Gas Laws 2

Pressure in the pleural space is always ________

negative

Intrapleural pressure (explanation & in mmHg)

Pressure in the resp system at rest

-4 mmHg

Transpulmonary pressure (explanation & calculation)

Difference between pressure on inside & outside of pleura

Pressure needed to keep lungs from collapsing (opposes the inward elastic recoil of the lung)

0mmHg - (-4mmHg) = 4mmHg

Alveolar pressure (same as atmospheric pressure) - (intra)pleural pressure = Transpulmonary pressure

760 - 756 = 4mmHg

FRC

functional residual capacity

What is happening at point 2

Air is moving into the lung - pressure begins to equalise

Tidal volume of lungs (explanation & mL)

Air that flows into the lungs during quiet breathing

About 500mL

What happens to intrapleural pressure & trans-pulmonary pressure during inspiration

Intrapleural becomes more negative (-7mmHg)

Trans-pulmonary pressure becomes more positive

What happens to intrapleural pressure & trans-pulmonary pressure during expiration

The intra-pleural Pressure Increases

The trans-pulmonary Pressure Decreases

What happens at 3

What happens at 4

Where is pressure inside the lung the most negative

The top of the lung

What is the transpulmonary pressure at the top, middle & bottom of the lung

Top: -10mmHg

Middle: -5mmHg

Bottom: -2.5 mmHg

Minute Ventilation (V_E) (explanation & formula)

Total ventilation per minute

Tidal Volume (V_t) x Respiratory Rate (f) per minute = Minute ventilation (V_E)

500ml (V_t) x 12/min (f) = 6000ml/min (V_E)

Alveolar ventilation

Portion of minute ventilation (V_E) that reaches the alveoli

More important than V_E for gas exchange

Anatomic dead space

Air that enters the resp system but doesn’t reach the alveoli

It ventilates the conducting airways and isn’t available for gas exchange

V_D

Dead space volume

Flow (F) formula

Flow (F) is _________ to pressure difference between 2 points & ______________ to the resistance

proportional

inversely proportional

Diffusion of gases across the alveolar membrane is influenced by what?

Laws of Diffusion

Diffusion and perfusion limitations

O2 uptake along the pulmonary capillary

Measurement of diffusing capacity

Interpretation of diffusing capacity for CO2

What does Fick’s Law describe

Diffusion through tissues

(Fick → Thick. What does thickness decide? Diffusion rate!)

Fick’s Law

The thicker the membrane, the less gas goes across

Put these in order of increasing %O2: Inspired air, Alveolar air, Expired air

Alveolar air, Expired air, Inspired air

NB Vapour pressure in alveolar air at body temp is _____

47mmHg

Why is the amount of oxygen in arterial blood not the same as in alveolar gas

Shunting

Volume of gas over time ( dot = over time)

Volume of gas transferred over time through tissue formula

Gas transferred formula using gas solubility

What gases can exert a partial pressure in the blood

Only gas that dissolves in plasma exerts a partial pressure

Perfusion

the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue

What is perfusion limited ventilation

Gas crosses the alveolar membrane and Dissolves in blood

The partial pressure of that gas in the capillary quickly equilibrates with the alveolar partial pressure

Further transfer will be possible if more blood passes with lower partial pressure passes through the alveolar capillary

What is diffusion limited ventilation

Gas crosses the alveolar membrane binds to haemoglobin - exerts NO Partial Pressure

NO equilibration of partial pressures

The gases ability to diffuse across the membrane is limiting factor

Does CO (carbon monoxide) in blood cause a big/small rise in partial pressure

very small

Is CO transfer perfusion/diffusion limited

diffusion limited

Laughing gas chemical formula & name

Nitrous Oxide (N₂O)

Is Nitrous Oxide transfer perfusion/diffusion limited

perfusion

How is Nitrous Oxide perfusion limited

It depends on capillary blood flow

N2O diffuses into capillary

It doesn’t bind to Hb

Hb has low avidity for N₂O

N₂O dissolves in plasma

PN₂O rises rapidly

Equilibrates with Alveolar PN₂O

No more N2O is transferred

Is oxygen transfer perfusion/diffusion limited, how?

Perfusion limited

Oxygen binds to haemoglobin & equilibrates with alveolar P_O2

How long does oxygen take to equilibrate with alveolar P_O2

0.25 sec

In an abnormal / non-healthy lung is transfer of oxygen diffusion/perfusion limited, how?

Diffusion

Thickened barrier

Diffusion impaired

Equilibrium may not be reached

Ability to diffuse across the membrane is limiting factor

What effect does severe exercise have on oxygen uptake in healthy and non-healthy lungs

During severe exercise, the RBC capillary time drops from 0.75 s - 0.25 s.

This is enough time for normal diffusion of oxygen in a healthy lung.

In a non-healthy lung with a thickened blood gas barrier & impaired diffusion blood P_O2 doesn’t reach alveolar P_O2

How are diffusion properties of the lungs measured

Single Breath Method

Single inspiration of CO mixture, hold for 10s, then expire

Calculate the rate of CO diffusion: Subtract CO in expired air from original CO concentration

How are diffusion properties of the lungs measured (formula)

Normal value of diffusing capacity for CO at rest

Units important!

value of diffusing capacity for CO during exercise is how many times that of rest

2 or 3 times

Why blood gas barrier in lungs is ideal for diffusion

Area of barrier is large (50-100 m²)

Thin membrane of 0.3 μm in many places

Why CO2 diffuses more readily than O2

CO2 diffuses 20 x more readily through tissue than O2, even though it has a similar MW, as it has a much higher solubility in the tissues

What diseases can increase/decrease diffusion capacity