Both inhalation and exhalation depend on pressure gradients between
the lungs and atmosphere, as well as the muscles in the thoracic
cavity.
LEARNING OBJECTIVE [ edit ]
Describe how the structures of the lungs and thoracic cavity control the mechanics of breathing
KEY POINTS [ edit ]
The mechanics of breathing follow Boyle's Law which states that pressure and volume have an
inverse relationship.
The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction
and chest wall expansion) which results in a decrease in lung pressure in comparison to the
atmosphere; thus, air rushes in the airway.
The process of exhalation occurs due to an elastic recoil of the lung tissue which causes a decrease
in volume, resulting in increased pressure in comparison to the atmosphere; thus, air rushes out
of the airway.
There is no contraction of muscles during exhalation; it is considered a passive process.
The lung is protected by layers of tissue referred to as thevisceral pleura and parietal pleura; the
intrapleural space contains a small amount of fluid that protects the tissue by reducing friction.
TERMS [ edit ]
visceral pleura
the portion of protective tissue that is attached directly to the lungs
parietal pleura
the portion of the protective tissue that lines the inner surface of the chest wall and covers the
diaphragm
Give us feedback on this content: FULL TEXT [ edit ]
The Mechanics of Human
Breathing
The relationship between gas pressure
and volume helps to explain the
mechanics of breathing. Boyle's Law is the
gas law which states that in a closed
space, pressure and volume are inversely
related. As volume decreases, pressure
increases and vice versa . When
discussing the detailed mechanics of
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breathing, it is important to keep this inverse relationship in mind.
Boyles law
This graph of data from Boyle's original 1662 experiment shows that pressure and volume are inversely
related. No units are given as Boyle used arbitrary units in his experiments.
Inhalation and Exhalation
The thoracic cavity, or chest cavity, always has a slight, negative pressure which aids in
keeping the airways of the lungs open. During the process of inhalation, the lung volume
expands as a result of the contraction of the diaphragm andintercostal muscles (the muscles
that are connected to the rib cage), thus expanding the thoracic cavity. Due to this increase in
volume, the pressure is decreased, based on the principles of Boyle's Law. This decrease of
pressure in the thoracic cavity relative to the environment makes the cavity pressure less
than the atmospheric pressure . This pressure gradient between the atmosphere and the
thoracic cavity allows air to rush into the lungs; inhalation occurs. The resulting increase in
volume is largely attributed to an increase in alveolar space because the bronchioles and
bronchi are stiff structures that do not change in size.
Inhalation and exhalation
The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b)
expiration.
During this process, the chest wall expands out and away from the lungs. The lungs are
elastic; therefore, when air fills the lungs, the elastic recoil within the tissues of the lung
exerts pressure back toward the interior of the lungs. These outward and inward forces
compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil
to force the air out of the lungs. The intercostal muscles relax, returning the chest wall to its
original position . During exhalation, the diaphragm also relaxes, moving higher into the
thoracic cavity. This increases the pressure within the thoracic cavity relative to the
environment. Air rushes out of the lungs due to the pressure gradient between the thoracic
cavity and the atmosphere. This movement of air out of the lungs is classified as a passive
event since there are no muscles contracting to expel the air.
Protection of the Lung
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and
dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the
interior of the thorax . The space between these layers, the intrapleural space, contains a
small amount of fluid that protects the tissue by reducing the friction generated from
rubbing the tissue layers together as the lungs contract and relax. If these layers of tissues
become inflamed, this is categorized as pleurisy: a painful inflammation that increases the
pressure within the thoracic cavity, reducing the volume of the lung.
Visceral pleura
A tissue layer called pleura surrounds the lung and interior of the thoracic cavity.