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رمضان كريم
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Chart for subscribers

1. Asthma
■ is an obstructive disease in which expiration is impaired.
■ is characterized by decreased FVC, decreased FEV1, and decreased FEV1/FVC.
■ Air that should have been expired is not, leading to air trapping and increased FRC.
2. COPD
■ is a combination of chronic bronchitis and emphysema.
■ is an obstructive disease with increased lung compliance in which expiration is
impaired.
■ is characterized by decreased FVC, decreased FEV1, and decreased FEV1/FVC.
■ Air that should have been expired is not, leading to air trapping, increased FRC, and a
barrel-shaped chest.
a. “Pink puffers” (primarily emphysema) have mild hypoxemia and, because they maintain
alveolar ventilation, normocapnia (normal Pco2).
b. “Blue bloaters” (primarily bronchitis) have severe hypoxemia with cyanosis and,
because they do not maintain alveolar ventilation, hypercapnia (increased Pco2). They
have right ventricular failure and systemic edema.
3. Fibrosis
■ is a restrictive disease with decreased lung compliance in which inspiration is impaired.
■ is characterized by a decrease in all lung volumes. Because FEV1 is decreased less than is
FVC, FEV1/FVC is increased (or may be normal).

Breathing cycle—description of pressures and airflow
1. At rest (before inspiration begins)
a. Alveolar pressure equals atmospheric pressure.
■ Because lung pressures are expressed relative to atmospheric pressure, alveolar
pressure is said to be zero.
b. Intrapleural pressure is negative.
■ At FRC, the opposing forces of the lungs trying to collapse and the chest wall trying to expand create a negative pressure in the intrapleural space between them.
■ Intrapleural pressure can be measured by a balloon catheter in the esophagus.
c. Lung volume is the FRC.
2. During inspiration
a. The inspiratory muscles contract and cause the volume of the thorax to increase.
■ As lung volume increases, alveolar pressure decreases to less than atmospheric pressure (i.e., becomes negative).
■ The pressure gradient between the atmosphere and the alveoli now causes air to flow
into the lungs; airflow will continue until the pressure gradient dissipates.
b. Intrapleural pressure becomes more negative.
■ Because lung volume increases during inspiration, the elastic recoil strength of the
lungs also increases. As a result, intrapleural pressure becomes even more negative
than it was at rest.
■ Changes in intrapleural pressure during inspiration are used to measure the dynamic
compliance of the lung
c. Lung volume increases by one Vt.
■ At the peak of inspiration, lung volume is the FRC plus one Vt.
3. During expiration
a. Alveolar pressure becomes greater than atmospheric pressure.
■ The alveolar pressure becomes greater (i.e., becomes positive) because alveolar gas
is compressed by the elastic forces of the lung.
■ Thus, alveolar pressure is now higher than atmospheric pressure, the pressure gradient is reversed, and air flows out of the lungs.
b. Intrapleural pressure returns to its resting value during a normal (passive) expiration.
■ However, during a forced expiration, intrapleural pressure actually becomes positive.
This positive intrapleural pressure compresses the airways and makes expiration
more difficult.
■ In COPD, in which airway resistance is increased, patients learn to expire slowly
with “pursed lips” to prevent the airway collapse that may occur with a forced
expiration.
c. Lung volume returns to FRC

Factors that change airway resistance
■ The major site of airway resistance is the medium-sized bronchi.
■ The smallest airways would seem to offer the highest resistance, but they do not
because of their parallel arrangement.
a. Contraction or relaxation of bronchial smooth muscle
■ changes airway resistance by altering the radius of the airways.
(1) Parasympathetic stimulation, irritants, and the slow-reacting substance of anaphylaxis (asthma) constrict the airways, decrease the radius, and increase the resistance
to airflow.
(2) Sympathetic stimulation and sympathetic agonists (isoproterenol) dilate the airways
via b2 receptors, increase the radius, and decrease the resistance to airflow.
b. Lung volume
■ alters airway resistance because of the radial traction exerted on the airways by surrounding lung tissue.
(1) High lung volumes are associated with greater traction on airways and decreased
airway resistance. Patients with increased airway resistance (e.g., asthma) “learn” to breathe at higher lung volumes to offset the high airway resistance associated
with their disease.
(2) Low lung volumes are associated with less traction and increased airway resistance,
even to the point of airway collapse.
c. Viscosity or density of inspired gas
■ changes the resistance to airflow.
■ During a deep-sea dive, both air density and resistance to airflow are increased.
■ Breathing a low-density gas, such as helium, reduces the resistance to airflow.

Surfactant
■ lines the alveoli.
■ reduces surface tension by disrupting the intermolecular forces between liquid molecules. This reduction in surface tension prevents small alveoli from collapsing and
increases compliance.
■ is synthesized by type II alveolar cells and consists primarily of the phospholipid
dipalmitoylphosphatidylcholine (DPPC).
■ In the fetus, surfactant synthesis is variable. Surfactant may be present as early as gestational week 24 and is almost always present by gestational week 35.
■ Generally, a lecithin:sphingomyelin ratio greater than 2:1 in amniotic fluid reflects
mature levels of surfactant.
■ Neonatal respiratory distress syndrome can occur in premature infants because of the
lack of surfactant. The infant exhibits atelectasis (lungs collapse), difficulty reinflating the lungs (as a result of decreased compliance), and hypoxemia (as a result of
decreased V/Q