Lecture 4: Properties of gases and gas exchange in the lung Flashcards
What is Boyle’s Law?
The pressure of a fixed quantity of gas at a constant temperature is inversely proportional to its volume
- as volume increases, pressure decreases
- as volume decreases, pressure increases
What is Dalton’s law?
In a mixture of gases, each component gas exerts a ‘partial pressure’ in proportion to its volume percentage in the mixture
-pressure is expressed as kPa
What does it mean when a system is in equilibrium for a gas and liquid?
Gas molecules enter the liquid to dissolve, at the same time some molecules which entered the liquid will leave to return to the gas phase
-when the rate of gas coming out of the solution is equal to the rate at which it enters the liquid,the system is in equilibrium for that gas and liquid
What is the partial pressure of a gas in a liquid?
When a gas is dissolved in a liquid, the collision of gas molecules within the walls of the container generates a pressure in the liquid.
At equilibrium, the partial pressure of the gas in the liquid is equal to the partial pressure of that gas in the gas phase
e.g. if partial pressure of oxygen in alveolar air is 13.3 kPa and this air is allowed to equilibriate with the blood in the pulmonary capillaries, the partial pressure of oxygen in the capillar is also 13.3 kPa
Is partial pressure of a gas and the content of gas dissolved in a liquid the same parameter?
No
Henry’s Law
-the amount/content of gas that dissolves in a specific volume of liquid, is proportional to the partial pressure of that gas in the gas phase AND its soluability coefficient
What is the soluability coefficient?
Amount (mmol) of a gas that will dissolve in a litre of plasma at 37 degrees, when exposed to a given partial pressure
What if the gas dissolved combines chemically with the liquid?
The content of gas= amount of gas chemically bound + amount of gas free in solution
e.g. amount of dossolve O2 in plasma is 0.13 mmol/l, and the amount bound to Hb is 8.8 mmol/l, total O2 content is 8.93 mmol/l
What is the saturated vapour pressure?
When a gas is in contact with water, water molecules evaporate to enter the gas phase, while some water vapour condenses to return to the liquid phase.
Equilibrium is reached when rate of evaporation=rate of condensation
-at equilibrium the gas mixture is saturated with water vapour, and the pressure it exerts is called the saturated vapour pressure
What factors does saturated vapour pressure (SVP) depend on?
Only temperature
What is the atmospheric pressure at sea level?
101 kPa
How much pressure does water vapour contribute to the pressure of the air in the airways?
6.28 kPa, while the other gases account for the remainder of 94.72 kPa of pressure
(since the other gases remain in the same proportion as in the dry air)
What is partial pressure of a gas?
The pressure exerted by an individual gas in a mixture of gases
- gases diffuse down a partial pressure gradient from high to low partial pressure
- use partial pressures of gas to calculate the amount of gas dissolved in a liquid
How do you calculate the partial pressure of a gas?
Multiply the fractional % (converted to a decimal) of the individual gas by the total pressure exerted by the mixture of gases
(assumes there are no chemical reactions between the gases)
-application of Dalton’s law
What is atmospheric pressure?
Pressure exerted by the weight of the air above the earth in the atmosphere
- atmospheric pressure changes with altitude (at high altitudes, atmospheric pressure is lower)
- at sea level 101 kilopascals/1 atm
What is the composition of atmospheric air?
78% nitrogen
21% oxygen
What is the partial pressure of oxygen in the atmospheric air?
101kPa x 0.21= 21.21 kPa
What happens to partial pressure of gases as we breath in?
-When we breath in the air is moistened-water vapour added and this exerts a pressure (SATURATED VAPOUR PRESSURE)
-saturated vapour pressure is dependent on body temp (37 degrees)
-we need to subtract the water vapour pressure from the total pressure of the dry gas mixture in a moist environment
: 101kPa - 6.28 kPa= 94.7 kPa = total pressure of gas mixture in moist environment
(other gases are still in the same ratios as in dry air)
What is the saturated vapour pressure?
Water vapour pressure= 6.28 kPa at body temp
-displaces a proportion of the total pressure of the mixture of gases
What is the partial pressure nomenclature?
pCO2/pO2: generic partial pressure
PaCO2/PaO2: partial pressure in arterial blood
PACO2/PAO2: partial pressure in alveoli
PvCO2/PvO2: partial pressure in venous blood
Why is there a difference in partial pressure of oxygen in upper resp tract compared to alveolar partial pressure?
Upper resp tract: 19.8 kPa
Alevolar: 13.3 kPa
Oxygen is constantly being extracted from alveolar air into pulmonary circulation
-not all the alveolar air is replaced (some stays in there)
How does alveolar ventilation differ from pulmonary ventilation?
Total amount of inhaled air is 450ml (tidal volume)
- 30% of tidal volume never reaches the alveoli as it stays in conducting portion of the resp tract= ANATOMICAL DEAD SPACE (150ml)
- 300ml of air therefore reaches the alveoli= ALVEOLAR VENTILATION
What is pulmonary minute ventilation and alveolar minute ventilation?
PMV: how much air is moved into your lungs per min
e.g. 12 breaths per min, 450 ml per breath= 450 x 12= 5400 ml/min
AMV: air reaching alveoli in a minute
e.g. 12 breaths per min, 300 ml reaches alveoli= 12 x 300= 3600 ml/min
What happens when inspired gases come into contact with body fluids?
Gas molecules will enter fluid and dissolve in the liquid
- amount of gas that dissolves is directionally proportional to the partial pressure of that gas
e. g.higher PP of that gas, the more gas will dissolve into that liquid - gas diffuses down a PP gradient
- diffusion will therefore continue until equilibrium is reached: when number of molecules of gas from air entering the fluid is matched by the number of molecules of air leaving the fluid: NO NET MOVEMENT
-once equilibrium is reached the partial pressure in the liquid is the same as in the air even though their concentrations aren’t the same
At equilibrium, how is the partial pressure of a gas in the air equal to the partial pressure of the gas in liquid when their concentrations aren’t the same?
At a low soluability, because these molecules leave the water much readily, equilibrium will be reached when there is a much lower conc of gas dissolved in the water
What is the rate of gas leaving the liquid determined by?
-concentration of dissolved gas
-soluability of gas for liquid medium
(low soluability= don’t have strong interaction with water molecules so these leave much more readily)
How do you work out the concentration of dissolved gas?
Conc of dissolved gas: PP of gas in liquid x soluability coefficient of gas
e.g. 0.01 mmol/L/kPa (solubility coef of O2) x 13.3 kPa = 0.13 mmol of O2 will dissolve in 1L
What happens to oxygen diffusion at the alveolar/venous blood barrier?
- alveolar PP of oxygen > PP of oxygen in venous blood arriving
- oxygen gas diffuses and dissolves down its PP gradient and equilibriates with blood
- PP of oxygen in blood is now equal to PP of oxygen in the alveoli
- therefore the alveolar PP of oxygen determines the alveolar PP of oxygen
(PP of CO2 is equal in blood and alveolus after equilibrium)
What is the soluability coefficient?
Volume of gas that can be dissolved in a fixed volume of solvent at a given temperature
Is oxygen soluable?
Yes, but its not very soluble.
Metabolic needs would not be met by the oxygen dissolved in the blood
What happens to the oxygen when it enters the blood?
-dissolved oxygen enters RBC and binds to Hb
-process continues till Hb is fully saturated (each Hb binds 4 O2 molecules)
(the oxygen bound to Hb does not participate to the partial pressure of oxygen)
-after Hb is fully saturated, oxygen continues to move down its PP gradient
-it will reach equilibrium, where PP of plasma O2 is equal to PP of alveolar air
By the Hb removing the free oxygen, it allows more oxygen to be delivered to the blood from the alveoli
How much free oxygen is found in the blood?
2%
98% is bound to Hb
What is the total content of oxygen in the blood?
Dissolved oxygen + Hb bound oxygen
How does carbon dioxide enter the blood?
- CO2 diffuses from blood to the alveoli along a partial pressure gradient
- PP of CO2 in mixed venous blood > PP of CO2 in alveoli
- CO2 equibrilates so that the PP in the alveolus is equal to the PP in the blood
What are the partial pressure of oxygen and carbon dioxide in the alveolus/mixed venous blood/arterial blood?
Alveolus
- pO2: 13.3
- pCO2: 5.3
Venous
- pO2: 6
- pCO2: 6
Arterial
- pO2: 13.3
- pCO2: 5.3
What factors affect the rate of diffusion?
Rate of diffusion is directionally proportional to:
A x D x (P1-P2)/T
A: SA available for diffusion
D: diffusion coefficient of the gas
P1-P2: partial pressure difference across the membrane
T: thickness (distance molecules must diffuse)
What factors affect the diffusion coefficient for a gas?
- soluability (affects diffusion much more)
- molecular weight of the gas
Is carbon dioxide more/less soluable than oxygen?
CO2 is much more soluable than oxygen
-even though it has a higher molecular weight than oxygen it diffuses much faster than oxygen (20x faster) due to the increased soluability
Body has adapted to this so that there is a greater PP gradient between oxygen compared to carbon dioxide (compensates for the slower diffusion of oxygen)
Why is gas exchange of oxygen impaired more than gas exchange of carbon dioxide in a diseased lung with lower alveolar PP?
Because there is a slower diffusion of oxygen compared to carbon dioxide and the compensatory mechanism of the greater PP gradient is diminished
What must the gas diffusing from the alveolar air to the RBC cross?
-fluid film lining the alveolus
-epithelial cell of alveolus
-interstitial space
-endothelial cell of capillary
-plasma
-RBC membrane
(5 cell membranes)
This barrier is very thin (<0.4 micrometers)
How quickly is oxygen exchange in the lung complete?
Oxygen exchange is complete in 1/3 of the time the blood spends in the capillary (due to large SA and thin barrier and large PP gradient)
-plently of reserve for exercise (where the blood flows much more quickly)
What factors affect the rate of gas diffusion in disease?
-thickness of the membrane (thicker: impaired diffusion)
-SA of the membrane (decreased SA: impaired diffusion)
In diseases diffusion of oxygen is affected more than diffusion of CO2 (you will get abnormal levels eventually of CO2, but this is is terminal phase)
Give some examples of some diseases where diffusion is impaired:
- fibrotic lung disease (thickened alveolar membrane)
- emphysema (destruction of alveoli reduces SA)
- pulmonary oedema (fluid in the interstitial space increasing diffusion distance)
What is vital for ventilation and perfusion in a healthy person?
Ventilation/Perfusion ratio= 1
e.g. alveolar ventilation (V): 4900ml and pulmonary blood flow (Q): 4900ml
What are some cases where there is no V/Q match?
Shunt: when the alveolus is not receiving any ventilation (V/Q=0)
Dead space: ventilation is normal but there is no perfusion (V/Q= infinity)- ALVEOLAR DEAD SPACE
Between these 2 extremes, there is V/Q mismatch
- ventilation excedes perfusion: V/Q >1 (wasted air)
- perfusion excedes ventilation: V/Q <1 (typically seen)
What happens to the arterial PP of oxygen and carbon dioxide when V/Q <1?
When V/Q <1
- arterial PP of oxygen is lower
- arterial PP of carbon dioxide is higher
How many alveoli are found in the lung?
300 million
- may have differing amounts of ventilation and perfusion
- alveoli with increased ventilation have increased perfusion
- alveoli with decreased ventilation have decreased perfusion
How are the alveoli adapted so ventilation and perfusion is matched throughout the lung?
When alveolar pO2 is low:
- hypoxic vasoconstriction of pulmonary arterioles occurs
- this diverts blood to better ventilated alveoli
Why is there V/Q mismatch in healthy lungs?
V/Q: 0.8/0.9
This is to act as a reserve during exercise
Top of lungs: greater ventilation relative to perfusion so V/Q >1
Bottom of lungs: increased perfusion relative to ventilation so V/Q <1
(due to gravity)
Therefore in exercise, for optimal gas exchange, areas of V/Q mismatch are eliminated
- increase perfusion to apex of lungs
- recuits alveoli to increase ventilation at the bottom of the lungs
What is the most common cause of systemic arterial hypoxaemia in people with cardio-pulmonary disease?
V/Q mismatch
What happens to V/Q mismatch with increasing age?
It increases with age, explaining the decreased exercise abilities in people of old age
What are some diseases that cause hypoxemia due to V/Q mismatch?
Asthma: airway narrowing, but not uniformly dispersed in the lung
Early COPD: airway narrowing, but not uniformly dispersed in the lung
Pneumonia: acute inflammatory exudate in affected alveoli can effect how much air gets into the alveoli
Pulmonary oedema: fluid in alveoli
Respiratory distress syndrome in newborns: not all alveoli open
Pulmonary embolism
(all cause hypoxemia)
What happens to oxygen concentration at high altitudes?
Atmospheric pressure decreases to 31.1 kPa
- therefore oxygen PP is 6.5 kPa
- this is low so you need oxygen or your body adapts
What is decompression sickness?
- in the sea, pressure increases to 202 kPa
- nitrogen moves from high pressure in lungs into the blood
- a slow return to the surface lets the nitrogen return to the lungs where it is breathed out
- swimming up too quickly doesn’t give nitrogen enough time to leave the blood so instead it forms painful bubbles
