Pressure Relationships in the Thoracic Cavity

Intrapulmonary pressure
is the pressure in the
alveoli, which rises and
falls during respiration,
but always eventually
equalizes with
atmospheric pressure.

Intrapleural pressure is
the pressure in the pleural
cavity. It also rises and
falls during respiration,
but is always about 4mm
Hg less than
intrapulmonary pressure.
Importance of Pressure Relationships

Transpulmonary pressure (Palv – Pip) keeps the airspaces of the
lungs open.

The negative pressure of the intrapleural space and the tight
coupling of the lungs to the thoracic walls is extremely
important.

If intrapleural pressure is equalized with intrapulmonary or
atmospheric pressure, lung collapse will occur immediately


Atelectasis (lung collapse) commonly occurs when air enters the
pleural cavity through a chest wound; it can also result from
ruptured visceral pleura (usually due to pneumonia) allowing
air to enter the pleural cavity from the respiratory tract
Pneumothorax (air in the intrapleural space) is reversed by
closing the hole and drawing air out of the intrapleural space
with chest tubes, allowing lungs to reinflate and resume normal
function.
Pulmonary Ventilation

Mechanical process causing gas flow into and out of the lungs
according to volume changes in the thoracic cavity. (A.K.A.
“Breathing”)

Consists of two phases:



Important physics rule to remember for breathing mechanics:



Inspiration: period of time when air flows into the lungs
Expiration: period of time when gases exit the lungs
Volume changes lead to pressure changes
Pressure changes lead to flow of gases to equalize pressure
Boyle’s Law: (when temp constant) P1V1 = P2V2



At a constant temperare, pressure varies inversely with volume
P = pressure in mm Hg
V = volume in cubic mm
Intrapulmonary & Intrapleural Pressure
Relationships During Pulmonary Ventilation

Gases, like liquids,
conform to the shape of
their container

Unlike liquids, gases
always fill their container

In a large volume, the gas
molecules will be far apart
and the pressure will be
low

If the volume is reduced,
the gas molecules will be
compressed and the
pressure will rise
Inspiration

Diaphragm and intercostals muscles contract
 Diaphragm moves inferiorly and flattens during
contraction, causing height of thoracic cavity to increase
 Intercostals contraction lifts the ribcage and thrusts
sternum forward, increasing anterioposterior and lateral
dimensions (circumference)

Lungs adhere tightly to the thorax walls (due to surface tension
of fluid between pleural membranes), they are stretched to the
new, larger size of the thorax.

As intrapulmonary volume increases, gases with in the lungs
spread out to fill the larger space.

Resulting decrease in the gas pressure in the lungs produces a
partial vacuum (pressure less than atmospheric pressure),
which sucks the air into the lungs.
Expiration

Passive process that depends mostly on natural elasticity of the
lungs than on muscle contraction.

As inspiratory muscles relax and resume normal resting length,
rib cage descends and lungs recoil.

As the thoracic and pulmonary volume to decrease, gases inside
the lungs are forced closer together and intrapulmonary
pressure rises to above atmospheric pressure.

This causes gases to flow out to equalize pressure inside and
outside of the lungs.

Normally this is a passive process, but if passageways are
narrowed due to spasms of bronchioles (asthma) or clogged with
mucus/fluid (bronchitis/pneumonia), it becomes an active
process, using intercostal muscles to help depress rib cage and
abdominal muscles to help squeeze air out of lungs.
Events of Inspiration
Steps of Expiration
Physical Influences on Pulmonary Ventilation:
Airway Resistance

Friction in the respiratory passageways is the major non-elastic
source of resistance to gas flow
F = ΔP/R

Gas flow in/out of the alveoli is directly proportional to the
difference in pressure between the atmosphere and the alveoli,
normally small changes in pressure cause large changes in volume
of gas flow (gradient ~2mmHg less, moves 500mL air in/out per
breath)

Gas flow inversely changes with resistance, which is mainly
determined by conducting tube diameter. Resistance is usually
insignificant because relatively speaking they are huge at the
initial part of the conducting zone and as the diameter gets small
diffusion takes over. Greatest resistance is in the bronchi.

Inhaled irritants & inflammatory chemicals can cause constriction
of the bronchioles and reduce air passage, accumulation of mucus
or infectious material can also increase resistance
Physical Influences on Pulmonary Ventilation:
Alveolar Surface Tension Forces

At a liquid-gas boundary, the molecules of liquid are more strongly
attracted to each other than to the gas.

This produces a surface tension at the liquid surface that draws the
liquid molecules even closer and reduces contact with gas molecules
and resists any force that tends to increase the area of the surface.

The liquid film that coats the alveolar walls is always acting to
reduce the alveoli to their smallest size.

This film contains surfactant (as opposed to being pure water) to
reduce surface tension. When too little surfactant is present, the
excess surface tension can collapse the alveoli requiring complete
reinflation with each breath, requiring tremendous energy
(IRDS – infant respiratory distress syndrome)
Physical Influences on Pulmonary Ventilation:
Lung Compliance

Healthy lungs are distensible (stretchy). The ease with which they
can be expanded is called lung compliance. CL

CL is a measure of the change of lung volume (ΔVL) that occurs with
a given change in transpulmonary pressure (Δ[Palv – Pip)

Meaning, the higher the lung compliance, the easier it is to expand
the lungs at any transpulmonary pressure.

The two factors that determine lung compliance are distensibility of
lung tissue and the surrounding thoracic cage, and surface tension
in the alveoli (contributes to elastic recoil).

Excessive compliance indicates a loss of elastic recoil of the lungs, as
in old age or emphysema. Decreased compliance means that a
greater change in pressure is needed for a given change in volume,
as in atelectasis, edema, fibrosis, pneumonia, or absence of
surfactant.
Respiratory Adjustments: Exercise

Ventilation can increase 10-20x during exercise

Breathing becomes deeper and more vigorous
(“Hyperpnea”), but respiratory rate may not be
significantly changed.

Any respiratory changes meet metablolic demands so
O2/CO2 levels in the blood are not altered.

Change in breathing is prompted by rising CO2 and
declining O2, which causes drop in blood pH.
Respiratory Adjustments: High Altitude

At elevation above 8000ft, air density and oxygen
pressure are lower, which may cause symptoms of
“acute mountain sickness” (AMS)

Respiratory and hematopoietic adjustments occur
called “acclimiatization”.

Chemoreceptors become more responsive to increases
in CO2/decreases in O2, resulting in increased
ventilation.

Within a few days, respiratory volume stabilizes at a
level 2-3 L/min higher than at sea level.