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Mechanics of Quiet Breathing Questions+ notes

How is the respiratory system a two-compartment model? What lies in between it?

It is separated into the lungs and the chest wall. Pleural cavity lies in between.

Specify the unit of pressure adopted when considering the mechanics
of breathing.

pressure of centimeters of H2O units are used.

relative pressure

Pressure changes are relative changes. We are assuming that the pressure of the atmosphere is 0cmH2O because it is different wherever we are depending on altitude, temperature, etc.. There is therefore no difference in pressure between the atmospheric pressure and the alveolar pressure = 0cmH2O

atmospheric pressure

Atmospheric pressure, aka barometric pressure is the pressure outside of the body. (The weight of the air in the atmosphere on objects on Earth's surface). 760 mmHg/1atm. at sea level.

alveolar pressure

alveolar pressure:aka intrapulmonary pressure: pressure within the alveoli (0 cmH2O)

intra-pleural pressure

intra-pleural pressure: pressure outside the lungs but still within the thoracic cavity (pressure within the pleural space/intrapleural pressure) = -5 cmH2O

-negative pressure due to the lungs wanting to collapse and the chest wall wanting to expand

Transmural pressures

transmural pressure: pressure inside - pressure outside
=0 cmH2O - -5 cmH2O = +5 cmH2O is the transmural pressure of the lungs.

Describe the relationship between the transmural and elastic recoil
pressure of the lungs.

Transmural pressure ( +5 cmH2O) represents that +5 cmH2O of pressure prevents the lungs from collapsing and the chest wall from recoiling outwards!

A hole in the lung (inner component) or in the chest wall (outer component) will result in what types of pneumothorax?

Traumatic pneumothorax and spontaneous pneumothorax, respectively.
(Collapsed lung)


Trauma of the lung or chestwall can result in hemothorax or blood in the pleural cavity.

Boyle's Law

P1V1 = P2V2. Inverse relationship with volume.

Pressure (equation)



1. Chest Wall expands
2. alveolar and intrapleural pressure decreases (creates vacuum)
3. air flows into the lungs until alveolar pressure = atmospheric/barometric pressure.

-Increased volume = decreased pressure = vacuum = inspiration

Driving pressure for inspiration?

The difference between alveolar and atmospheric pressure

What created this driving pressure?

The intercostal muscles and chest wall expanding.* check notes. inspiratory muscles contracting.*

How come alveolar pressure
decreases and then swings back
up but pleural pressure
decreases continuously during

Alveoli allows air in so the pressure increases, but the pleural cavity is unable to allow air in.

We are ______ pressure breathers

We are negative pressure breathers

Iron lung

The whole body goes inside the iron lung and decreases the pressure around the body, thus sucking the chest wall out and allowing inspiration.


1. inspiratory muscles stop contracting
2. lungs recoil inwards (volume decreases)
3. alveolar and intrapleural pressures increase.-alveolar pressure rises above atmospheric pressure so air is expelled.
This expiration is passive.

The transmural pressure
of the lungs is always
positive during a breath
cycle; what does this
imply about the state of
the lungs?

Alveolar pressure will always be greater than intrapleural pressure no matter how much the alveolar pressure drops or increases. Intrapleural pressure is equal to or less than -5 cmH2O throughout the cycle

What is the relationship
between transmural
pressure & the elastic
recoil pressure of the

The more positive the transmural pressure, the more elastic recoil pressure is required to keep the lungs from collapsing.

Major muscles of inspiration

Diaphragm; external intercostal muscles

Accessory muscles of inspiration

sternocleidomastoid - lifts the sternum
scalenus - lifts the first 2 ribs

Major muscles of expiration

Abs; internal intercostal muscles

What spinal roots innervate the diaphragm?


The diaphragm is responsible for __% of the air inhaled.


What spinal roots innervate the external intercostal muscles?


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