The Thoracic Cavity
The structure and functions of the major
organs of the respiratory system are described below:
(Right
median saggital section of the head showing the upper portion of the
respiratory tract)
The structures of the nose are divided into
the external nose and the internal nasal cavity.
The surface features of the external nose
include the root (area between the eyebrows), bridge, and dorsum nasi (anterior
margin). The external openings of the nose, the nostrils or nares, are bounded
laterally by the flared alae.
The internal nasal cavity lies in and posterior
to the external nose. The nasal cavity is divided by a midline nasal septum,
The nasal cavity continues posteriorly with the nasal portion of the pharynx
through the posterior nasal apertures. It is lined by the mucosa which secretes
mucus. The roof of the cavity has olfactory (smell) receptors.
During breathing,
air passer through the nostrils, or nares to enter the nasal cavity.
The roof of the
nasal cavity is formed by the bones of the skull. The walls of the nasal cavity
are formed as follows:
The floor is formed by the palate, which
separates the nasal cavity from the oral cavity below. Anteriorly, where the
palate is supported by the hard palate. The unsupported posterior portion is
the muscular soft palate.
The hairs in the nostrils filter large
particles (dust, pollen) from the inspired air.
The rest of the nasal cavity is lined with
two types of mucous membranes.
1.
The olfactory mucosa:
It
lines the upper part (roof) of the nasal cavity, and contains smell (olfactory)
receptors.
2.
The respiratory mucosa:
It
lines the balance of the nasal cavity mucosa. The respiratory mucosa is a
pseudo-stratified ciliated columnar epithelium, containing scattered goblet
cells, mucous and serous glands. Mucous cells secrete mucus, and serous cells
secrete a watery fluid containing enzymes. Each day, these glands secrete mucus
containing lysozyme, (an antibacterial enzyme). The sticky mucus traps inspired dust,
bacteria, and other debris, while lysozyme attacks and destroys bacteria
chemically. The epithelial cells of the respiratory mucosa also secrete
secretary Immunoglobulins (IgA) that help destroy invading microbes (non
specific defense mechanism). The high water content of the mucus film
humidifies the inhaled air. The ciliated cells of the
respiratory mucosa create a gentle current that moves the sheet of contaminated
mucus toward the throat, where it is swallowed and digested by stomach juices.Three mucosa-covered projections, the superior, middle, and inferior nasal conchae are located in the lateral wall of the nasal cavity. The curved conchae greatly increase the mucosal surface area exposed to air, and act as air filters to filter out particles larger than 6 µm in the inspired air.
Pharynx
The pharynx is divided into three regions from superior to inferior, namely the naso-pharynx, oro-pharynx, and laryngo-pharynx.The muscular pharyngeal wall is composed of skeletal muscle throughout its length, but the cellular composition of the mucosa varies from one pharyngeal region to another.
The Nasopharynx
The nasopharynx is posterior to the nasal cavity. It lies above the point where food enters the body. Therefore it functions only as an air passageway. During swallowing, the soft palate and the uvula move superiorly, and close off the nasopharynx and prevent food from entering the nasal cavity. The nasopharynx is continuous with the nasal cavity. On the posterior wall is the pharyngeal tonsil (or adenoids), which traps and destroys pathogens entering the nasopharynx through the inhaled air.
The larynx has three functions. They are listed below
The elastic vocal ligaments attach the arytenoid cartilages to the thyroid cartilage. They are the core of the vocal cords, which are pearly white because they lack blood vessels. The vocal cords vibrate, produce sounds as air rushes up from the lungs. The vocal cords and the medial opening between them through which air passes are called the glottis. The variations in vibration of the vocal cords, as air passes through them produce the “variations in the voice” or “sounds”.
The nasal cilia, when exposed to cold air
become sluggish, therefore allowing mucus to accumulate in the nasal cavity.
This mucus then dribbles out of the nostrils. This and the water vapor in
expired air condense at lower temperatures. This is the reason why you might
have a “runny” nose on a dry, cold day.
The nasal mucosa is richly
supplied with sensory nerve endings. When, irritating particles (dust, pollen,
and the like) contact with it triggers a sneeze reflex. The sneeze forces air
outward in a violent burst and expels irritants from the nose.
A large number of capillaries and
thin-walled veins are located just beneath the nasal epithelium. These blood
vessels warm the incoming air as it flows across the mucosal surface. Since
many of these blood vessels are located superficially, nose bleeds are common
and bleed heavily even with a minor injury.
Is a funnel shaped
tube that connects the superiorly
located nasal cavity and mouth to the inferiorly located larynx and oesophagus.
It is commonly called the throat, and extends from the base of the skull to the
level of the sixth cervical vertebra.
Clinically Important Anatomy
Infected and
swollen tonsils (adenoids) block the passage of air in the nasopharynx, making
it difficult to breathe. Therefore the affected person has to breathe through
the mouth. As a result, the air is not properly moistened, warmed, or filtered
before reaching the lungs. When the adenoids are enlarged, speech is affected and sleep may be
disturbed by snoring.
The pharyngo-tympanic
(Eustacian) tubes, open into the lateral walls of the nasopharynx. These tubes
drain the middle ear cavities and allow the middle ear pressure to equalize
with atmospheric pressure. The tubal and pharyngeal tonsils near the opening of these
tubes, helps protect the middle ear against infections.
When we have a cold the pharyngo-tympanic (Eustacian)
tube may be inflamed and therefore we may have blocked ears and poor hearing as
the pressures in the tubes may not be equalised.
(Larynx)
Larynx
The
larynx or voice box extends for about 5 cm (2 inches) from the level of the
third to the sixth cervical vertebra. Superiorly it attaches to the hyoid bone
and opens into the laryngopharynx. Inferiorly it is continuous with the trachea.
The larynx has three functions. They are
listed below
-
to
provide an open (patent) airway from the laryngopharynx to the trachea for
air
-
Provide a food passage from the pharynx to the oesophagus
-
Voice
production.
The
larynx is made up of nine cartilages connected by membranes and ligaments. Except for the
epiglottis, all laryngeal cartilages are hyaline cartilages.
The ninth cartilage, is the flexible,
spoon-shaped epiglottis. It is
composed of elastic cartilage and is almost entirely covered by a taste
bud–containing mucosa. When only air is flowing into the larynx, the inlet to
the larynx is open wide and the free edge of the epiglottis projects upward.
During swallowing, the larynx is pulled superiorly and the epiglottis tips to
cover the laryngeal opening . Due to this action food does not enter the lower
respiratory passages; therefore the epiglottis has been called the “guardian of
the airways”.
Substances
(other than air entering the larynx initiates the cough reflex, which acts to
expel the substance. The protective cough reflex does not work when we are
unconscious, therefore as a nurse; never
give liquids when attempting to revive an unconscious person.
The elastic vocal
ligaments attach the arytenoid cartilages to the thyroid
cartilage. They are the core of the vocal cords,
which are pearly white because they lack blood vessels. The vocal cords
vibrate, produce sounds as air rushes up from the lungs. The vocal cords
and the medial opening between them through which air passes are called the glottis.
The variations in vibration of the vocal cords, as air passes through them
produce the “variations in the voice” or “sounds”.
The Trachea (Windpipe)
The trachea is
about 2.5 cm in diameter and is about 10 – 12 cm long. It is the tube through
which air travels between the larynx and the bronchi of the lungs. It is kept
open by bands of cartilage- The trachea is felt if you run your fingers up and
down the front of your neck along the midline, the bumps you feel are formed by
the bands of cartilage surrounding the trachea. The trachea is lined with
mucous-secreting ciliated epithelial cells. The cilia remove foreign particles
which are trapped in the mucous are moved upwards, away from the lungs.
Conducting Zone Passages
The air pathway beyond the trachea consists
of the main bronchi, lobar bronchi, and
segmental bronchi, which branch into the smaller bronchi and bronchioles until
the terminal bronchioles of the lungs are reached.
The
right and left main (primary) bronchi are formed by the division of the
trachea. Each bronchus runs obliquely in the mediastinum before entering into
the medial depression (hilum) of the lung.The right main bronchus is wider,
shorter, and more vertical than the left. Therefore if a foreign body
is inhaled the right main bronchus is the more common site where the foreign body
may become stuck.
Once inside the lungs, each main bronchus subdivides into lobar
(secondary) bronchi—three on the right and two on the left—each supply one lung
lobe. The lobar bronchi divide repeatedly into smaller and smaller bronchi. Air
passages, which are smaller than 1 mm in diameter called bronchioles (“little
bronchi”), and the smallest bronchiole is the, terminal bronchiole.
(Structure
of bronchi and bronchioles)
(Structure of respiratory
zone)
The tissue
composition of the wall of the main bronchus is similar to the trachea. As the
airways become smaller, the following structural changes occur along the air passages:
1. Support structures change. The cartilage rings in the trachea are
replaced by irregular plates of cartilage. At the level of the bronchioles
there is no supportive cartilage in the airway walls. Elastic fibers line the
airway walls throughout the bronchial tree.
2. Epithelium type changes. The mucosal
epithelium changes from pseudostratified columnar to columnar and then to
cuboidal in the terminal bronchioles. Cilia gradually decrease and
mucus-producing cells are absent in the bronchioles.
3. Amount of smooth muscle increases. The relative amount of smooth muscles in
the bronchial walls increases as the air passage ways become smaller. A
complete layer of circular smooth muscle in the bronchioles without supporting
cartilage produces substantial resistance to the passage of air in some
diseases.
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