Session 3 - Gas exchange + O2 in the blood Flashcards
Explain the term partial pressure and its importance
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
How do we calculate partial pressure?
= 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
How does the partial pressure of oxygen change from the atmosphere to alveoli?
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
What is…
Dalton’s law?
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.
What is…
Henry’s law?
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
What is…
Kh - Henry’s constant?
Solubility of a gas in liquid at body temperature (37 deg)
Different for each gas.
In the blood what does pO2 represent
Dissolved Oxygen
Haemoglobin bound oxygen does not contribute to partial pressure
What is the normal PCO2 in arterial blood?
5.3kPa
What is the normal PO2 in arterial blood?
13.3kPa
What is the percentage of Oxygen, nitrogen and CO2 in the atmosphere?
Nitrogen 78%
Oxygen 20.9%
CO2 0.03%
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%
PO2 = 101 x 20.9% = 21.1 kPa
PCO2 = 101 x 0.03% = 0.0303 kPa
PN2 = 101 x 78% = 78.7 kPa
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
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
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?
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.
List the factors that affect diffusion of gas and consider the formula of diffusion
Diffusion distance (T) Surface area (A) Partial pressure gradient (P1-P2) Diffusion coefficient (D)
Diffusion ∝ A x D x (P1-P2)
T
Describe what is meant by diffusion coefficient of a gas (D)
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
Discuss the difference in rate of diffusion of Oxygen compared to Carbon dioxide and why they are different.
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
Diffusion of alveolar gas to RBC in a capillary must cross…
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
Give 3 conditions which would lead to impaired diffusion of gases
What feature in each case has affected diffusion?
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
Outline the importance of V:Q ratio
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.
Think of 3 conditions which show V:Q mismatch and what has led to it in each case.
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
Dissolved O2 =
= Solubility factor x PO2
Where the solubility factor of Oxygen is 0.01mmol/L/kPa
Grossly describe the structure of Haemoglobin
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
Draw a normal Oxygen Haemoglobin Dissociation curve.
Accurately label the axes
…
What does an OxyHb dissociation curve tell us?
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
If OxyHb curve shifts right, what does this tell us about Hb
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
Factors that lead to right shift of OxyHb curve are:
↑ 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
Factors that lead to left shift of OxyHb curve are:
↓ CO2 ↓ H+ ↓ 2,3 - BPG ↓ temperature
↑ pH
What does Percentage saturation of O2 mean?
Proportion of O2 bound Hb to total Hb expressed as a percentage
This is measured using a pulse-oximeter
Describe the two conformational states of Haemoglobin and how they relate to oxygen binding
Relaxed state - high affinity, easier for O2 to bind
Tense state - low affinity, harder for O2 to bind
What factors impact the conformation of Hb
Low O2, high CO2 → Tense (low affinity for O2)
High O2, low CO2 - relaxed (high affinity for O2)
What is the Bohr shift?
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.
