ENDOCRINE SYSTEM AND DIGESTIVE AND RESPIRATORY TRACT
The respiratory tract is the pathway through
which air travels from the external
environment to the lungs, where gas
exchange occurs. It can be divided into two
main parts: the upper respiratory tract and
the lower respiratory tract. The upper
respiratory tract consists of the nose, pharynx
and larynx. The lower respiratory tract
consists of the trachea, bronchi, bronchioles
and alveoli.
The lumen of the trachea and bronchi is wide,
and the walls are supported by C-shaped
cartilage. The smooth muscle layer is relaxed.
The lumen slightly narrows due to contraction of
the smooth muscle, particularly in the posterior
part of the trachea where there is no cartilage.
The red arrows indicate the direction of
contraction.
The lumen of the intrapulmonary bronchi has a star-shaped appearance,
supported by cartilage plates and surrounded by smooth muscle. The lumen of the
bronchi becomes much smaller due to the contraction of the smooth muscle, which
encircles (omsluiten) the airway.
The cartilage in the trachea and main bronchi prevents collapse during breathing, but
the posterior part of the trachea and the bronchioles (which have less or no cartilage)
are more affected by muscle contraction. The lower the respiratory tract, the less
cartilage.
Submucosal glands are predominantly found in the upper airways. It is composed of
serous cells (producing a watery secretion) and mucous cells (producing mucus).
These glands produce mucus that helps trap inhaled particles (like dust, pathogens,
and pollutants) and keep the airway surface moist. The mucus contains enzymes and
antibodies (e.g. lysozyme and IgA) that help in defending against infections.
Ciliated cells are present throughout the upper airways and small airways. The
cells have hair-like projections called cilia that extend from the cell surface. Cilia beat
rhythmically to move the mucus layer towards the pharynx, where it can be
swallowed. This mechanism helps to clear inhaled particles and pathogens from the
respiratory tract.
1
,Goblet cells are also present throughout the upper airways and small airways.
Goblet cells secrete a thinner mucus.
Basal cells are located in the upper airways and small airways. Basal cells are
progenitor cells (stem cells) that can differentiate into ciliated cells, goblet cells, or
other types of epithelial cells to maintain and repair the airway epithelium.
Club cells (Clara cells) are predominantly found in the small airways. Club cells
secrete a variety of proteins which help protect the bronchiolar epithelium and
reduce surface tension. Club cells can also act as progenitor cells, regenerating the
bronchiolar epithelium after injury. They also have enzymatic activity that helps
detoxify harmful substances inhaled into the lungs (antimicrobial function).
Submucosal glands, ciliated cells, goblet cells, and basal cells work together to
produce, move, and regulate mucus, providing a primary defense mechanism against
inhaled pathogens and particles. Club cells take on a more prominent role in
secreting protective substances, detoxifying inhalants, and repairing the bronchiolar
epithelium. The amount of produced mucus is higher in the upper airways than
in the small airways !
The distal lung, specifically the alveoli, is where gas exchange takes place. The
alveoli are tiny, balloon-like structures that provide a large surface area for oxygen
and carbon dioxide exchange between the air and the blood. Each alveolus is
surrounded by a network of capillaries, ensuring close contact between air in the
alveolus and blood in the capillaries.
The walls of the alveoli are extremely thin (one cell thick) to facilitate efficient gas
exchange. These walls are composed of two main types of cells: Type I alveolar
cells and Type II alveolar cells.
• Type I alveolar cells are flat and extremely thin, covering about 95% of the
alveolar surface area. They form the majority of the alveolar wall, creating the
thin barrier through which gas exchange occurs.
• Type II alveolar cells are cuboidal, making up about 5% of the alveolar surface
area. Type II cells secrete surfactant, a substance composed of lipids and
proteins that reduces surface tension within the alveoli. This prevents the
alveoli from collapsing during exhalation and ensures they remain open for
efficient gas exchange. Type II cells also serve as progenitor cells. They can
proliferate and differentiate into Type I cells, especially after injury, helping to
repair the alveolar epithelium.
According to PP:
• Ciliated cell - mucocilairy transport
• Goblet cell / mucous cell – mucus production
• Basal cell – airway progenitor
• Club cell – secretory proteins, small airway progenitor, antimicrobial activity
2
,Mucociliary transport is a defense mechanism of the respiratory tract that helps
remove inhaled particles, pathogens, and mucus from the airways, keeping them
clean and preventing infections. This process involves ciliated cells and goblet
cells.
Goblet cells produce mucus proteins. Ciliated cells help control the hydration of the
mucus by regulating the movement of ions, particularly sodium (Na⁺) and chloride
(Cl⁻), across the epithelial membrane. Cl⁻ are transported into the lumen of the
airway through ion channels, such as the Cystic Fibrosis
Transmembrane Conductance Regulator (CFTR). Water
follows the movement of chloride ions passively, drawn
into the lumen due to osmotic gradients. This process
ensures that the mucus has the right consistency.
(Cl- enters the cell in combination with Na+, but there is a
recycling of Na+ out of the cell while Cl- enters the lumen)
Without functional CFTR channels, Cl- cannot be transported into the airway lumen
effectively. The absence of Cl- movement means that water does not follow into the
lumen, resulting in dehydrated, thickened mucus. The thick mucus is difficult to clear,
leading to the accumulation of mucus in the airways. The CFTR is thus an important
channel of the respiratory tract.
Diseases of the respiratory tract –
• Asthma
• Allergic rhinitis
• Cough
• COPD
• Pulmonary fibrosis
• Cystic fibrosis
Asthma & allergies –
Asthma is a heterogeneous disease, usually characterized by chronic airway
inflammation. It is defined by respiratory symptoms such as wheeze, shortness of
breath, chest tightness and cough that vary over time and in intensity, together with
variable expiratory airflow limitation.
In the graph it is seen that asthma &
allergies are the most common chronic
condition. However, these 2 conditions
should be distinguished as asthma is not
always an allergic reaction.
For a condition to be asthma, there have to be symptoms from the lung, not simply
from the nose.
3
, The prevalence of asthma and allergies has
increased over the years, which may be
explained by the hygiene hypothesis. The
Hygiene Hypothesis posits that a lack of
exposure to microbes and infections in early
childhood, due to a cleaner, more sanitized
environment, results in an underdeveloped
or improperly regulated immune system.
This underexposure leads to an increased
susceptibility to allergic diseases and asthma.
The prevalence of asthma and allergies is higher in children than in adults.
The gender paradox in the prevalence of
asthma and allergies refers to the observation
that asthma is more common in boys than in
girls during childhood, but this pattern
reverses in adulthood, with a higher
prevalence in women than in men. This
paradox is influenced by a combination of
hormonal differences and variations in lung development between genders.
Airway hyperresponsiveness is a hallmark feature of asthma. It refers to the
increased sensitivity and exaggerated narrowing (bronchoconstriction) of the airways
in response to stimuli that would typically not cause
such a reaction in healthy individuals.
The dashed lines mark the concentration points
where each category of individuals shows a
significant (20%) change in FEV₁. Asthmatic
individuals show a significant reduction in FEV₁ at
very low concentrations of the bronchoconstrictor,
indicating airway hyperresponsiveness.
Individuals with a genetic predisposition may be
more susceptible to developing asthma when
exposed to certain environmental factors.
Common environmental triggers include
allergens, viruses, and occupational sensitizers.
These factors can contribute to airway
inflammation. In the early stages, inflammation
causes a temporary increase in airway
responsiveness to various stimuli. Persistent
inflammation can lead to structural changes in
the airways, such as thickening of the airway
walls, which can result in permanent airway hyperresponsiveness. Allergens, viruses,
exercise and cold air can induce symptoms of asthma and airflow obstruction.
4
The respiratory tract is the pathway through
which air travels from the external
environment to the lungs, where gas
exchange occurs. It can be divided into two
main parts: the upper respiratory tract and
the lower respiratory tract. The upper
respiratory tract consists of the nose, pharynx
and larynx. The lower respiratory tract
consists of the trachea, bronchi, bronchioles
and alveoli.
The lumen of the trachea and bronchi is wide,
and the walls are supported by C-shaped
cartilage. The smooth muscle layer is relaxed.
The lumen slightly narrows due to contraction of
the smooth muscle, particularly in the posterior
part of the trachea where there is no cartilage.
The red arrows indicate the direction of
contraction.
The lumen of the intrapulmonary bronchi has a star-shaped appearance,
supported by cartilage plates and surrounded by smooth muscle. The lumen of the
bronchi becomes much smaller due to the contraction of the smooth muscle, which
encircles (omsluiten) the airway.
The cartilage in the trachea and main bronchi prevents collapse during breathing, but
the posterior part of the trachea and the bronchioles (which have less or no cartilage)
are more affected by muscle contraction. The lower the respiratory tract, the less
cartilage.
Submucosal glands are predominantly found in the upper airways. It is composed of
serous cells (producing a watery secretion) and mucous cells (producing mucus).
These glands produce mucus that helps trap inhaled particles (like dust, pathogens,
and pollutants) and keep the airway surface moist. The mucus contains enzymes and
antibodies (e.g. lysozyme and IgA) that help in defending against infections.
Ciliated cells are present throughout the upper airways and small airways. The
cells have hair-like projections called cilia that extend from the cell surface. Cilia beat
rhythmically to move the mucus layer towards the pharynx, where it can be
swallowed. This mechanism helps to clear inhaled particles and pathogens from the
respiratory tract.
1
,Goblet cells are also present throughout the upper airways and small airways.
Goblet cells secrete a thinner mucus.
Basal cells are located in the upper airways and small airways. Basal cells are
progenitor cells (stem cells) that can differentiate into ciliated cells, goblet cells, or
other types of epithelial cells to maintain and repair the airway epithelium.
Club cells (Clara cells) are predominantly found in the small airways. Club cells
secrete a variety of proteins which help protect the bronchiolar epithelium and
reduce surface tension. Club cells can also act as progenitor cells, regenerating the
bronchiolar epithelium after injury. They also have enzymatic activity that helps
detoxify harmful substances inhaled into the lungs (antimicrobial function).
Submucosal glands, ciliated cells, goblet cells, and basal cells work together to
produce, move, and regulate mucus, providing a primary defense mechanism against
inhaled pathogens and particles. Club cells take on a more prominent role in
secreting protective substances, detoxifying inhalants, and repairing the bronchiolar
epithelium. The amount of produced mucus is higher in the upper airways than
in the small airways !
The distal lung, specifically the alveoli, is where gas exchange takes place. The
alveoli are tiny, balloon-like structures that provide a large surface area for oxygen
and carbon dioxide exchange between the air and the blood. Each alveolus is
surrounded by a network of capillaries, ensuring close contact between air in the
alveolus and blood in the capillaries.
The walls of the alveoli are extremely thin (one cell thick) to facilitate efficient gas
exchange. These walls are composed of two main types of cells: Type I alveolar
cells and Type II alveolar cells.
• Type I alveolar cells are flat and extremely thin, covering about 95% of the
alveolar surface area. They form the majority of the alveolar wall, creating the
thin barrier through which gas exchange occurs.
• Type II alveolar cells are cuboidal, making up about 5% of the alveolar surface
area. Type II cells secrete surfactant, a substance composed of lipids and
proteins that reduces surface tension within the alveoli. This prevents the
alveoli from collapsing during exhalation and ensures they remain open for
efficient gas exchange. Type II cells also serve as progenitor cells. They can
proliferate and differentiate into Type I cells, especially after injury, helping to
repair the alveolar epithelium.
According to PP:
• Ciliated cell - mucocilairy transport
• Goblet cell / mucous cell – mucus production
• Basal cell – airway progenitor
• Club cell – secretory proteins, small airway progenitor, antimicrobial activity
2
,Mucociliary transport is a defense mechanism of the respiratory tract that helps
remove inhaled particles, pathogens, and mucus from the airways, keeping them
clean and preventing infections. This process involves ciliated cells and goblet
cells.
Goblet cells produce mucus proteins. Ciliated cells help control the hydration of the
mucus by regulating the movement of ions, particularly sodium (Na⁺) and chloride
(Cl⁻), across the epithelial membrane. Cl⁻ are transported into the lumen of the
airway through ion channels, such as the Cystic Fibrosis
Transmembrane Conductance Regulator (CFTR). Water
follows the movement of chloride ions passively, drawn
into the lumen due to osmotic gradients. This process
ensures that the mucus has the right consistency.
(Cl- enters the cell in combination with Na+, but there is a
recycling of Na+ out of the cell while Cl- enters the lumen)
Without functional CFTR channels, Cl- cannot be transported into the airway lumen
effectively. The absence of Cl- movement means that water does not follow into the
lumen, resulting in dehydrated, thickened mucus. The thick mucus is difficult to clear,
leading to the accumulation of mucus in the airways. The CFTR is thus an important
channel of the respiratory tract.
Diseases of the respiratory tract –
• Asthma
• Allergic rhinitis
• Cough
• COPD
• Pulmonary fibrosis
• Cystic fibrosis
Asthma & allergies –
Asthma is a heterogeneous disease, usually characterized by chronic airway
inflammation. It is defined by respiratory symptoms such as wheeze, shortness of
breath, chest tightness and cough that vary over time and in intensity, together with
variable expiratory airflow limitation.
In the graph it is seen that asthma &
allergies are the most common chronic
condition. However, these 2 conditions
should be distinguished as asthma is not
always an allergic reaction.
For a condition to be asthma, there have to be symptoms from the lung, not simply
from the nose.
3
, The prevalence of asthma and allergies has
increased over the years, which may be
explained by the hygiene hypothesis. The
Hygiene Hypothesis posits that a lack of
exposure to microbes and infections in early
childhood, due to a cleaner, more sanitized
environment, results in an underdeveloped
or improperly regulated immune system.
This underexposure leads to an increased
susceptibility to allergic diseases and asthma.
The prevalence of asthma and allergies is higher in children than in adults.
The gender paradox in the prevalence of
asthma and allergies refers to the observation
that asthma is more common in boys than in
girls during childhood, but this pattern
reverses in adulthood, with a higher
prevalence in women than in men. This
paradox is influenced by a combination of
hormonal differences and variations in lung development between genders.
Airway hyperresponsiveness is a hallmark feature of asthma. It refers to the
increased sensitivity and exaggerated narrowing (bronchoconstriction) of the airways
in response to stimuli that would typically not cause
such a reaction in healthy individuals.
The dashed lines mark the concentration points
where each category of individuals shows a
significant (20%) change in FEV₁. Asthmatic
individuals show a significant reduction in FEV₁ at
very low concentrations of the bronchoconstrictor,
indicating airway hyperresponsiveness.
Individuals with a genetic predisposition may be
more susceptible to developing asthma when
exposed to certain environmental factors.
Common environmental triggers include
allergens, viruses, and occupational sensitizers.
These factors can contribute to airway
inflammation. In the early stages, inflammation
causes a temporary increase in airway
responsiveness to various stimuli. Persistent
inflammation can lead to structural changes in
the airways, such as thickening of the airway
walls, which can result in permanent airway hyperresponsiveness. Allergens, viruses,
exercise and cold air can induce symptoms of asthma and airflow obstruction.
4
