Gas exchange

• Movement of air in the lungs: 

  • Mechanics of breathing produce inspiration and air flow  

  • During inspiration air initially moves into lungs via convection  

  • Air passes through the conducting zone to respiratory zone of the bronchial tree  

  • Air in smallest airways move into the alveoli sacs by diffusion  

  • The actual exchange of gases occurs across the capillaries in the alveoli sacs 
• Convection: movement of currents within fluids (i.e. liquids, gases) 
• Diffusion: process by which molecules intermingle as a result of their kinetic energy of random motion. It's an extremely rapid process and can only occur over very small distances 

   
• Diffusion and gas exchange: 

  • Gas exchange takes place in alveoli sac across the alveolar membrane  

    – a boundary between the external environment and interior of the body  

   

  • Gases cross the respiratory membrane by diffusion  

  • In accordance with Fick’s Law 
• Fick’s Law: 

  • The rate of transfer of a gas through a sheet of tissue is proportional to  

    – tissue area  

    – difference in gas partial pressure between the 2 sides  

    – diffusion constant  

   

  • inversely proportional to tissue thickness 

   
• Diffusion across alveolar membrane: In accordance with Fick’s law diffusion is dependent on  

  – Concentration / pressure gradient  

  – Gas solubility  

  – Thickness of alveolar membrane  

  – Surface area of alveolar membrane  

  – Ventilation / perfusion coupling 

   
• Partial pressure: 

  • Partial pressure describes the amount of gas dissolved in the plasma  

  – E.g. PaO2 amount of O2 dissolved in plasma of arterial blood  

  – PvCO2 amount of CO2 dissolved in plasma of venous blood 

   
• Gas solubility: 

  • Both O2 and CO2 are soluble gases  

  • CO2 is 20 times more soluble than O2 

  • High solubility of CO2 facilitates rapid diffusion despite small CO2 gradient  

  • Equal amounts of CO2 & O2 diffuse across the respiratory membrane in the same time period 
• Clinical relevance solubility: 

  • In respiratory disease when diffusion is impaired O2 will be primarily affected as it is less soluble  

  • A significant impairment is required to lead to poor CO2 transfer 

   
• Thickness of alveolar membrane: 

  • 0.5 to 1 nanometer thick  

  • Diffusion efficiency is highly dependent on distances involved  

  • As membrane ultra-thin gas exchange rapid and efficient 

   
• Clinical relevance membrane thickness: 

  • If membrane thickened there is greater distance between alveoli and capillary  

  • Diffusion slower and / or impaired  

  • Leading to hypoxia  

  • Membrane thickening may occur due to 

  – Inflammation  

  – Infection  

  – Fibrosis 

   
• Surface area of membrane: 

  • Adult lung contains around 300 million alveoli  

  • Gives gas exchange surface area 70- 80m2  

  • Huge capacity which can be utilised to maintain acid-base balance  

   
• Clinical relevance surface area: 

  • If surface area reduced, despite adequate diffusion sufficient gas exchange may not be possible  

   

  • ↓ O2 and ↑ CO2 and ↓ pH --> acidity  

   

  • Temporary loss surface area  

  – Bronchial obstruction (tumour, mucus plug)  

  – Atelectasis (partial collapsed lung) 

  – Consolidation  

   

  • Permanent loss surface area  

  – Emphysematous bullae 
• Ventilation perfusion coupling: 

  • V/Q ratio is measurement used to describe efficiency and adequacy of matching two variables  

   

  • V ventilation  

  – the air which reaches the lungs  

   

  • Q perfusion  

  – the blood which reaches the lungs  
• V/Q differences in normal lung: 

  • The lungs are centered vertically around the heart  

  • Part of the lung is superior to the heart and part is inferior  

  • This has major impact on the V/Q ratio  

  • Lower part of lung is better ventilated and better perfused than apex 

   
• Clinical relevance V/Q: 

  • For efficient gas exchange there needs to be maximum coupling between ventilation and perfusion  

  • Inadequacy of either V or Q will have significant impact on removal of CO2& oxygenation of blood  

  • The V/Q ratio can be measured with a ventilation/perfusion scan 

   
• Shunt – no ventilation, but perfusion 

  alveolar dead space – ventilation but no perfusion 

   
• Maintaining V/Q matching: 

  • Vital to maintain V/Q ratio to achieve adequate gas exchange  

  • In respiratory disease V/Q mismatch triggers auto-regulatory homeostatic mechanisms to minimise deficit  

  – Hypoxic pulmonary vasoconstriction (HPV) or dilation  

  – Bronchiole response (constriction or dilation) 

   
• Hypoxic pulmonary vasoconstriction: 

  • In respiratory disease areas of poor ventilation lead to ↓ gas exchange & hypoxia 

  • When PaO2 falls to ~ 6Kpa / SaO2 low 80s hypoxia is sensed by receptors in arterioles 

  • Arterioles passing through area of poor ventilation constrict to minimise V/Q mismatch  

  • Blood flow is redirected to area with good ventilation to facilitate gas exchange 

   
• Clinical relevance HPVC: 

  • Persistent HPVC occurs in chronic respiratory disease (COPD)  

  • Pulmonary arterioles have ↓ diameter due vasoconstriction  

  • Therefore ↑ resistance to blood flow 

  • To maintain blood flow to lungs R side heart must work hard  

  • R side heart hypertrophy  

  • Ultimately --> R sided heart failure  

  • Cor Pulmonale – need long term 2 therapy  

   
• Pulmonary capillary recruitment: In areas where ventilation is high additional vessels in the pulmonary arteriole bed are recruited to optimise V/Q matching and maximise gas exchange 
• Bronchiole Response: 

  • Bronchioles are highly sensitive to PACO2 (alveolar levels of carbon dioxide) 

  • High PACO2 produces bronchodilation  

    – ↑ CO2 excretion  

    – normalisation of PACO2 & PaCO2  

   

  • Low PACO2 produces bronchoconstriction  

    – ↓ CO2 excretion  

    – normalisation of PACO2 & PaCO2