CHAPTER 2 Respiratory System^87
Air is then drawn through the pharynx, larynx, trachea, and bronchi. The bronchi
branches (bifurcates) right and left into smaller tubes called bronchioles that ter-
minate in alveoli. The airways are lined with mucous membranes to add moisture
to the inhaled air. There is a thin layer of mucous in the airways that helps to trap
foreign particles, such as dust, pollen, or bacteria. Cilia—small, hair-like projec-
tions—help to move the mucous with the foreign material upward so it can be
coughed out.
Alveoli are air-filled sacs containing membranes coated with surfactant. The
surfactant helps the alveoli to expand evenly on inspiration and prevents collapse
on exhalation. Carbon dioxide and oxygen are exchanged; a higher concentration
of gas moves to the lower area of concentration. A higher concentration of carbon
dioxide in the hemoglobin moves across the membrane into the alveoli and is
expired by the lung. Higher concentration of oxygen in the alveoli crosses the
membrane and attaches to the hemoglobin which is then distributed by the circu-
latory system throughout the body.
Lungs are contained within a pleural sac in the thoracic cavity and operate on
negative pressure. The visceral pleura is close to the lungs and the parietal pleura
is close to the chest wall. There is a pleural space between these two layers that
contains a small amount of fluid to prevent friction with chest movement on inspi-
ration and expiration.
Just the Facts
Acute Respiratory Distress Syndrome (ARDS)
WHAT WENT WRONG?
Patients develop acute respiratory failure. Lungs stiffen as a result of a build-up of
fluid in the lungs. Fluid builds up in the tissue of the lungs (interstitium) and the
alveoli. This fluid and stiffness impairs the lungs’ ability to move air in and out
(ventilation). There is an inflammatory response in the tissues of the lungs. Damage
to the surfactant within the alveoli leads to alveolar collapse, further impairing gas
exchange. An attempt to repair the alveolar damage may lead to fibrosis within
the lung. Even as the respiratory rate increases, sufficient oxygen cannot enter the
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