is muscular and elastic, and the internal is the continuation
THE RESPIRATORY SYSTEM
The respiratory system has an essential role in the body,
bringing in outside air with oxygen necessary for all physi-
ological processes, and exhaling air with carbon dioxide
as the result of these processes. The respiratory system is
responsible for the gas exchanges between the blood and
the surrounding air. It consists of two categories of ana-
tomical structures: the air (respiratory) passages and the
essential organs.
The Air (Respiratory) Passages
T
HE
N
ASAL
C
AVITY
The nasal cavity consists of the nostrils, the nasal cavity
proper, and the choanae. The nostrils are the rostral open-
ings of the nasal cavity, and are sculpted in an area called
the nasal plane. They lead to the entrance of the nasal
cavity called the nasal vestibule. The roof, lateral walls,
