Functional Anatomy of the Respiratory
Developmental Aspects of the Respiratory System
- The nasal cavity, the chamber within the nose, is divided medially
by a nasal septum and separated from the oral cavity by the palate(Figure
13.1). The nasal cavity is lined with a mucosa, which warms, filters,
and moistens incoming air. The mucosa also contains receptors for sense
of smell. Paranasal sinuses and nasolacrimal ducts drain into the nasal
- The pharynx (throat) is a mucosa-lined, muscular tube with three
regions nasopharynx, oropharynx, and laryngopharynx. The nasopharynx
functions in respiration only; the others serve both respiratory and
digestive functions. The pharynx contains tonsils, which act as part
of the body's defense system.
- The larynx (voice box) is a cartilage structure; most prominent
is the thyroid cartilage (Adam's apple). The larynx connects the pharynx
with the trachea below. The laryngeal opening (glottis) is hooded by
the epiglottis, which prevents entry of food or drink into respiratory
passages when swallowing. The larynx contains the true vocal cords,
which produce sounds used in speech.
- The trachea (windpipe) extends from larynx to primary bronchi.
The trachea is a smooth muscle tube lined with a ciliated mucosa and
reinforced with C-shaped cartilage rings, which keep the trachea open.
- Right and left primary bronchi result from subdivision of the trachea.
Each plunges into the hilus of the lung on its side.
- The lungs are paired organs flanking the mediastinum in the thoracic
cavity. The lungs are covered with visceral pleura; the thorax wall
is lined with parietal pleura. Pleural secretions decrease friction
during breathing. The lungs are primarily elastic tissue, plus passageways
of the respiratory tree. The smallest passageways end in clusters of
- The conducting zone includes all respiratory passages from the
nasal cavity to the terminal bronchioles; they conduct air to and from
the lungs. Respiratory bronchioles, alveolar ducts and sacs, and alveoli
which have thin walls through which gas exchanges are made with pulmonary
capillary blood are respiratory zone structures (Figure
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- Mechanics of breathing: Gas travels from high-pressure to low-pressure
areas. Pressure outside the body is atmospheric pressure; pressure inside
the lungs is intrapulmonary pressure; pressure in the intrapleural space
is intrapleural pressure (which is always negative). Movement of air
into and out of the lungs is called pulmonary ventilation, or breathing
(Figure 13.3). When inspiratory
muscles contract, intrapulmonary volume increases, its pressure decreases,
and air rushes in (inspiration). When inspiratory muscles relax, the
lungs recoil and air rushes out (expiration). Expansion of the lungs
is helped by cohesion between pleurae and by the presence of surfactant
- Nonrespiratory air movements: Nonrespiratory air movements are
voluntary or reflex activities that move air into or out of the lungs.
These include coughing, sneezing, laughing, crying, hiccuping, yawning.
- Respiratory volumes and capacities: Air volumes exchanged during
breathing are tidal volume, inspiratory reserve volume, expiratory reserve
volume, and vital capacity (Figure
13.4). Residual volume is non-exchangeable respiratory volume and
allows gas exchange to go on continually.
- Respiratory sounds: Bronchial sounds are sounds of air passing
through large respiratory passageways. Vesicular breathing sounds occur
as air fills alveoli.
- External respiration, gas transport, and internal respiration:
Gases move according to laws of diffusion (Figure
13.5). Oxygen moves from alveolar air into pulmonary blood. Most
oxygen is transported bound to hemoglobin inside RBCs. Carbon dioxide
moves from pulmonary blood into alveolar air. Most carbon dioxide is
transported as bicarbonate ion in plasma. At body tissues, oxygen moves
from blood to the tissues, whereas carbon dioxide moves from the tissues
- Control of respiration
- Nervous control: Neural centers for control of respiratory
rhythm are in the medulla and pons. Reflex arcs initiated by stretch
receptors in the lungs also play a role in respiration by notifying
neural centers of excessive overinflation.
- Physical factors: Increased body temperature, exercise, speech,
singing, and nonrespiratory air movements modify both rate and depth
- Volition: To a degree, breathing may be consciously controlled
if it does not interfere with homeostasis.
- Emotional factors: Some emotional stimuli can modify breathing.
Examples are fear, anger, and excitement.
- Chemical factors: Changes in blood levels of carbon dioxide
are the most important stimuli affecting respiratory rhythm and
depth. Carbon dioxide acts directly on the medulla via its effect
on reducing blood pH. Rising levels of carbon dioxide in blood result
in faster, deeper breathing; falling levels lead to shallow, slow
breathing. Hyperventilation may result in apnea and dizziness, due
to alkalosis. Oxygen is less important as a respiratory stimulus
in normal, healthy people, but it is the stimulus for those whose
systems have become accustomed to high levels of carbon dioxide.
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- The major respiratory disorders are emphysema, chronic bronchitis,
and lung cancer. A significant cause is cigarette smoking.
- Emphysema is characterized by permanent enlargement and destruction
of alveoli. The lungs lose their elasticity, and expiration becomes
an active process.
- Chronic bronchitis is characterized by excessive mucus production
and its pooling in lower respiratory passageways, which severely impairs
ventilation and gas exchange. Patients may become cyanotic as a result
of chronic hypoxia.
- Lung cancer is extremely aggressive and metastasizes rapidly. The
three most common lung cancers are squamous cell carcinoma, adenocarcinoma,
and small cell carcinoma.
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- Premature infants have problems keeping their lungs inflated due
to lack of surfactant in their alveoli. (Surfactant is formed late in
- The most important birth defects of the respiratory system are
cleft palate and cystic fibrosis.
- The lungs continue to mature until young adulthood.
- During youth and middle age, most respiratory system problems are
a result of external factors, such as infections and substances that
physically block respiratory passageways.
- In old age, the thorax becomes more rigid and lungs become less
elastic, leading to decreased vital capacity. Protective mechanisms
of the respiratory system decrease in effectiveness in elderly persons,
predisposing them to more respiratory tract infections.