lOMoARcPSD|51648332
lOMoARcPSD|51648332
Physiology Review
The main function of the respiratory system is gas exchange. The primary goal is to get oxygen into the
blood while also expelling carbon dioxide from the body. The respiratory system can be divided into the
conducting airways and the respiratory tissues. The nasal passages, mouth and pharynx, larynx,
trachea, bronchi, and bronchioles are the structures that make up the conducting airways. Air is
brought in from the atmosphere and directed to the lungs through these structures. Once in the lung,
the respiratory tissues are where gas exchange takes place.
As a review, most of the conducting airways are lined with ciliated pseudostratified columnar
epithelium. The ciliated cells protect the airway by entrapping particles and redirecting them back
toward the oropharynx where they can be expectorated or swallowed. Within the ciliated cells, mucus-
secreting glands also secrete antibacterial enzymes. The bronchus also contains smooth muscle cells,
mucus glands, connective tissue, and cartilage.
The smaller bronchioles are made of simple epithelium. They lack cartilage and the wall is thinner.
Importantly, the alveolar wall is made for gas exchange, not structural support. These structural
differences are summarized in Figure 5.1.
The tracheobronchial tree starts at the conducting airway and branches into the lobes of the lungs,
ending in the respiratory airway where gas exchange takes place. Altogether, as the name implies the
tracheobronchial tree consists of the trachea, bronchi, and bronchioles.
The trachea connects the larynx to the bronchi. It contains C-shaped rings of hyaline cartilage which
prevent it from collapsing. At its base, the trachea divides to form the right and left main, or primary,
bronchi. Each main bronchus contains pulmonary arteries, veins, and lymph vessels and together
enter the lung through a slit called
the hilum. Between the main bronchi is a ridge, called the carina. It is a highly sensitive tissue that
initiates violent coughing when contacted by a foreign object, such as by food.
Upon entering the lungs, the main bronchi divide into secondary or lobular bronchi, supplying each
lobe of the lungs. As shown in Figure 5.2, the right lung has 3 lobes, while the left lung only has 2 lobes
because of the position of the heart. The secondary bronchi then further divide to form the segmental
bronchi.
The segmental bronchi will further branch into smaller bronchi until they become terminal bronchi—
the smallest structures within the conducting airways. Notably, the
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structural composition of bronchi also changes as it branches and decreases in size. Although initially
composed primarily of cartilage, its composition is gradually replaced by smooth muscle and elastic
tissue. In fact, by the time the bronchi branches into bronchioles no cartilage is present. Instead, the
bronchioles are primarily composed of smooth muscle and elastic tissue.
Figure 5.3 Generations of the conducting and respiratory airways
The levels of branching are shown in Figure 5.3. The first 16 generations (Z) make up the conducting
airways. Generations 17 to 23 make up the respiratory airways.
Throughout childhood, the airways increase in diameter and length. The number and size of the alveoli
increase until adolescence when respiratory development matures to that of an adult.
Gas exchange occurs in the respiratory bronchioles as well as the alveolar ducts and sacs which are
located in the lobules of the lungs. Deoxygenated blood enters the lungs through a pulmonary artery,
and oxygenated blood exits through a pulmonary vein.
The alveoli are the actual sites of gas exchange between the air and blood.
The alveolus is a sac that fills with oxygenated air when one breathes in and allows that air to pass
across a membrane into a blood vessel (alveolar capillary). The alveolus contracts back down like a
deflating balloon to let carbon dioxide out and to begin the cycle again. Within this space, capillaries are
abundant, allowing blood and air to constantly mix. Oxygen from the alveoli diffuses into the blood, and
carbon dioxide from the blood diffuses into the alveoli. Alveolar epithelium consists of type I and type II
alveolar cells, as well as macrophages.
Type I alveolar cells make up 95% of the surface area and are composed of thin squamous cells. Most
notably, Type I cells cannot divide.
Type II alveolar cells are cuboidal cells and cover about 5% of the alveolar surface area, though they
are just as numerous. Type II cells synthesize surfactant, which decreases the surface tension in the
alveoli and allows for greater ease in lung inflation. Upon lung injury, type II cells are capable of
proliferating into both type I and type II cells. The macrophages are responsible for removing offending
substances from the alveoli.
Mechanics of Breathing
Ventilation is a mechanical process that uses pressure differences to move air in and out of the lungs.
As gas (air) always moves from areas of greater pressure and into areas of lesser pressure, during the
process of inspiration, the respiratory muscles work to expand the thoracic cavity, which lowers the
pressure inside the lung relative to the atmosphere, causing air to flow into the lungs. Conversely, during
the process
of expiration, the respiratory muscles relax, and as the thoracic cavity retracts
a greater internal pressure relative to the outside atmosphere is created. The result of which causes the
high-pressure air in the lung to be expelled from the body towards the area outside the body that is of
lesser pressure (Figure 5.4).
As previously mentioned, ventilation is dependent upon pressure differences to allow airflow to
occur between the atmosphere and the lungs. The intrapulmonary pressure or alveolar pressure is
the measured pressure inside the airways and the alveoli. When air is neither being inspired nor
expired, the intrapulmonary pressure is equal to atmospheric pressure, or zero. The intrapleural
pressure is the measured
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pressure in the pleural cavity. In a normal inflated lung, the intrapleural pressure is negative relative to
the intrapulmonary pressure. In other words, there is a negative pressure in the pleural cavities. This is
caused by an oppositional pull that occurs between the parietal and visceral layers of the pleura. As the
elastic recoil of the lungs increases, the intrapleural pressure becomes more negative during inspiration
than expiration. This negative pressure holds the lungs against the chest wall. Without it, the elastic
recoil of the lungs would cause them to collapse. The intrathoracic pressure is the pressure in the
thoracic cavity and is equal to the intrapleural pressure.
The lungs are contained within the thoracic cavity, along with the heart, great vessels, and esophagus.
For the reasons of pressure control described above, the act of breathing depends on this closed cavity.
The diaphragm is the main muscle of inspiration. When the diaphragm contracts (inspiration), the chest
expands. In a minor role, the external intercostal muscles, scalene and sternocleidomastoid muscles aid
in breathing. Each is associated along the ribs, raising and spreading them during inhalation, although
most commonly exerting aid during exercise or difficulty breathing.
The diaphragm is innervated primarily by C4 but also by C3, and C5 of the spinal cord. Thus, people
who sustain a spinal cord injury above C3 lose diaphragm function and require mechanical ventilation.
Paralysis of one side of the diaphragm causes the chest to move up on that side rather than down
during inspiration because of the negative pressure in the chest. This is referred to as paradoxical
movement.
Figure 5.4 Chest volume and pressure changes during inspiration and expiration and movement of the diaphragm
Lung compliance is the ease with which lungs can inflate. For example, blowing up a new balloon is
hard because it is noncompliant. In contrast, blowing up a balloon that has already been inflated is
easier because it has become compliant. Thus, it takes more pressure to move air into a noncompliant
lung than a compliant one. Lung compliance depends on multiple factors including overall water content
and surface tension, as well as the amount of elastin and collagen fibers that are present.
Surface tension refers to the force exerted by water molecules on the surface of alveoli. As water
molecules are held together by strong covalent bonds, this force is stronger than the force holding air
molecules within the alveolar space. As such, as air leaves during exhalation, the strong surface tension
the water exerts on the surrounding
Downloaded by Benjamin Luca ()
lOMoARcPSD|51648332
Physiology Review
The main function of the respiratory system is gas exchange. The primary goal is to get oxygen into the
blood while also expelling carbon dioxide from the body. The respiratory system can be divided into the
conducting airways and the respiratory tissues. The nasal passages, mouth and pharynx, larynx,
trachea, bronchi, and bronchioles are the structures that make up the conducting airways. Air is
brought in from the atmosphere and directed to the lungs through these structures. Once in the lung,
the respiratory tissues are where gas exchange takes place.
As a review, most of the conducting airways are lined with ciliated pseudostratified columnar
epithelium. The ciliated cells protect the airway by entrapping particles and redirecting them back
toward the oropharynx where they can be expectorated or swallowed. Within the ciliated cells, mucus-
secreting glands also secrete antibacterial enzymes. The bronchus also contains smooth muscle cells,
mucus glands, connective tissue, and cartilage.
The smaller bronchioles are made of simple epithelium. They lack cartilage and the wall is thinner.
Importantly, the alveolar wall is made for gas exchange, not structural support. These structural
differences are summarized in Figure 5.1.
The tracheobronchial tree starts at the conducting airway and branches into the lobes of the lungs,
ending in the respiratory airway where gas exchange takes place. Altogether, as the name implies the
tracheobronchial tree consists of the trachea, bronchi, and bronchioles.
The trachea connects the larynx to the bronchi. It contains C-shaped rings of hyaline cartilage which
prevent it from collapsing. At its base, the trachea divides to form the right and left main, or primary,
bronchi. Each main bronchus contains pulmonary arteries, veins, and lymph vessels and together
enter the lung through a slit called
the hilum. Between the main bronchi is a ridge, called the carina. It is a highly sensitive tissue that
initiates violent coughing when contacted by a foreign object, such as by food.
Upon entering the lungs, the main bronchi divide into secondary or lobular bronchi, supplying each
lobe of the lungs. As shown in Figure 5.2, the right lung has 3 lobes, while the left lung only has 2 lobes
because of the position of the heart. The secondary bronchi then further divide to form the segmental
bronchi.
The segmental bronchi will further branch into smaller bronchi until they become terminal bronchi—
the smallest structures within the conducting airways. Notably, the
Downloaded by Benjamin Luca ()
, lOMoARcPSD|51648332
structural composition of bronchi also changes as it branches and decreases in size. Although initially
composed primarily of cartilage, its composition is gradually replaced by smooth muscle and elastic
tissue. In fact, by the time the bronchi branches into bronchioles no cartilage is present. Instead, the
bronchioles are primarily composed of smooth muscle and elastic tissue.
Figure 5.3 Generations of the conducting and respiratory airways
The levels of branching are shown in Figure 5.3. The first 16 generations (Z) make up the conducting
airways. Generations 17 to 23 make up the respiratory airways.
Throughout childhood, the airways increase in diameter and length. The number and size of the alveoli
increase until adolescence when respiratory development matures to that of an adult.
Gas exchange occurs in the respiratory bronchioles as well as the alveolar ducts and sacs which are
located in the lobules of the lungs. Deoxygenated blood enters the lungs through a pulmonary artery,
and oxygenated blood exits through a pulmonary vein.
The alveoli are the actual sites of gas exchange between the air and blood.
The alveolus is a sac that fills with oxygenated air when one breathes in and allows that air to pass
across a membrane into a blood vessel (alveolar capillary). The alveolus contracts back down like a
deflating balloon to let carbon dioxide out and to begin the cycle again. Within this space, capillaries are
abundant, allowing blood and air to constantly mix. Oxygen from the alveoli diffuses into the blood, and
carbon dioxide from the blood diffuses into the alveoli. Alveolar epithelium consists of type I and type II
alveolar cells, as well as macrophages.
Type I alveolar cells make up 95% of the surface area and are composed of thin squamous cells. Most
notably, Type I cells cannot divide.
Type II alveolar cells are cuboidal cells and cover about 5% of the alveolar surface area, though they
are just as numerous. Type II cells synthesize surfactant, which decreases the surface tension in the
alveoli and allows for greater ease in lung inflation. Upon lung injury, type II cells are capable of
proliferating into both type I and type II cells. The macrophages are responsible for removing offending
substances from the alveoli.
Mechanics of Breathing
Ventilation is a mechanical process that uses pressure differences to move air in and out of the lungs.
As gas (air) always moves from areas of greater pressure and into areas of lesser pressure, during the
process of inspiration, the respiratory muscles work to expand the thoracic cavity, which lowers the
pressure inside the lung relative to the atmosphere, causing air to flow into the lungs. Conversely, during
the process
of expiration, the respiratory muscles relax, and as the thoracic cavity retracts
a greater internal pressure relative to the outside atmosphere is created. The result of which causes the
high-pressure air in the lung to be expelled from the body towards the area outside the body that is of
lesser pressure (Figure 5.4).
As previously mentioned, ventilation is dependent upon pressure differences to allow airflow to
occur between the atmosphere and the lungs. The intrapulmonary pressure or alveolar pressure is
the measured pressure inside the airways and the alveoli. When air is neither being inspired nor
expired, the intrapulmonary pressure is equal to atmospheric pressure, or zero. The intrapleural
pressure is the measured
Downloaded by Benjamin Luca ()
, lOMoARcPSD|51648332
pressure in the pleural cavity. In a normal inflated lung, the intrapleural pressure is negative relative to
the intrapulmonary pressure. In other words, there is a negative pressure in the pleural cavities. This is
caused by an oppositional pull that occurs between the parietal and visceral layers of the pleura. As the
elastic recoil of the lungs increases, the intrapleural pressure becomes more negative during inspiration
than expiration. This negative pressure holds the lungs against the chest wall. Without it, the elastic
recoil of the lungs would cause them to collapse. The intrathoracic pressure is the pressure in the
thoracic cavity and is equal to the intrapleural pressure.
The lungs are contained within the thoracic cavity, along with the heart, great vessels, and esophagus.
For the reasons of pressure control described above, the act of breathing depends on this closed cavity.
The diaphragm is the main muscle of inspiration. When the diaphragm contracts (inspiration), the chest
expands. In a minor role, the external intercostal muscles, scalene and sternocleidomastoid muscles aid
in breathing. Each is associated along the ribs, raising and spreading them during inhalation, although
most commonly exerting aid during exercise or difficulty breathing.
The diaphragm is innervated primarily by C4 but also by C3, and C5 of the spinal cord. Thus, people
who sustain a spinal cord injury above C3 lose diaphragm function and require mechanical ventilation.
Paralysis of one side of the diaphragm causes the chest to move up on that side rather than down
during inspiration because of the negative pressure in the chest. This is referred to as paradoxical
movement.
Figure 5.4 Chest volume and pressure changes during inspiration and expiration and movement of the diaphragm
Lung compliance is the ease with which lungs can inflate. For example, blowing up a new balloon is
hard because it is noncompliant. In contrast, blowing up a balloon that has already been inflated is
easier because it has become compliant. Thus, it takes more pressure to move air into a noncompliant
lung than a compliant one. Lung compliance depends on multiple factors including overall water content
and surface tension, as well as the amount of elastin and collagen fibers that are present.
Surface tension refers to the force exerted by water molecules on the surface of alveoli. As water
molecules are held together by strong covalent bonds, this force is stronger than the force holding air
molecules within the alveolar space. As such, as air leaves during exhalation, the strong surface tension
the water exerts on the surrounding
Downloaded by Benjamin Luca ()
