Why/how multicellular organisms have evolved gaseous exchange

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Why/how multicellular organisms have evolved gaseous exchange
- metabolic needs are proportional to volume. Larger organisms need more oxygen
- Larger organisms external surface insufficient for gas exchange
- diffusion of respiratory gases proportional to surface area
- surface area : volume ratio is too small/ larger animals have a smaller SA: vol ratio to supply metabolic needs
- diffusion distances too large
- not enough O2 can diffuse / O2 cannot diffuse fast enough to the cells furthest from surface to meet metabolic needs
- gas exchange surface folded/ large number of alveoli - to increase surface area
- internal lungs
- to reduce water / heat loss
- gaseous exchange takes place in the alveoli
- thin walls - reduce diffusion distance
- layer of moisture – for gases to dissolve in
- blood supply in capillaries – maintain concentration gradient between alveolar air and blood
- haemoglobin in erythrocytes – transport of oxygen
- ventilation mechanism e.g. lungs
  - -intercostal muscles contract. Diaphragm contracts.
    - Ribs pulled up+out. Diaphragm flattens
      - Increased volume of thorax. Decrece pressure in lungs
        Air in to replace stale air with fresh air to enable continuous exchange of gases
Ribs pulled up+out. Diaphragm flattens
- Increased volume of thorax. Decrece pressure in lungs
  - Air in to replace stale air with fresh air to enable continuous exchange of gases

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