Gas exchange in the lungs

Question 1

Describe what air consists of?

Answer 1

Air consists of a mixture of gases
They behave in accordance with their partial pressure rather than conc.

Question 2

What is the definition of partial pressure of a gas dissolved in a liquid?

Answer 2

reflects the amount of gas that would dissolve (at equilibrium) if the liquid was placed in contact with a gas phase of equivalent partial
pressure.

Essentially the local amount of specific gas present in the liquid

Question 3

State the 2 equations for partial pressure?

Answer 3

Total pressure x mole fraction of gas
OR
(Atmospheric pressure - water vapour pressure) x mole fraction of gas

WV pressure can be 0

Question 4

State equation for total pressure?

Answer 4

Water vapour pressure + Sum of all partial pressure of individual gasses

Question 5

What determines the concentration of a gas dissolved within a liquid?

Answer 5

Partial pressure x solubility

Solubility= Ability to dissolve
PP takes into account many factors

Question 6

Define gas exchange

Answer 6

Diffusion of resp. gases between air and blood

Question 7

How does oxygen get from the atmosphere to cells?

Answer 7

“O2 inhaled from atmosphere into alveoli within lungs
↓
O2 diffuses from alveoli into blood within pulmonary capillaries
↓
O2 transported in blood predominantly bound to haemoglobin
↓
O2 diffuses into cells/tissues for use in aerobic respiration
↓
CO2 diffuses from respiring tissues to blood - exchanged at the lungs “

Question 8

“What are the structures that blood gases have to move from alveolar air
into capillary blood?”

Answer 8

“1) Airspace

2) Alveolar lining fluid
3) Alveolar epithelial layer
4) Basement membrane + interstitial fluid
5) Capillary endothelial layer

”

Question 9

“If size of structures/layers that blood gases pass through increase,
what will happen?”

Answer 9

“Gas exchange will be impaired
- As diffusion will take much longer”

Question 10

What are the steps O2 has to take to bind to Hb from the atmosphere?

Answer 10

O2 enters the alveolar airspace from the atmosphere
↓
O2 dissolves in ALF
↓
O2 diffuses through alveolar epithelium, basement membrane + capillary endothelial cells
↓
O2 dissolves in blood plasma
↓
O2 binds to Hb molecule

Question 11

Adaptations of alveoli which makes it specialised for gas exchange?

Answer 11

”- Large surface area (lungs have high surface area-volume ratio due to 3D structure)

• Wall = one cell layer thick + basement membrane fused with blood vessel
• Richly innervated by capillaries (adequate blood supply)

These collectively maximise the rate of diffusion”

Question 12

“Describe the importance of Oxygenation times for blood during rest and
excercise
“

Answer 12

“Oxygenation of blood (diffusion) must be rapid when the RBC is flowing through pulmonary capillaries
In typical conditions (rest) it takes 0.75 seconds for a RBC to pass through a pulmonary capillary
During intensive exercise it takes 0.25 seconds.”

Question 13

What is the equation for rate of diffusion and define?

Answer 13

Rate of diffusion ∝ Surface area/(Distance)^2 x (Pa - Pc)

SURFACE AREA : Alveolar surface area
DISTANCE = Epithelial + endothelial cell thickness + basement membrane thickness + fluid layer depth
(PA-PC) = Partial pressure gradient between alveolar air and capillary blood.

Question 14

What is the rate of diffusion determined by?

Answer 14

“Partial pressure gradient between two areas
Size of the diffusion distance
Surface area”

Question 15

How would Abnormal diffusion occur in terms of the blood-gas barrier?

Answer 15

Thickening of blood-gas barrier.

Question 16

Criteria for maximum diffusion

Answer 16

↑ Surface area
↑ partial pressure gradient
↓ Distance ( barrier thickness )

Question 17

State how Emphysema affects diffusion factors

Answer 17

↓Surface area
- Emphysema= Over-swelling of the alveoli

Question 18

State how Fibrosis affects diffusion factors

Answer 18

↑ Basement membrane thickness

Question 19

State how Pulmonary oedema affects diffusion factors

Answer 19

↑ Increased thickness of fluid layer via oedema

Question 20

State how Hypoventilation affects diffusion factors

Answer 20

“Don’t breathe at a sufficient rate so the partial pressure gradient decreases.
- Decreases alveolar pressure
Hypoventilation= Type II respiratory failure”

Question 21

State how Hypoperfusion affects diffusion factors

Answer 21

Hypoperfusion= Type I resp. failure

• Decreases capillary pressure
• partial pressure gradient decreases.

Question 22

What 2 factors need to be matched for efficient gas exchange?

Answer 22

“Blood flow through pulmonary capillaries (perfusion) needs to be matched to alveolar ventilation (supply of O2)
- So sufficient blood to absorb oxygen arriving at alveoli”

Question 23

Why does perfusion need to be matched to ventilation?

Answer 23

“Each unit of blood has a finite amount of Hb and can only transport a limited amount of oxygen.
- Ensures that gas exchange is occuring efficiently”

Question 24

Define V/Q ratio

Answer 24

“The relationship between ventilation and perfusion of the resp system.
- Known as V/Q coupling”

Question 25

Normal V/Q ratio

Answer 25

“one
Both sides of lung or areas of resp. system have same perfusion and ventilation ratio”

Question 26

When is V/Q >1

Answer 26

”
Hypoperfusion- ‘dead space effect’”

Question 27

When is V/Q < 1

Answer 27

Hypoventilation- ‘shunt’

Question 28

How is ventilation-perfusion coupling maintained?

Answer 28

“Hypoxic vasoconstriction
- Reduces ventilation-perfusion mismatching”

Question 29

What is Hypoxic vasoconstriction

Answer 29

“Constriction of capillaries which diverts blood flow from poor to well ventilated alveoli
When ventilation to an alveoli decreases”

Question 30

Hypoxic vasoconstriction mechanism

Answer 30

“1. Under normal conditions, blood flow and ventilation are matched
2. If ventilation of a specific alveolus decreases
Pa Co2 will rise
Pa O2 will fall
Less oxygenation of blood flowing through innervating capillaries
3. Decreased Pa O2 induces vasoconstriction of smooth muscle of nearby capillaries = decreased blood flow
Blood is diverted to alveoli with more ventilation”

Question 31

What is ventilation-perfusion ineqaulity/mismatch?

Answer 31

“In situations where ventilation and perfusion to individual alveolar units are not matched, gas exchange will be reduced.
- Where V/Q ratios vary substantially between alveolar units or different regions of the lung”

Question 32

What can a substantial level of VQ inequality lead to?

Answer 32

“Inducing hypoxaemia (Abnormal low conc. of O2 within the blood)
- Regardless of whether deadspace effect or shunting”

Question 33

How does V/Q ineqaulity affect CO2 and O2 exchange

Answer 33

“In theory, V-Q inequality affects both O2 and CO2 exchange.
However in most cases, ↑PaCO2 will induce a reflex hyperventilation that clears the excess CO2 (but doesn’t ↑PaO2)”

Question 34

Describe what reduced perfusion of lung region leads to

Answer 34

“Increase in V/Q ratio
Affected alveoli: Physiological dead-space as no/reduced gas exchange occuring “

Question 35

Physiological dead space

Answer 35

“Ventilation but not perfusion in terms of not occuring at same rate

• Presence of inspired O2 which is ‘wasted’
• No participation in gas exchange”

Question 36

Diseases that causing physiological dead space

Answer 36

“Heart failure (cardiac arrest)
Blocked vessels (pulmonary embolism)
Loss/damage to capillaries (emphysema)”

Question 37

Describe V/Q of underperfused alveoli and how it can be compensated?

Answer 37

“V/Q ratio increases in alveoli that is not being perfused
Increased perfusion to other alveoli and their V/Q ratio decreases
you can compensate reduction in gas exchange by ventilating the divertd regions more”

Question 38

Describe what occurs in pulmonary embolism

Answer 38

“1. Embolism occludes pulmonary artery supplying a region of the lung

2. Underperfused alveoli - physiologic dead-space
   - Blood is diverted through other pulmonary arteries/capillaries
   - Perfusion to these vessels/alveoli increases as cardiac output diverted
3. Unless ventilation of these alveoli (with greater perfusion) increase to match ventilation,
   - Hypoxaemia and hypercapnia (elevated CO2 levels in blood) will occur”

Question 39

Shunt

Answer 39

“Reduced ventilation of alveoli or limits to diffusion

• > decrease in V/Q ratio
• > Deoxygenated Blood will travel from the right side of the heart to the left, without taking part in gas exchange
• > Perfusion without ventilaiton
• > Treatment of hypoxamia has limited response to O2 therapt compared to other V/Q inequalties”

Question 40

What disease can shunt lead to?

Answer 40

“Cardiac shunts
Pneumonia, acute lung injury, respiratory distress syndrome, atelectasis
“

Question 41

“State the effect of O2 therapy on hypoxaemia comparing shunt and other V/Q
inequalities”

Answer 41

“Hypoxaemia caused by shunt is associated with a much more limited response to supplemental oxygen therapy than that associated with other V/Q inequalities.
This is because regardless of degree of oxygenation occurring in blood perfusing well-ventilated alveoli
it will eventually mix with deoxygenated blood returning from areas affected by shunt, reducing the overall PaO2.
. The vast majority (>98%) of oxygen carried in blood is transported bound to haemoglobin.
As saturation of haemoglobin is typically >95% , administering supplemental oxygen cannot increase oxygen saturation in well-ventilated regions of the lung sufficiently to compensate for the deoxygenated blood with which it will eventually mix.”