ANATOMY OF RESPIRATION
The organs of the respiratory system include the nose, pharynx,
larynx, trachea, bronchi, and their smaller branches, and the
lungs, which contain the alveoli.
The Nose
The nose is the only externally visible part of the respiratory
system.
- Nostrils. During breathing, air enters the
nose by passing through the nostrils, or nares.
- Nasal cavity. The interior of the nose
consists of the nasal cavity, divided by a midline nasal
septum.
- Olfactory receptors. The olfactory receptors
for the sense of smell are located in the mucosa in the slitlike
superior part of the nasal cavity, just beneath the ethmoid
bone.
- Respiratory mucosa. The rest of the mucosal
lining, the nasal cavity called the respiratory mucosa, rests on a
rich network of thin-walled veins that warms the air as it flows
past.
- Mucus. In addition, the sticky mucus produced
by the mucosa’s glands moistens the air and traps incoming bacteria
and other foreign debris, and lysozyme enzymes in
the mucus destroy bacteria chemically.
- Ciliated cells. The ciliated cells of the
nasal mucosa create a gentle current that moves the sheet of
contaminated mucus posteriorly toward the throat, where it is
swallowed and digested by stomach juices.
- Conchae. The lateral walls of the nasal cavity
are uneven owing to three mucosa-covered projections, or lobes
called conchae, which greatly increase the surface area of the
mucosa exposed to the air, and also increase the air turbulence in
the nasal cavity.
- Palate. The nasal cavity is separated from the
oral cavity below by a partition, the palate; anteriorly, where the
palate is supported by bone, is the hard palate;
the unsupported posterior part is the soft
palate.
- Paranasal sinuses. The nasal cavity is
surrounded by a ring of paranasal sinuses located in the frontal,
sphenoid, ethmoid, and maxillary bones; theses sinuses lighten the
skull, and they act as a resonance chamber for speech.
Pharynx
- Size. The pharynx is a muscular passageway
about 13 cm (5 inches) long that vaguely resembles
a short length of red garden hose.
- Function. Commonly called the
throat, the pharynx serves as a common passageway
for food and air.
- Portions of the pharynx. Air enters the
superior portion, the nasopharynx, from the nasal
cavity and then descends through the oropharynx
and laryngopharynx to enter the larynx below.
- Pharyngotympanic tube. The pharyngotympanic
tubes, which drain the middle ear open into the nasopharynx.
- Pharyngeal tonsil. The pharyngeal tonsil,
often called adenoid is located high in the
nasopharynx.
- Palatine tonsils. The palatine tonsils are in
the oropharynx at the end of the soft palate.
- Lingual tonsils. The lingual tonsils lie at
the base of the tongue.
Larynx
The larynx or voice box routes air and food
into the proper channels and plays a role in speech.
- Structure. Located inferior to the pharynx, it
is formed by eight rigid hyaline cartilages and a spoon-shaped flap
of elastic cartilage, the epiglottis.
- Thyroid cartilage. The largest of the hyaline
cartilages is the shield-shaped thyroid cartilage, which protrudes
anteriorly and is commonly called Adam’s
apple.
- Epiglottis. Sometimes referred to as the
“guardian of the airways”, the epiglottis protects
the superior opening of the larynx.
- Vocal folds. Part of the mucous membrane of
the larynx forms a pair of folds, called the vocal folds, or
true vocal cords, which vibrate with expelled air
and allows us to speak.
- Glottis. The slitlike passageway between the
vocal folds is the glottis.
Trachea
- Length. Air entering the trachea or
windpipe from the larynx travels down its length
(10 to 12 cm or about 4 inches) to the level of the fifth
thoracic vertebra, which is approximately midchest.
- Structure. The trachea is fairly rigid because
its walls are reinforced with C-shaped rings of
hyaline cartilage; the open parts of the rings abut the esophagus
and allow it to expand anteriorly when we swallow a large piece of
food, while the solid portions support the trachea walls and keep
it patent, or open, in spite of the pressure changes that occur
during breathing.
- Cilia. The trachea is lined with ciliated
mucosa that beat continuously and in a direction opposite to that
of the incoming air as they propel mucus, loaded with dust
particles and other debris away from the lungs to the throat, where
it can be swallowed or spat out.
Main Bronchi
- Structure. The right and left main (primary)
bronchi are formed by the division of the trachea.
- Location. Each main bronchus runs obliquely
before it plunges into the medial depression of the lung on its own
side.
- Size. The right main bronchus is wider,
shorter, and straighter than the left.
- Lungs
-
Location. The lungs occupy the entire thoracic
cavity except for the most central area, the
mediastinum, which houses the heart, the great
blood vessels, bronchi, esophagus, and other organs.
- Apex. The narrow, superior portion of each
lung, the apex, is just deep to the clavicle.
- Base. The broad lung area resting on the
diaphragm is the base.
- Division. Each lung is divided into lobes by
fissures; the left lung has two lobes, and the
right lung has three.
- Pleura. The surface of each lung is covered
with a visceral serosa called the pulmonary, or
visceral pleura and the walls of the thoracic
cavity are lined by the parietal pleura.
- Pleural fluid. The pleural membranes produce
pleural fluid, a slippery serous secretion which allows the lungs
to glide easily over the thorax wall during breathing movements and
causes the two pleural layers to cling together.
- Pleural space. The lungs are held tightly to
the thorax wall, and the pleural space is more of a potential space
than an actual one.
- Bronchioles. The smallest of the conducting
passageways are the bronchioles.
- eventually terminate in alveoli, or air sacs.
- Respiratory zone. The respiratory zone, which
includes the respiratory bronchioles, alveolar ducts, alveolar
sacs, and alveoli, is the only site of gas exchange.
- Conducting zone structures. All other
respiratory passages are conducting zone structures that serve as
conduits to and from the respiratory zone.
- Stroma. The balance of the lung tissue, its
stroma, is mainly elastic connective tissue that allows the lungs
to recoil passively as we exhale.
-
The Respiratory Membrane
- Wall structure. The walls of the alveoli are
composed largely of a single, thin layer of squamous epithelial
cells.
- Alveolar pores. Alveolar pores connecting
neighboring air sacs and provide alternative routes for air to
reach alveoli whose feeder bronchioles have been clogged by mucus
or otherwise blocked.
- Respiratory membrane. Together, the alveolar
and capillary walls, their fused basement membranes, and occasional
elastic fibers construct the respiratory membrane (air-blood
barrier), which has gas (air) flowing past on one side and blood
flowing past on the other.
- Alveolar macrophages. Remarkably efficient
alveolar macrophages sometimes called “dust
cells”, wander in and out of the alveoli picking up
bacteria, carbon particles, and other debris.
- Cuboidal cells. Also scattered amid the
epithelial cells that form most of the alveolar walls are chunky
cuboidal cells, which produce a lipid (fat) molecule called
surfactant, which coats the gas-exposed alveolar
surfaces and is very important in lung function.
-
Physiology of the Respiratory
System
The major function of the respiratory system is to supply the
body with oxygen and to dispose of carbon dioxide. To do this, at
least four distinct events, collectively called respiration, must
occur.
Respiration
- Pulmonary ventilation. Air must move into and
out of the lungs so that gasses in the air sacs are continuously
refreshed, and this process is commonly called breathing.
- External respiration. Gas exchange between the
pulmonary blood and alveoli must take place.
- Respiratory gas transport. Oxygen and carbon
dioxide must be transported to and from the lungs and tissue cells
of the body via the bloodstream.
- Internal respiration. At systemic capillaries,
gas exchanges must be made between the blood and tissue cells.
-
Mechanics of Breathing
- Rule. Volume changes lead to pressure changes,
which lead to the flow of gasses to equalize pressure.
- Inspiration. Air is flowing into the lungs;
chest is expanded laterally, the rib cage is elevated, and the
diaphragm is depressed and flattened; lungs are stretched to the
larger thoracic volume, causing the intrapulmonary pressure to fall
and air to flow into the lungs.
- Expiration. Air is leaving the lungs; the
chest is depressed and the lateral dimension is reduced, the rib
cage is descended, and the diaphragm is elevated and dome-shaped;
lungs recoil to a smaller volume, intrapulmonary pressure rises,
and air flows out of the lung.
- Intrapulmonary volume. Intrapulmonary volume
is the volume within the lungs.
- Intrapleural pressure. The normal pressure
within the pleural space, the intrapleural pressure, is always
negative, and this is the major factor preventing the collapse of
the lungs.
- Nonrespiratory air movements. Nonrespiratory
movements are a result of reflex activity, but some may be produced
voluntarily such as cough, sneeze, crying, laughing, hiccups, and
yawn.