Ventilation and diffusion Flashcards
Learning outcomes
- Define the term “partial pressure“ and know how to calculate a gas partial pressure
- Explain what factors determine how much gas dissolves in a liquid
- Explain what factors affect the diffusion of gases across the air-blood barrier, and the consequences of such factors on arterial blood gases
- State the normal partial pressures of nitrogen, oxygen, carbon dioxide and water vapour in atmospheric air and alveolar air (in mmHg and kPa)
- Describe the processes involved in an oxygen molecule moving from alveolar air to the blood
- Describe the processes involved in a carbon dioxide molecule moving from blood to the alveolar air
- State how different disease states can affect the diffusion of O2 and CO2 across the alveolar barrier
- Describe how altitude affects PO2
- Describe the effects of high pressure on blood gases
Discuss the conducting airways
• The conducting airways
– Cartilage, few smooth muscles
– Collapse rare
• Respiratory bronchioles & alveolar ducts
– No cartilage, lots of smooth muscle
– Susceptible to collapse during expiration
• Anatomical Dead Space – 150mls
– Up to generation 17
• Humans have ~ 300 million alveoli and cross sectional area increases from 2.5cm2 at trachea to around 100m2
What are the primary functions of the respiratory and cardiovascular systems
- One of the primary functions of the cardiovascular systems is to transport O2 from the lungs to all tissues in the body
- And CO2 from the tissues to the lungs
- The lungs expire this CO2 to the atmosphere
- Both gases move by diffusion down their concentration gradients
What is Dalton’s law of partial pressures?
- “Total pressure (PTotal) of a mixture of gases is the sum of their individual partial pressures (Px)”
- Atmospheric Pressure (PB) at sea level is 760mmHg or 101.325 kPa
• Nitrogen (PN2): 78.09% x 760 mmHg = 593.48 mmHg
• Oxygen (PO2): 20.95% x 760 mmHg = 159.22 mmHg
• Carbon dioxide (PCO2): 0.030% x 760 mmHg = 0.23 mmHg
Argon (P ): 0.930% x 760 mmHg = 7.07 mmHg
- If atmospheric pressure changes then partial pressure changes
- If proportion of gas changes its partial pressure changes
What is Henry’s Law
• States that the concentration of O2 dissolved in water ([O2]dis) is proportional to the Partial pressure (PO2) in the gas phase
[O2]dis = s x PO2
where s = solubility of O2 in water
For blood plasma s = 0.0013 mM/mmHg at 37oC, so
[O2]dis = 0.0013 x 100 mmHg = 0.13 mM (arterial blood) [O2]dis = 0.0013 x 40 mmHg = 0.05 mM (mixed venous blood)
• CO2 is the most soluble, O2 is about 1/20th as soluble and N2 is barely soluble at atmospheric pressure
Discuss alveolar gas composition
• Alveolar air is warmed and humidified
- Nitrogen (PN2): 78.09% x (760 – 47) mmHg = 556.78 mmHg
- Oxygen (PO2): 20.95% x (760 – 47) mmHg = 149.37 mmHg
- Carbon dioxide (PCO2): 0.030% x (760 – 47) mmHg = 0.21 mmHg
- Argon (P ): 0.930% x (760 – 47) mmHg = 6.63 mmHg
• O2 in lungs is actually lower (104 mmHg), CO2 is higher (40 mmHg) and water vapour is higher (as a consequence of this N2 is lower)
• Why?
– alveolar air is made up of ‘fresh air’ plus the air that remains in the lungs after the last breath
Discuss gas exchange between alveolar and blood
• O2 has to:
– dissolve in an aqueous layer
– diffuse across the membranes
– enter the blood
Flow = (∆P x A x D) / T
• Rate of diffusion is proportional to
– Partial pressure difference (∆P)
– Surface area (A)
– Solubility (D, diffusion coefficient)
– molecular mass (D, diffusion coefficient)
– Inversely proportional to tissue thickness (T)
• Surface area of lungs is large 50 -100 m2
• Large number of alveoli (~500 million)
• Thickness is small (0.2 – 0.5 μm)
• Concentration gradient is large
– PO2 alveolar air is 100 mmHg
– PO2 of venous blood is 40 mmHg, so diffusion rapid
• Molecular mass insignificant, but solubility very important, with CO2 diffusing 20 x more rapidly than O2
• At rest, takes about 0.75 - 1 second for blood to pass through pulmonary capillaries
• O2 equilibrium only takes about 0.25 s (so not normally diffusion-limited)
• In exercise, capillary transit time can be reduced to as little as 0.3 s (so now diffusion-limited)
• CO2 moves in the other direction, from blood capillary into alveoli
• Smaller concentration gradient
– alveolar PCO2 is 40 mmHg
– venous PCO2 is 45 mmHg
• However, greater solubility, so CO2 diffusing 20 x more rapidly than O2
• Same amount of gas moves
Discuss diffusion limitations of gas exchange
• Flow = (∆P x A x D) / T
• In oedema, T (thickness of barrier) increases
• Transit time through capillary may not be sufficient to complete full gas exchange
– gas exchange reduced
– More marked effect on O2 than CO2, due to greater solubility of CO2
• In emphysema, A reduced (breakdown of tissue and alveolar sacs)
– Gas exchange reduced
• In pulmonary fibrosis, T increased (deposition of fibrotic tissue) – Gas exchange reduced
• Mucus, inflammation of airway, tumours, reduce gas entry – Gas exchange reduced
Discuss the impact of altitude on ventilation
- At altitude, atmospheric pressure is reduced
- Hence, PO2 is reduced
• Denver, Colorado, 1620m (5300 ft)
above sea level
– PB is 632 mmHg (down from 760 mmHg)
– Inspired PO2 – 125 mmHg (down from 149 mmHg)
– Alveolar PO2 – 84 mmHg (down from 104 mmHg)
– Alveolar PCO2 – 34 mmHg (down from 40 mmHg)
Discuss physiological adaptations to altitude
• Acute
– Hypoxia sensed by peripheral
chemoreceptors
– Ventilatory drive increases initially but blunted by central chemoreceptors that respond to decreased PaCO2 due to increased ventilation
– CO increases due to suppression of cardioinhibitory centre
• Adaptive
– Central chemoreceptors adapt so
ventilation rate continues to increase
– PaCO2 drops leading to respiratory alkalosis, kidneys compensate by reducing acid excretion blood pH normalises
– Alkalosis stimulates 2,3 DPG production – leads to rightward shift of O2 dissociation curve
• Acclimation
– Blood
– Erythropoietin release stimulated
– Hb conc. increases to 200 g/L from 150 g/L
– Vasculature
– Hypoxia stimulates angiogenesis
– Capillary density increases throughout body
– Cardiopulmonary system
– Vascular and ventricular remodelling
– Smooth muscle growth increase vascular wall thickness
– Right ventricle hypertrophies
Discuss diving and ventilation
Atmospheric pressure increases by 760 mmHg (1 atmosphere) every 10 m depth
Effects of Depth
– Increase in partial pressure, N2 and O2 dissolve into blood at lethal excess
– Volume decrease
Gas toxicity
– N2 narcosis – partial pressure of N2 (40m and below) rises and starts to dissolve in body tissues
– O2 poisoning – again tightly regulated at sea level and system essentially saturated
– At high pressure O2 dissolves in blood in excess of the buffering capacity of Hb
– Heliox – N2 replaced by helium and percentage of O2 tailored to reduce harm
– He less readily dissolves in body tissues and less narcotic.
Summary
- O2 and CO2 exchange occurs by diffusion , driven by partial pressures gradients for both gases
- O2 has limited water solubility
- CO2 is 20 x more soluble than O2 in blood
- Atmospheric pressure decreases with altitude above sea level
- PO2 also decreases causing hypoxia
- Diving increases partial pressure of inspired gases
- O2 and N2 become potentially toxic as dissolve into tissues at depth
