Session 3 - Gas exchange + O2 in the blood

Question 1

Explain the term partial pressure and its importance

Answer 1

The force exerted by an individual gas on the walls of its container, in a mixture of gases.

In a container with multiple gases there is an individual partial pressure of each gas - and they are independent of each other.

Sum of the partial pressures of all the gases in a mixture equals the total pressure.

Gases move from an area of high partial pressure to low - along a partial pressure gradient

Question 2

How do we calculate partial pressure?

Answer 2

= Total pressure x percentage of a gas in a mixture

e.g. Partial pressure of O2 in the air

= 101kPa x 20.9% = 21.109 = 21.1 kPa

Question 3

How does the partial pressure of oxygen change from the atmosphere to alveoli?

Answer 3

Reduced in the upper airways due to humidification in the nostrils which introduces water vapour - it displaces 6.28kPa of the air. pO2 goes from 21.1 to 19.8.

Reduced again in the alveoli as it mixes with ‘old’ air which does not completely leave with expiration to 13.3kPa.

Also, there is always some O2 being continuously absorbed into the blood - therefore a balance is achieved at 13.3kPa

Question 4

What is…

Dalton’s law?

Answer 4

PT = P1 + P2 + P3+ P4…

States that in a mixture of gases the sum of all partial pressures equals the total pressure

Each component gas exerts a partial pressure in proportion with its volume percentage in the mixture

This law is also known as Dalton’s law of partial pressures.

Question 5

What is…

Henry’s law?

Answer 5

Describes the behaviour of gases when they come into contact with a liquid.

The amount of dissolved gas in a liquid is proportional to the partial pressure above the liquid and the gas solubility in the liquid.

An equilibrium is reached between the amount the gas dissolves and the amount that leaves the liquid.

Concentration of a gas = solubility of the gas (Kh, henry’s constant) x partial pressure of the gas above the liquid

Question 6

What is…

Kh - Henry’s constant?

Answer 6

Solubility of a gas in liquid at body temperature (37 deg)

Different for each gas.

Question 7

In the blood what does pO2 represent

Answer 7

Dissolved Oxygen

Haemoglobin bound oxygen does not contribute to partial pressure

Question 8

What is the normal PCO2 in arterial blood?

Answer 8

5.3kPa

Question 9

What is the normal PO2 in arterial blood?

Answer 9

13.3kPa

Question 10

What is the percentage of Oxygen, nitrogen and CO2 in the atmosphere?

Answer 10

Nitrogen 78%
Oxygen 20.9%
CO2 0.03%

Question 11

Using the following values calculate the pO2, pCO2 and pN2 in the atmosphere

Atmospheric pressure = 101kPa
Percent O2 = 20.9%
Percent CO2 = 0.03%
Percent N2 = 78%

Answer 11

PO2 = 101 x 20.9% = 21.1 kPa

PCO2 = 101 x 0.03% = 0.0303 kPa

PN2 = 101 x 78% = 78.7 kPa

Question 12

Using the following values calculate the PO2, PCO2 and PN2 in the upper respiratory tract after humidification has taken place

Atmospheric pressure = 101kPa
Percent O2 = 20.9%
Percent CO2 = 0.03%
Percent N2 = 78%

Water vapour pressure = 6.28kPa

Answer 12

PO2 = (101 - 6.28) x 20.9% = 19.8 kPa

PCO2 = (101-6.28) x 0.03% = 0.0284kPa

PNO2 = (101 - 6.28) x 78% = 73.8 kPa

Question 13

How do you explain the difference between the PO2 in the upper airways being 19.8kPa and the PO2 in the alveoli being 13.3kPa?

Answer 13

In alveoli, O2 is constantly diffusing into the capillaries so the resultant PO2 in the alveoli is less than we have in the upper airways.

Question 14

List the factors that affect diffusion of gas and consider the formula of diffusion

Answer 14

Diffusion distance (T)
Surface area (A)
Partial pressure gradient (P1-P2)
Diffusion coefficient (D)

Diffusion ∝ A x D x (P1-P2)
T

Question 15

Describe what is meant by diffusion coefficient of a gas (D)

Answer 15

A value attributed to a gas which represents the relative rate of diffusion of a gas across a membrane, at a certain pressure

Higher the coefficient the faster the rate of diffusion

Diffusion coefficient ∝ solubility/√molecular weight

Question 16

Discuss the difference in rate of diffusion of Oxygen compared to Carbon dioxide and why they are different.

Answer 16

Diffusion ∝ A x D x (P1-P2)
T
D ∝ solubility/√molecular weight

CO2 ~20x more soluble than O2 (comparing Henry’s constants)

Molecular weight of CO2 is 44 and O2 is 32 - CO2 weighs only 1.375x more.

Partial pressure gradient greater for CO2 than O2 as (between capillaries and alveoli)

Overall CO2 still diffuses ~20x faster than O2

Question 17

Diffusion of alveolar gas to RBC in a capillary must cross…

Answer 17

Fluid film lining alveolus
Epithelial cell (type 1 pneumocyte)
Interstitial space
Endothelial cell of capillary
Plasma
Red cell membrane
This totals 5 cell membranes, 3 layers of cytoplasm and two layers of tissue fluid + plasma

Question 18

Give 3 conditions which would lead to impaired diffusion of gases

What feature in each case has affected diffusion?

Answer 18

ARDS - interstitial fluid, loss of alveolar/capillary membrane

Pulmonary fibrosis - interstitial fibrosis and thickening

Pneumonia - (inflammatory) alveolar fluid +/- interstitial oedema due to inflammation-

COPD - reduced surface area

note that V:Q mismatch also happens in different ways too, this is just testing you on diffusion factors

Question 19

Outline the importance of V:Q ratio

Answer 19

Matching ventilation to perfusion ensures optimal gas exchange

Ideally alveoli with good ventilation are well perfused and alveoli with poor ventilation are less perfused

In particular, when alveolar O2 is low you get vasoconstriction of that alveoli - this diverts blood to better ventilated alveoli. This is not a perfect corrective measure - you can’t divert all blood.

Question 20

Think of 3 conditions which show V:Q mismatch and what has led to it in each case.

Answer 20

Asthma - variable airway narrowing
COPD - variable airway narrowing, air trapping/poorly ventilated alveoli
Pneumonia - inflammatory exudate in alveoli
ARDS newborn - some alveolar collapse
Pulmonary Oedema - fluid in alveoli
Pulmonary Embolism - alveoli ventilated not perfused

Question 21

Dissolved O2 =

Answer 21

= Solubility factor x PO2

Where the solubility factor of Oxygen is 0.01mmol/L/kPa

Question 22

Grossly describe the structure of Haemoglobin

Answer 22

Quaternary protein
2 alpha 2 beta subunits
Each subunit has one haem group which can bind one O2 molecule
So each Hb can bind 4x O2 molecules at maximum

Question 23

Draw a normal Oxygen Haemoglobin Dissociation curve.

Accurately label the axes

Answer 23

…

Question 24

What does an OxyHb dissociation curve tell us?

Answer 24

Ultimately shows us how much O2 will be bound or given up by Hb at a given PO2

In areas of low PO2 you have less SatO2
As PO2 rises so too does SatO2
Plateaus at 100% - sigmoidal shaped curve

Also allows us to work out the difference in percentage O2 saturations between two PO2s.

This curve varies from leftward shift to rightward shift based on various factors - which reflect Haemoglobin’s ability to bind and dissociate with O2

Question 25

If OxyHb curve shifts right, what does this tell us about Hb

Answer 25

Takes higher PO2 to saturate Hb than normal

This is because Hb dissociates O2 more readily - which is important to do near to metabolically active tissues

Question 26

Factors that lead to right shift of OxyHb curve are:

Answer 26

↑ CO2 ↑ H+ ↑ 2,3 - BPG ↑ Temperature
↓ pH

Imagine standing right next to a respiring cell - these are the conditions you are likely to see…and you would want Hb to dissociate their O2 to supply the respiring cell

Question 27

Factors that lead to left shift of OxyHb curve are:

Answer 27

↓ CO2 ↓ H+ ↓ 2,3 - BPG ↓ temperature

↑ pH

Question 28

What does Percentage saturation of O2 mean?

Answer 28

Proportion of O2 bound Hb to total Hb expressed as a percentage

This is measured using a pulse-oximeter

Question 29

Describe the two conformational states of Haemoglobin and how they relate to oxygen binding

Answer 29

Relaxed state - high affinity, easier for O2 to bind

Tense state - low affinity, harder for O2 to bind

Question 30

What factors impact the conformation of Hb

Answer 30

Low O2, high CO2 → Tense (low affinity for O2)

High O2, low CO2 - relaxed (high affinity for O2)

Question 31

What is the Bohr shift?

Answer 31

This is a rightward shift of the Oxygen Haemoglobin curve - signifying an increased readiness for Hb to dissociate with O2 in areas of high CO2, low pH etc.