Session 3 Flashcards
How do you calculate total pressure from partial pressures?
In a mixture of gases, gas mixtures follow Dalton’s Law that the total pressure of the gases = the sum of the partial pressures of the individual gases.
E.g Partial pressure of O2 = 1/3 of Total pressure
Partial pressure of N2 = 2/3 of Total pressure
Total pressure = Partial pressure of O2 + Partial pressure of N2
What is atmospheric pressure?
Atmospheric pressure – the sum of the partial pressures of the gases in the atmosphere
- Pressure exerted by the weight of the air above the earth in the atmosphere
- At sea level: 101 kilopascals (kPa) = 1 atmosphere = 760 mm Hg of pressure
- At high altitudes atmospheric pressure is lower (weight of air pressing down is less)
Gas partial pressures in the air we breath in, and in air in our lungs
- Ambient air is a gas mixture of mostly nitrogen (79%) and oxygen (21%) and a bit of carbon dioxide (0.04%)
- Remembering Dalton’s Law (sum of the partial pressures of gases = total pressure)
- Atmospheric pressure (at sea level) = 101 kPa – sum of atmospheric partial pressures
- Partial Pressure of Oxygen (PO2) = Fraction of Oxygen in the air (FiO2)×Atmospheric pressure (Patm)
- PO2 = FiO2 ×Patm= (0.21 ×101 kPa) = 21 kPa - partial pressure of O2 in the air we breath
- BUT Inspired gas (air) is warmed and humidified in the upper respiratory tract - that is we add water to the air we breath in, in the form of water vapour - so need to subtract the water vapour pressure from the atmospheric pressure 101 kPa- 6.3 kPa = 94.7 kPa
- Therefore, PO2 of air once inspired and moistened (i.e. in our conducting part of Resp Tract) is 0.21 ×94.7kPa= 20 kPa
Alveolar gas pressure
ALVEOLARGAS PRESSURES determine SYSTEMIC ARTERIAL BLOOD GAS PRESSURES
- Atmosphere PO2 = 21.3 kPa; PCO2 = 0.04 kPa
- Remember PO2 in conducting airways of our Respiratory Tract = 20 kPa ; PCO2 = 0.04 kPa
- Alveolar partial pressure oxygen is 13.3 kPa
Describe alveolar ventilation
- Typical amount of air inspired/exhaled at rest = 450 ml (called the Tidal Volume) Typically 30% of the normal tidal volume fills Anatomical dead space ~30% of 450 ml = 150 ml
- Anatomical dead space is Air conducting space of our respiratory tract
- Conducting airways extend from the nostrils/nose, nasopharynx, trachea, bronchi, to the distal end of terminal bronchiole (or the start of/proximal end of respiratory bronchiole – No gas exchange occurs here
- Therefore, 300 ml of air reaches the respiratory portion of our lung – this is alveolar ventilation
- Typical total air volume in our lung is 3 litres – 3000 ml
- Therefore that 300 ml of NEW (fresh) air represents about 10% of all the air in our lungs
- Fresh air is DILUTED by old air in lung –old air is having O2 continually extracted by blood exchange, and CO2 constantly being added
Conducting airways, alveolar, arterial and vein partial gas pressures:
- Conducting airways partial pressures – PO2 = 20 kPa; PCO2 = 0.04 kPa
- Alveoli partial pressure O2(denoted PAO2) = 13.3 kPa; • Alveoli partial pressure CO2 (denoted PACO2) = 5.3 kPa
- Arteries partial pressure of O2 (denoted PaO2 or pO2) =13.3 kPa
- Arteries partial pressure of CO2 (denoted PaCO2 or pCO2) = 5.3 kPa
- Veins partial pressure O2 (denoted PvO2 ) = 5.3 kPa • Veins partial pressure CO2 (denoted PvCO2)= 6.1 kPa
How does gas exchange occur?
In the body gases diffuse down their partial pressure gradient – from area of high partial pressure – to low partial pressure
e. g. movement of oxygen from alveolar air to blood
e. g. movement of carbon dioxide from blood to alveoli
- Partial pressures (rather than concentrations) used to describe gases in the body
- Denoted by ‘p’ - as in pO2, pCO2, pN2
When inspired gases come in contact with body fluids (made up mostly of water) - gas molecules will enter fluid and dissolve in the liquid
Gas dissolves in body fluids
Dissolved gas molecules also exert pressure in the liquid –i.e. water
• Equilibrium is reached when: rate of gas entering water = rate of gas leaving the water. At equilibrium, the partial pressure of the gas dissolved in the liquid = partial pressure of the gas in the air above it
– This occurs at the alveoluscapillary border
- Blood oxygen and CO2 partial pressures equilibrate to the alveolar levels because “the partial pressure of the gas in the liquid = partial pressure of the gas in the air above it.”
- –NB:another term used for partial pressure of a gas in the liquid is ‘tension’ (e.g. oxygen tension in blood)
Summary alveolar gas partial pressures
- Alveolar air pO2 =13.3 kPa (lower than inhaled air)
- pCO2 =5.3 kPa (higher than in inhaled air)
- Because pO2 in inhaled air lower because it mixes with residual volume
- Effect of O2 diffusing OUT across the alveolar wall
- Effect of CO2 entering INTO the alveoli
- Alveolar air composition stays constant around this level;
- Blood equilibrates to this level –because “the partial pressure of the gas in the liquid = partial pressure of the gas in the air above it.”
How is partial pressure different form the amount of gas dissolved?
Amount of a gas dissolved (mmol/L) = partial pressure (kPa) x solubility coefficient of gas
Solubility coefficient – is a constant for the individual gas and the solvent (the substance in which it is dissolved) – which in our body is blood, plasma, ECF – i.e. mostly water!
Solubility coefficient of O2 in plasma = 0.01 mmol/ L /kPa (at 37°C) \When exposed to a pO2 of 13.3 kPa (as in alveolar air) the equation is: 0.01mmol/L/kPa x 13.3 kPa = 0.13 mmol/L of O2 will dissolve
Plasma has 0.13 mmol dissolved oxygen /per litre BUT –need to remember O2 binding to haemoglobin (Hb)
If a gas reacts with a substance within a liquid -e.g. O2 binding to haemoglobin (Hb) -in addition to dissolving, then this reaction must complete before equilibrium is reached and partial pressure is established –the oxygen binding to Hb does NOT contribute to pO2 in the blood
- O2 enters plasma and dissolves in plasma, dissolved O2 enters RBC to bind to Hb
- Process continues till Hb fully saturated (each Hb molecules binds 4 O2 molecules)
- After Hb is fully saturated, O2 continues to dissolve till equilibrium is reached • At equilibrium, pO2 of plasma = pO2 of alveolar air
- Blood contains both dissolved and Hb bound oxygen
- The pO2 is a measure of dissolved O2 in the blood.
- Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
- As dissolved O2 leaves the blood, it will be replaced by O2 bound to Hb.
- In this way, the oxygen bound to Hb will be downloaded and diffuse into tissues
- 98-99% oxygen bound to haemoglobin, 1-2% oxygen dissolved in the blood
- In this way we very significantly increase the total available oxygen in our blood
Total content of gas = dissolved gas + gas bound to or reacted with a component ( e.g. O2 dissolved plasma + O2 bound to Hb)
What do we mean by “MIXED” venous blood? Mixed from what?
In mixed venous blood
- PO2 typically ~6.0 kPa
- PCO2 typically ~6.1 kPa
- but varies with metabolism ratio of carbohydrates to fats eaten –respiratory quotient
Mixed venous blood is blood from IVC SVC and coronary vessels
Gradients of partial pressure of gases in the lung alveoli
PO2=~6.0 PCO2=~6.1 kPa in mixed venous blood and kPa PO2=13.3 kPa PCO2=5.3 kPa in alveolar gas
- Alveolar PO2 > PO2 in mixed venous blood
- Alveolar PCO2 < PCO2 in mixed venous blood
- Therefore oxygen will diffuse into blood and carbon dioxide out
Factors affecting rate of diffusion
- Partial pressure difference (gradient) across membrane (P1–P2) AND
- A -the surface area available for diffusion
- T –(thickness) i.e. distance molecules must diffuse
- Diffusion coefficient of the individual gas Diffusion also depends on properties of the individual gas:
- The solubility of the gas in the liquid : greater the solubility, faster the rate of diffusion
- Molecular weight of gas: – Higher the molecular weight slower the rate of diffusion
Diffusion coefficient (D) - used to determine the relative rates at which different gases will diffuse across the same membrane at the same pressures;
Diffusion of CO2versus diffusion of O2
- Solubility: CO2 much more soluble than O2 -so diffuses faster than O2
- Molecular weight: molecular weight of CO2 > O2 -molecular weight slows down CO2
Combine the two factors Oxygen is small and thus fast, but CO2 is more soluble
Overall, the effect of solubility is greater CO2 diffuses 20 times faster than O2
- Larger difference in partial pressures (ΔP) compensates for slower diffusion of O2
- In a diseased lung with lower oxygen levels, O2 gas exchange is more impaired than CO2 because of O2 slower diffusion rate
- HOWEVER, in order to exhale CO2, air needs to be delivered to the lungs for gas exchange to occur – therefore in HYPOVENTILATION (reduced amount of air entering alveoli 2º to either too few breathes or too shallow or both) pCO2 in the blood goes up as well as pO2 in blood decreasing
0Diffusion barriers between alveolar lumen and blood
Diffusion from alveolar air to RBC in capillary must cross:
– Fluid film lining alveolus
– epithelial cell of alveolus
– Interstitial space
– endothelial cell of capillary
– plasma
– red cell membrane
- 5 cell membranes • 3 layers of cytoplasm • 2 layers of tissue fluid +plasma
- The surface area of the alveolar capillary membrane about 100 m2
- Barrier < 0.4 μM thick
- Oxygen exchange complete in 1/3 of time blood spends in the lung capillary bed
- So plenty of reserve – needed for when we exercise
Factors affecting rate of gas diffusion –in disease
- Thickness of the membrane/space – Increase as a result of oedema fluid in the interstitial space and in alveoli – Lung fibrosis -increased thickness of alveolar and capillary membranes and interstium
- Surface area of the membrane – Decreased by removal of an entire lung – Emphysema -decreased surface area
- Diffusion coefficient of the gas: – CO2 always diffuses much faster than O2 – So, diffusion of O2 affected pO2 is low – Diffusion of CO2 not affected àpCO2 normal until late stage disease Diffusion impairment problems with alveolar capillary membrane
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