Physics of the lungs and breathing
We breathe (6 liter) of air per minute (this is also
about the volume of the blood the heart pumps each
minute. Men breathe 12 times per minute at rest
women 20 times and infant 60, the air we inspired
about 80% N2 +20% O2, the air we expired about
80%N2 + 16%O2 + 4%CO2, the large convoluted
surface of the lungs with a surface area about 80m2
have a greater exposure to the environment than any
other part included skin. Its surprising we don't have
more diseases of the lungs.
The airways
The air passes through the windpipe (trachea). Each
bronchus divided and redivides about 15 times there
are alveoli like bubbles of 0.2 mm diameter and walls
0.4μm each alveolus surrounding by blood, so O2 can
diffuse from alveolus into R.B.C. and CO2 diffuses
from the blood into the air in the alveolus. The body
has two mechanisms for cleaning the airways of
foreign particles.
•
Large chunks removed by coughing
•
Small particles carried upward to the mouth by
million of small hairs or cilia of 0.1 mm long. Each
cilia vibrate about 1000 times a minute. The mucous
moves 1-2 cm/min. Cilia as an escalator system of
trachea it takes 30 minutes for particle of dust to be
cleared out of the bronchi and trachea into the throat,
where it is expelled or swallowed.
How blood and lungs interact
About (1 liter) of the blood supply in the lungs, but 70
ml is in the capillaries of the lungs getting O2. The
transfer of O2 and CO2 into and out of the blood is
controlled by law of diffusion. Molecules diffuse from
a region of higher concentration to lower on until the
concentration uniform.
A molecule of O2 diffuses faster than CO2 because of
its smaller mass. The lungs are not emptied during
expiration during normal breathing the lungs retain
about 30% of their volume at the end of each
expiration.
Measurement of lung volume
During normal breathing we inhale 500 cm3of air
with each breath. If a person cough or sneezes hard
the velocity of air in the trachea can reach the velocity
of sound in air this high velocity can cause partial
collapse of the airways because of the Bernoulli effect
(define).
In coughing to dislodge foreign object this partial
collapse increases air velocity and increase the force
on a foreign object.
Physics of the alveoli
The alveoli like millions of small, interconnected
bubbles they have the tendency to get smaller due to
the surface tension of unique fluid lining, this lineage
called the surfactant. The absence of surfactant in the
lungs of some newborn infant is the cause of
Respiratory Distress Syndrome (RDS). The called
hyaline membrane disease causes death. To
understand the physics of alveoli we have to
understand the physics of bubbles.
The (P) inside the bubble is inversely proportional to
radius and directly to surface tension.
Laplaces law: P = 4γ/R where R radius, γ surface
tension.
Two forces keep the lungs from collapsing:
• Surface tension between the lungs and the chest
wall
• Air pressure inside the lungs
Since each lung is its own sealed compartment it is
possible to collapse one lung only.
This done by inserting a hollow needle between ribs
and allowing air to flow into intra thoracic space.
The air trapped in the space is gradually absorbed
by the tissues and lung expand to normal after a
few weeks.
Sometimes lung collapse spontaneously with no
known cause. The lung returns to normal size as
the air is absorbed into the surrounding tissues.
Since both lung and chest wall are elastic they look
like springs.
Not all air we inspire adds O2 to blood. The volume of
trachea and bronchi called anatomic dead space since air
in space is not exposed to blood in pulmonary capillaries
the space about 150 cm3.
Airway Resistance:
During inspiration the forces on airway tend to open
them further, during expiration the forces tend to
close the airways and restrict air flow.
Airway resistance Rg = ∆P/V
∆P = pressure difference = voltage
V= rate of air flow
The resistance depends on the dimension of the tube,
viscosity of gas, most of Rg in the upper airway
passages Thus, diseases that affect terminal airways
(bronchioles and alveoli) do not affect airway
resistance until they are far advanced.
Physics of common lung diseases
Emphysema: the divisions between alveoli break
down producing larger lung spaces. This destruction
of lung tissue reduces the springiness of the lungs.
The lungs become more compliant, small change in
pressure produces larger than normal change in
volume. Emphysema produces two changes:
1. The lungs become flabby and expands
2. The tissues do not pull very hard on the airways
Permitting the narrowed airways to collapse easily
during expiration