Download Gross Anatomy and the Lower Respiratory Tract II

Gross Anatomy and the Lower Respiratory
Tract II
Lower Respiratory Tract
Trachea
The flow of air continues from the larynx to the trachea or windpipe.
The trachea is about 10 cm (4 in.) long and 2 cm (3/4 in.) wide and
extends from the larynx to the level of the fifth thoracic vertebrae. The
presence of 16 to 20 C-shaped pieces of hyaline cartilage located along
the trachea prevents this airway from closing. The last cartilage in the
trachea exhibits a projection from the anterior surface that extends into
the lumen. This projection, the carina, is very sensitive to particulate
material and causes coughing when stimulated. The openings of the Cshaped cartilages face the posterior of the trachea and contain the
trachealis smooth muscle. This muscle constricts when someone coughs,
increasing the force of the cough. The trachealis muscle also constricts
during an asthmatic reaction, shrinking the airway and making it harder
to breathe.
Structurally the trachea consists of three concentric tissue layers: the
mucosa, submucosa, and adventitia. This arrangement is continuous
throughout the remainder of the upper respiratory system. The layer
facing the lumen, the mucosa, is an epithelial layer. The cell types found
in the trachea are pseudostratified, ciliated, columnar epithelia. In
addition prominent elastic fibers can be found in the lamina propria.
The submucosa contains blood vessels, nerves, assorted connective
tissues, and mucus glands. The outer adventitia is mostly areolar
connective tissue that is continuous with the hyaline cartilage and
connects the trachea to the surrounding structures and tissues.
The Bronchi and Bronchioles
The bronchial tree is so named because it resembles a tree that has been
turned upside-down. The trachea would be the trunk, the progressively
smaller bronchi and bronchioles are the branches, and the alveoli are
the leaves. Branching of the trachea into the right and left primary
bronchi occurs after the last cartilage in the trachea at the level of the
seventh thoracic vertebrae. The right primary bronchus is wider,
shorter, and more vertical than the left primary bronchus because the
left primary bronchus and lung must accommodate the heart. The
primary bronchi branch to form the secondary or lobar bronchi. There
are three secondary bronchi on the right and two on the left. The
secondary bronchi continue to branch into the tertiary or segmented
bronchi, which also continue the branching pattern. Approximately 23
successive branches lead to the bronchioles. A bronchiole is a tube with
a diameter of less than 1 millimeter (mm). When the measurement
reaches less than 0.5 mm, a bronchiole is termed a terminal bronchiole.
The bronchial tree is lined by pseudostratified, ciliated, columnar
epithelia with goblet cells dispersed among the columnar cells. Plates of
cartilage are found in the walls of the bronchial tree, with the amount of
cartilage and the number of plates decreasing as the bronchi and
bronchioles become smaller. The terminal bronchioles do not contain
cartilage plates; these tubes are small enough to stay open without
cartilage. Smooth muscle is found throughout the system, even into the
respiratory zone.
Lungs, detail aveoli. This work by Cenveo is licensed
under a Creative Commons Attribution 3.0 United
Detail aveoli. This work by Cenveo is licensed under a
Creative Commons Attribution 3.0 United States
(http://creativecommons.org/licenses/by/3.0/us/).
The Lungs
The lungs are two of the largest organs of the body, but they are among
the lightest. Along with the heart, the lungs take up nearly all of the
space in the thorax, superior to the diaphragm. As an organ, the lung is
made up of airway tubes and alveoli, giving it little weight. Elastic
connective tissues in the stroma of the lungs allow them to expand with
incoming air and recoil when expelling air. The lungs contain a large
amount of surface area in order to efficiently support the exchange of
oxygen and carbon dioxide.
The hilus (meaning depression or pit) of the lungs is an indentation on
the medial side of the lungs and the point of entry of blood vessels,
primary bronchi, nerves, and lymphatics. This collection of vessels and
nerves makes up the root of the lung. The tip or apex of the lungs is a
blunted point found just above the clavicles. The posterior, lateral, and
anterior sides of the lungs are surrounded by the ribs. These areas are
called the costal surfaces of the lungs referring to the costal cartilage
surrounding them. The flat, inferior surface of the lungs is found
superior to the diaphragm and referred to as the base of the lung. Since,
the liver is found on the right side of the body and inferior to the
diaphragm, the insertion of the diaphragm is slightly raised on the right.
Consequently the right lung is usually slightly shorter than the left. The
lungs extend from the first costal cartilage to the tenth thoracic
vertebrae.
The lungs consist of a right lung and a left lung. Even though the right
lung is slightly shorter than the left, the left lung has about 10 percent
less mass than the right due to the cardiac notch on the medial side of
the left lung. The heart is tucked into this notch. The heart, the right
lung, and the left lung, are each located in their own anatomical
compartment in the upper thorax. The right lung is divided into three
lobes. A horizontal fissure separates the superior and middle lobes, and
an oblique fissure separates the middle and inferior lobes. The smaller
left lung contains only two lobes. An oblique fissure separates the
superior and inferior lobes on this side.
Each lobe is divided into bronchopulmonary segments separated by
connective tissue septa. There are a total of 10 of these segments and
each contains a tertiary bronchiole, a pulmonary and bronchial artery,
and a lymphatic branch. The presence of these segments aids in further
isolating parts of the lungs to prevent the spread of infection or disease.
Connective tissue further divides the segments into lung lobules, the
smallest anatomical unit in the lungs. A lobule is hexagonal in shape and
less than a centimeter in diameter. Each lobule contains a terminal
bronchiole and its associated alveoli. The connective tissue associated
with lobules may be blackened by tobacco smoke or pollution from the
environment.
Image below compares the lungs from a smoker with those from a nonsmoker.
Lungs.
Usually, lungs are a pink with a relatively smooth surface. The lungs on
the right are blackened and irregularly shaped from the accumulation of
material inhaled with tobacco smoke. This material can be cleared from
the lungs, but only over a long period of time and only if the person
stops smoking. Cigarette smoking causes the deaths of over 440,000
people in the United States each year, including people exposed to
second-hand smoke. The financial losses associated with these deaths
are in excess of 200 billion dollars a year.
There are over 4,000 chemical compounds that are created when the
more than 600 chemical ingredients in cigarettes are burned and
inhaled. More than 50 of the chemicals produced by the burning of a
cigarette are classified as carcinogens. A partial list of these chemicals
includes: acetone, acetic acid, ammonia, arsenic, benzene, butane,
cadmium, carbon monoxide, formaldehyde, lead, naphthalene,
methanol, nicotine, tar, and toluene.
The direct effect of many of the chemicals in the smoke is to paralyze
and destroy cilia. When the cilia cannot function, mucus accumulates in
the lung tubules. Opportunistic bacterial and other microorganisms
utilize the mucus to grow and colonize the lungs. Many of these
organisms create pathologic conditions inside the lung.
Smoker’s cough is a condition that exists because the mucus and other
debris accumulate in the lung tissue. These accumulations stimulate the
cough reflex as means to clear the lungs.
The inhaled chemicals can also contribute to the breakdown of
connective tissue fibers, especially elastin. The degeneration of the
connective tissue can lead to emphysema, a form of chronic obstructive
pulmonary disease (COPD).
Blood and nerve supply
The lungs have a dual blood supply. The pulmonary artery brings
oxygen-poor blood from the right ventricle of the heart. This blood
passes through the pulmonary capillaries, where some carbon dioxide
will leave the blood and a large amount of oxygen will be acquired. The
newly oxygenated blood enters the pulmonary veins and returns back to
the left side of heart. The pulmonary circulation holds about 500
milliliters (ml) of blood, or about 10 percent of the body’s supply. About
75 ml of blood is in the pulmonary capillaries for gas exchange at any
one time. The blood supply that nourishes the tissues of the lungs
arrives through the bronchial artery, which branches off of the aorta and
carries oxygen-rich blood to support the lung tissues. The bronchial
supply anastomoses with the pulmonary vessels, and a mixture of blood
leaves through the bronchial and pulmonary veins. Blood passes
through the lungs at a rate equal to cardiac output, or about five liters
per minute.
Blood pressure measured in the pulmonary circulation is less than it is
in corresponding vessels of the systemic circulation. To some extent this
decrease results from the decreased resistance found in this shorter
pulmonary pathway. The mean pulmonary arterial pressure of about 15
mmHg is adequate to push blood through the pulmonary capillary
network and into the left side of the heart. The low hydrostatic
pulmonary capillary pressure of 7-9 mmHg only produces a small
amount of fluid filtration across the capillary wall. Under normal
conditions, the lymphatic vessels readily remove this filtrate. Under
conditions where left atrial pressure rises dramatically, such as in mitral
valve stenosis or congestive heart failure, pulmonary capillary pressure
will also rise, increasing the rate of capillary filtration. If the lymphatic
system cannot keep up with the higher filtration rate, fluid will
accumulate in the alveoli as pulmonary edema. This can interfere with
the exchange of oxygen, leading to cyanosis, decreased activity
tolerance, etc.
The pulmonary circulation responds to hypoxia differently than the
systemic circulation. The systemic circulation dilates under hypoxic
conditions causing an increased blood flow through the tissues. The
arterioles of pulmonary circulation constrict selectively in cases of
alveolar hypoxia. This constriction diverts blood to areas in the lungs
that are better ventilated. This helps ensure adequate gas exchange.
Nerves from the pulmonary plexus enter the lungs at the hilus. These
nerves contain a mixture of visceral sensory and autonomic nerve fibers
that follow the bronchial tree and blood vessels. Parasympathetic nerve
stimulation results in bronchoconstriction, constriction of the
bronchioles, while sympathetic nerve stimulation results in
bronchodilation, dilation of the bronchioles.
Pleura
Each lung is found in a pleural cavity bounded by the pleural
membrane, a double sided membrane that contains a thin layer
of pleural fluid. The visceral pleural is a mucus membrane that covers
the lungs and folds over at the hilus. The folded membrane continues
and becomes the parietal pleura, which lines the inner wall of the
thoracic cavity. The space between the two membranes is called the
pleural cavity or space. From 1 to 15 ml of pleural fluid is found on the
facing surfaces of the pleural membranes. This fluid helps to lubricate
the membrane surfaces so that the movement of the lungs during
inhaling and exhaling does not cause frictional damage to the tissues.
The fluid also lightly holds the two membranes together so that they
move together as the chest wall expands and contracts.
Since pleural fluid is mainly a filtrate of plasma, the factors that affect
the amount of pleural fluid production and removal are the same factors
that govern interstitial fluid volumes in most regions of the body. The
main factors include capillary hydrostatic pressure, capillary colloid
osmotic pressure, the permeability (“leakiness”) of the capillaries, and
the rate of fluid removal by the lymphatics. Under normal conditions,
there is a slow but steady turnover of pleural fluid, but under certain
conditions excess fluid can build up, and impede lung expansion. This
condition is known as a pleural effusion and may be secondary to
blockade of venous drainage by tumors, increased capillary permeability
because of infections, etc.
The pleural sac extends below the lungs, to the level of the twelfth
thoracic vertebrae. Samples of the pleural fluid can be safely taken from
this area. Normal pleural fluid is clear and pale yellow in color. It has
very few cells free in the fluid. The majority (75 percent) of these cells
are macrophages. About 23 percent of the cells are lymphocytes, with an
assortment of cells making up the remaining 2 percent.
Pleural Transudates vs. Exudates
Since a pleural effusion may be caused by a number of different
conditions, inspection of the pleural fluid is often be used to help
determine the cause of the effusion. The fluid can either be classified
as a transudate or an exudate, depending on if the protein content is
significantly increased (exudates) or not (transudate). A transudate
forms when the forces contributing to capillary exchange are altered,
such as with venous blockade that increases hydrostatic pressure, or
when lymphatic drainage is impeded. An exudate occurs when the
permeability of the capillaries increases, such as with infections or
localized cancers.
Gross examination of pleural fluids can help to distinguish between
transudates and exudates. Both conditions produce an increase in
fluid volume. Transudates will appear similar to normal pleural
fluid. It will appear clear and slightly yellow in color. Exudates will
be cloudy and gray in appearance. Transudates will also have a
normal cell count and distribution. Exudates will have an increased
total cell count. For example, an increase in lymphocytes occurs in
tuberculosis, and an increase in neutrophils occurs in bacterial
infections.