18 - Lungs

• The lungs occupy all of the thoracic cavity except
for the mediastinum
• Each lung is suspended within its own pleural cavity
and connected to the mediastinum by vascular and
bronchial attachments called the lung root.
• The left lung is smaller than the right because the
position of the heart is shifted slightly to the left.
• Each lung is divided into lobes, separated from
each other by fissures

Blood supply and innervation:

• Lung tissue consists largely of air spaces, with the
majority of lung tissue, called the lung stroma,
composed primarily of elastic connective tissue.
• Two circulatory routes go to the lungs: pulmonary
arteries (pulmonary circuit) carry blood to the lungs
for oxygenation, and bronchial arteries (systemic
circuit) carry oxygenated blood to the lung tissue.
• The lungs are innervated by parasympathetic and
sympathetic motor fibres that constrict or dilate the
airways, as well as visceral sensory fibres

Parietal and visceral pleura:

• The parietal pleura covers the thoracic wall, the
diaphragm and around the heart between the
lungs. The visceral pleura covers the external
lung surface. Pleural fluid lubricates the space
between the pleurae (the pleural cavity) to reduce
friction when the lungs move during breathing.
• The pleurae divide the thoracic cavity into three
discrete chambers, preventing one organ’s
movement from interfering with another’s, as well
as limiting the spread of infection

Lung compliance and resilience:

• Lung compliance (lung stretch) refers to how easily
the lungs can expand or inflate when we inhale.
• Lung resilience (lung recoil) is the opposite of
compliance: it is the lungs’ ability to return to their
original shape after being stretched. This should be
high enough to help us exhale without much effort.
• Factors such as chronic inflammation, buildup of
non-elastic scar tissue, and decreased surfactant
can reduce lung compliance. Breakdown of alveolar
walls and elastic fibres reduces lung resilience

Respiratory volumes:

• Tidal volume (TV) is the amount of air that
moves in and out of the lungs with each breath
during quiet breathing. It averages 500 mL per
breath.
• The inspiratory reserve volume (IRV) is the
amount of air that can be inspired beyond the tidal
volume.
• The expiratory reserve volume (ERV) is the
amount of air that can be evacuated from the lungs
after tidal expiration

Vital and total lung volumes:

• Vital capacity is the sum of tidal volume,
inspiratory reserve, and expiratory reserve volumes
and is the total amount of exchangeable air.
• Residual volume is the volume of air remaining
in the lungs after maximal forced expiration.
• Total lung capacity is the sum of all lung volumes.
• The anatomical dead space is the volume of the
conducting zone conduits, roughly 150 mL, that
doesn’t contribute to gas exchange in the lungs

Ventilation and Boyle’s Law:

• Pulmonary ventilation is a mechanical process
causing gas flow into and out of the lungs according
to volume changes in the thoracic cavity.
• Boyle’s law states that at a constant temperature,
the pressure of a gas varies inversely with its
volume. Pressure changes lead to gas flow.

Ex. If the air inside the lungs is at higher pressure than
the air outside of the body, air will move out of the
lungs and up the respiratory tract. If the air inside
the lungs is at lower pressure, air will be drawn in

Quiet and forced inspiration:

• During quiet inspiration, the diaphragm and
intercostal muscles contract, increasing the thoracic
volume. Intrapulmonary pressure to drop below
atmospheric pressure, and air flows into the lungs.
• During forced inspiration, accessory muscles of
the neck and thorax contract, further increasing
thoracic volume.
• Alveolar surface tension acts to draw the walls of
the alveoli together, presenting a force that must be
overcome in order to expand the lungs

Quiet and forced expiration:

Quiet expiration is a passive process that relies
mostly on elastic recoil of the lungs as the thoracic
muscles relax.
• Forced expiration is an active process relying on
contraction of abdominal muscles to increase intra-
abdominal pressure and depress the rib cage.
• Nonrespiratory air movements cause movement of
air into or out of the lungs, but are not related to
breathing (coughing, sneezing, crying, laughing,
hiccups, and yawning)

Respiratory pressures:

• Intrapulmonary pressure, also called alveolar
pressure, is the air pressure in the lung alveoli.
This rises and falls during respiration, but always
eventually equalizes with atmospheric pressure.
• Intrapleural pressure is the pressure inside
the pleural cavity (between the visceral pleura and
parietal pleura). It also rises and falls, but is always
lower than intrapulmonary pressure.
•Transpulmonary pressure is the difference
between intrapulmonary and intrapleural pressure

• Intrapleural pressure is always slightly lower than
atmospheric pressure and intrapulmonary pressure
because of two opposing forces:
• The inward pull of recoil force and surface tension
of alveolar fluid in the lungs.
• The outward pull from the curve of the chest wall.
• Some textbooks describe respiratory pressures
relative to atmospheric pressure. With this math,
intrapulmonary pressure is 0 mmHg (equal to
atmospheric) and intrapleural pressure is −4 mmHg

Airway resistance:

Airway resistance is the friction encountered by air
in the airways. Gas flow is reduced as airway
resistance increases.
• Airway resistance is normally insignificant because:
• Upper airways are very large in diameter.
• Lower airways, while smaller, are very numerous.
• Airway resistance can become a problem if the large
or medium-sized airways get much narrower, or if
many of the smaller airways become blocked.