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CHAPTER OUTLINE
15.1 The Respiratory System
The primary function of the respiratory system is to allow oxygen from the air to enter the blood
and carbon dioxide from the blood to exit into the air. Inspiration, or inhalation, is breathing in
and expiration, or exhalation, is breathing out.
The Respiratory Tract
As air moves in along the airways of the respiratory tract, it is cleansed, warmed, and
moistened.
The Nose
The nose is the only external portion of the respiratory system and is part of the
upper respiratory tract. Air enters the nose through external openings called
nostrils, into two nasal cavities, which are lined with mucous membranes and
contain odor receptors.
The Pharynx
The pharynx connects the nasal and oral cavities to the larynx. The larynx leads
to the trachea, both of which are normally open, allowing air to pass.
The Larynx
The larynx is a cartilaginous structure that serves as a passageway for air
between the pharynx and the trachea. It is called the voice box because it houses
the vocal cords.
The Trachea
The trachea, commonly called the windpipe, is a tube connecting the larynx to
the primary bronchi. It is lined with a mucosa that contains cilia for sweeping
mucus and debris toward the pharynx, keeping the lungs clean.
The Bronchial Tree
The trachea divides into right and left primary bronchi, which lead into the right
and left lungs. The bronchi branch into secondary bronchi that lead to
bronchioles. Each bronchiole leads to a multitude of air sacs called alveoli. The
components of the bronchial tree beyond the primary bronchi compose the lungs.
The Lungs
The right lung has three lobes and the left has two, which further divide into
lobules, and each lobule has a bronchiole serving many alveoli. Each alveolus is
surrounded by an extensive capillary network where gas exchange occurs.
15.2 Mechanism of Breathing
During ventilation, a free flow of air is vitally important. A spirometer can be used to record the
volume of air exchanged during both normal and deep breathing.
Respiratory Volumes
Normally the amount of air that moves into and out of the lungs during each breath is
much less than the maximum volume that can be inhaled or exhaled.
Inspiration and Expiration
There is a continuous column of air from the pharynx to the alveoli. The lungs lie within
the sealed-off thoracic cavity and adhere to the thoracic wall by way of the pleura.
Inspiration
Inspiration is the active phase of ventilation. The diaphragm contracts and
lowers and the external intercostal muscles contract, and the rib cage moves
upward and outward. The volume of the thoracic cavity is larger than it was
before and air rushes into the lungs.
Expiration
Expiration is the passive phase of breathing and no effort is required to bring it
about. The elastic properties of the thoracic wall and lungs cause them to recoil.
Control of Breathing
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Normally adults have a breathing rate of 12–20 ventilations per minute. The
rhythm of ventilation is controlled by a respiratory center in the medulla
oblongata of the brain.
Chemical Input
The respiratory center is sensitive to the levels of carbon dioxide and
hydrogen ions in the blood, when either rises, the respiratory center
increases the rate and depth of breathing.
15.3 Gas Exchanges in the Body
Respiration includes the exchange of gases in the lungs and the exchange of gases in the tissues.
External Respiration
External respiration refers to the exchange of gases between air in the alveoli and blood
in the pulmonary capillaries. Gases exert pressure and the blood in the pulmonary
capillaries has a higher partial pressure than atmospheric air does, so CO2 diffuses out of
the plasma into the lungs.
Internal Respiration
Internal respiration refers to the exchange of gases between the blood in systemic
capillaries and the tissue fluid. Oxygen diffuses out of the blood into the tissues because
the partial pressure of oxygen of tissue fluid is lower than that of blood.
15.4 Disorders of the Respiratory System
The respiratory tract is susceptible to various infectious agents because it is constantly exposed to
the air in our environment.
Disorders of the Upper Respiratory Tract
Because it is responsible for filtering out many of the pathogens and other materials that
may be present in the air, the upper respiratory tract is susceptible to a variety of viral and
bacterial infections.
The Common Cold
Most “colds” are relatively mild infections of the upper respiratory tract
characterized by sneezing, a runny nose, and perhaps a mild fever.
Pharyngitis, Tonsillitis, and Laryngitis
Pharyngitis, tonsillitis, and laryngitis are caused by inflammation of the
pharynx, tonsils, and larynx, respectively, all of which are usually the result of an
infection.
Sinusitis
Sinusitis is an inflammation of the cranial sinuses, it develops when nasal
congestion blocks the tiny openings leading to the sinuses.
Otitis Media
Otitis media is an inflammation of the middle ear. Although not part of the
respiratory tract, the middle ear can become infected from a nasal infection
through the eustachian tubes. Pain is the primary symptom.
Disorders of the Lower Respiratory Tract
Several disorders of the lower respiratory tract cause problems by obstructing normal
airflow.
Disorders of the Trachea and Bronchi
One of the most life-threatening disorders that affects the trachea is choking.
Acute bronchitis is an inflammation of the primary and secondary bronchi,
usually preceded by a viral infection. In chronic bronchitis, the airways are
inflamed and filled with mucus, the bronchi have undergone degenerative
changes. Asthma is a disease of the bronchi and bronchioles marked by
wheezing, breathlessness, and sometimes a cough and expectoration of mucus
due to an unusual sensitivity to specific irritants.
Diseases of the Lungs
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Pneumonia is an infection of the lungs in which the bronchi or alveoli fill with
fluid, its symptoms are high fever and chills, with headache and chest pain.
Pulmonary tuberculosis is caused by a bacterial infection, cells accumulate
around the bacteria into what is called a tubercle. Emphysema is a chronic and
incurable disorder in which the alveoli are distended and their walls damaged so
that the surface area available for gas exchange is reduced. Cystic fibrosis is a
genetic lung disease in which mucus secretions can form plugs that interfere
with breathing, symptoms include coughing and shortness of breath. Pulmonary
fibrosis is a common lung disease in which fibrous connective tissue builds up in
the lungs, causing a loss of elasticity, restricting the ability of the lungs to expand
and therefore reducing lung volume. Lung cancer is the leading cause of cancer
death and is correlated with smoking. Smoking damages cilia, making it possible
for dust and dirt to settle in the lungs.
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