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what is pulmonary ventilation and why is it required?

What does it enable?


Pulmonary ventilation (movement of air from the atmosphere to gas exchange surfaces within the lung) is required to maintain O2 and CO2 gradients between alveolar air and arterial blood

This enables a sufficient level of gas exchange to take place, ensuring adequate O2 supply/CO2 removal to/from respiring tissues (via blood).


what determines that adequate transport of O2 from atmosphere to respiring tissues can take place?


healthy levels of alveolar ventilation, gas exchange, and cardiac output


How is the pressure gradient between alveoli and blood maintained?


maintained by adequate ventilation


How does the body vary levels of gas exchange to supply o2 and remove co2 for chnaging metabolic demands by the body (exercise, injury or infection)


achieved by changing the rate of alveolar ventilation which will modulate the partial pressure gradients between alveoli and blood


What is hyper/hypoventilation?

describe in levels of breathing and how to detect this with arterial blood?


Hypoventilation and hyperventilation are defined as insufficient/excessive levels of breathing relative to that required to meet the metabolic demands of the body, and can be identified by the level of CO2 present within the blood .

Hypoventilation results in excessive levels of CO2 within arterial blood (PaCO2 > 6.0 kPa)

Hyperventilation results in reduced levels of CO2 within arterial blood (PaCO2 < 4.9 kPa)


effect of increasing ventilation


Increasing the rate of ventilation increases alveolar oxygen partial pressure (PAO2), and decreases alveolar carbon dioxide partial pressure (PACO2).


calculating total volume of air that in inspired over a given time period - total level of ventilation

depends on which 2 factors?
what is the equation?


the total level of ventilation (i.e. the total volume of air that is inspired over a given time period) depends on both the volume of air inspired,
the frequency of breathing (also known as respiratory rate, how many breaths per minute, etc.)

V = Vt (tidal volume) x f (number of breaths per min)


Alveolar air ≠ Inspired air: the lungs contain a mixture of ‘fresh’ & ‘stale’ air

what else are the airways called?
what kind of system is the resp system?
why does air remain in lungs after expiration?
how much volume of air is this and significance of this air?


Gas exchange only takes place in alveoli, but air must first pass through the airways (airways = “anatomic dead space”)

The respiratory system is two-way system; air enters and leaves via the same path. Also, a residual volume of air remains in the airway & lungs at the end of expiration. (so alveoli don’t collapse)

This means that the final ≈150mL (dead space volume) of each inspiration never reaches the alveoli or takes place in gas exchange.


how much is dead-space volume typically?




what happens during inspiration with fresh and stale air?

how much reaches alveoli with a tidal volume of 500ml?
what first reaches the alveoli?


dead space is filled with 150ml air, only 350ml of fresh air reaches alveoli and the first 150ml into alveoli is stale air from dead space

hence during inspiration, 500ml (tidal volume) of ‘fresh’ air is inspired but only 350ml reaches the alveoli.


what happens during expiration with fresh and stale air?


at the end of inspiration, the dead space is fille dwith ‘fresh’ air.

the first 150ml of an expiration will be the air previously occupying the dead space that has not taken part in gas exchange


how do we calculate alveolar ventilation

new equation taking into account of dead space?


Va = (Vt - Vd) x f

Va = alveolar minute volume
Vt = tidal volume
Vd = dead space volume 
Vt - Vd = the volume of fresh air entering the alveoli in each breath
f = frequency

How do gases move?

what will stop the movement?


Gases naturally move from (connected) areas of high pressure to low pressure until an equilibrium is re-established.

Therefore gas can be moved two connected spaces by creating a pressure gradient


what is boyle’s law?

what is the equation?
If n stays constant, what is the relationship between p and v?


Boyle’s law (derived from the ideal gas law) describes the relationship between pressure (P), volume (V) and molar quantity (n, the number of gas molecules present):

p = n/v

Pressure = the number of gas molecules within a given volume
If n remains constant,
↑ Volume = ↓ Pressure


How does air move into and out of the lungs?

how does gases move generally between areas?
using this, how do air move into lungs? and out of lungs?


pressure gradients are required to move gases (gases move from areas of high pressure to low pressure)

To move air into the lungs during inspiration, alveolar pressure must be fall below atmospheric pressure (so air moves down the pressure gradient), and

during expiration, alveolar pressure must rise above atmospheric pressure.


pressure difference and air movement during inspiration and expiration

what happens during inspiration?
how does pressure change? effect of this?

what happens during expiration?
how does pressure change? effect of this?


At the end of expiration, P alveoli = P atmosphere, therefore there is no movement of air

During inspiration
Diaphragm contracts & thoracic cavity expands. Alveolar pressure decreases
The outer surfaces of the lung are pulled outwards (expansion).
↑volume = ↓alveolar pressure.
Palveoli < Patmosphere
Air flows from high (atmosphere) to low (alveoli) pressure.

Durinfg expiration
Diaphragm relaxes (and lung recoils). Thoracic cavity volume decreases, alveolar pressure increases.
Air within the lung is compressed.
↓volume = ↑alveolar pressure.
Palveoli > Patmosphere
Air flows from high (alveoli) to low (atmosphere) pressure.


What is the pleural cavity?
effect of this?
how does lungs relate to walls of thoracic cavity?


Pleural cavity = fluid filled space between the membranes (pleura) that line the chest wall and each lung - helps to reduce friction between lungs and chest.

hence the lungs are not directly attached to the wall of the thoracic cavity. Respiratory muscles are not attached to or pull on the lungs directly


functions of the pleural cavity (2 functions)


1) Provide a frictionless surface, aiding movement of the lungs
2) Ensure that changes in the volume of the thoracic cavity (generated by respiratory muscle contraction) result in corresponding changes in lung volume.


How does pleural cavity achieve functions?

what does it do being a fluid filled sealed compartment? effect of this?


the pleural space is a fluid filled, sealed compartment –

this means that it resists changes in volume (relative to gas-filled compartments such as the lungs) Thus, changes in the volume of the thoracic cavity (due to resp. muscle activity) result in changes in lung volume.

Also, it removes potential friction that would be produced by the lungs rubbing on the chest wall as they inflate/deflate.


How is the pleura organised? 2 pleuras? what do they surround?

what is between the 2 pleuras?


The (inner) visceral pleura lines each lung, whereas the (outer) parietal pleura lines the thoracic cavity, surrounding the chest, diaphragm and mediastinum (which contains the heart).

Between the two pleura is the sealed, fluid-filled pleural cavity.


How is the pressure within the pleural cavity sub-atmospheric?


The opposing elastic recoil of the chest wall (outward) and lungs (inward) results in the pressure within the pleural cavity being sub-atmospheric


what is negative pressure?

effect of negative pressure?


Negative pressure = lower number of molecules per volume (relative to surroundings) → generates collapsing force (pulls surfaces of contained space together).


process of inspiration

effect of this?


Respiratory muscles (e.g. diaphragm) contract

Volume of thoracic cavity increases

Intrapleural pressure becomes more negative

Lungs expand, increasing volume

PAlv (alveolar pressure) decreases below PAtm (atmospheric pressure)

Air moves down pressure gradient, through airways into alveoli, expanding the lungs


process of expiration

effect of this?

what happens during forced expiration?

Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres
Volume of thoracic cavity decreases
Intrapleural pressure increases
Lungs compressed*, volume decreases
PAlv increases above PAtm
Air moves down pressure gradient, into atmosphere, deflating lungs

*Compression of the lungs due to increased intrapleural pressure only occurs during forced expiration. In quiet breathing, elastic recoil is sufficient to decrease lung volume.


summary of lung volume, intrapleural pressure, alveolar pressure and air flow

what happens when lung volume increases during inspiration? effects on pressure? how does this change as air enters the lungs?

what is the speed of airflow dependent upon?


1) As lung volume increases during inspiration, intrapleural pressure becomes more negative due to the elastic properties of the lung generating increasing recoiling force.
2) As the lungs expand, the increase in volume decreases PAlv. As air enters the lungs, the pressure re-equilibrates once again as the increased concentration of gas molecules compensates for the increased volume (P = n/V)
3) When PAlv < PAtm the pressure gradient causes air to move into the lungs. Where PAlv > PAtm air moves out. The speed of airflow is dependent on the pressure gradient and level of airway resistance present.
4) Entry of air into the lungs due to (3) leads to inflation and increased volume, which is reversed during expiration.


what is pneumothorax? effect of this?


If either of the either of the pleural membranes is ruptured (e.g. due to trauma, bleb formation, or disease), the pressure gradient between the pleural cavity and the atmosphere (or lungs, depending on the particular injury) will cause air to enter the pleural space (‘pneumothorax’).


what happens during pneumothorax?

what two things will reduce lung volume?
why will the lungs collapse? (2 reasons)


Entry of air into the pleural cavity results in its volume increasing, at the expense of the lung.

Furthermore, recoil of the lungs and expansion of the chest wall during breathing will potentially draw additional air into the cavity, further decreasing lung volume.

With the loss of negative intrapleural pressure, elastic recoil of the chest wall and lungs is no longer resisted. This will cause affected regions of the lungs to collapse, with the overall effect depending on the site and extent of the injury.


breathing and pneumothorax

when will air stop entering pleural cavity during pneumothorax?

what will increase pleural cavity volume? effect of this?


If either pleural membrane is ruptured, the pressure gradient between the pleural cavity and surrounding environment will cause air to enter (pneumothorax) until intrapleural pressure = atmospheric pressure.

Entry of air = ↑ pleural cavity volume (at the expense of the lung volume, which decreases). Elastic recoil of lung tissue, and expansion of the chest during inspiration can then potentially draw further air into the pleural space.


summary of pneumothorax

why does air enter pleaural space? effect of this?
what can further increase air entering pleural space?
why can the other lung function if one is experiencing pneumothorax?


A pneumothorax (presence of air within the pleural space) can occur if either pleural membrane is damaged, resulting in a passageway for air to enter (either from the atmosphere or lungs).

Air enters the pleural space as intrapleural pressure is lower than atmospheric pressure and there is thus a volume gradient resulting in movement of air.

As air enters the pleural space, its volume will increase at the expense of the lungs, which will thus decrease (causing atelectasis or collapse of lung tissue).

Elastic recoil of the lungs and expansion of the thoracic cavity during breathing can cause further air to enter the pleural space, further reducing lung volume.

Fortunately each lung is surrounded by a separate pleural space, and thus only one lung may be affected, allowing the individual to continue breathing (albeit in a laboured fashion).