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Learning Objectives
• What are the essential features of exchange surfaces?
• How are gases exchanged in the alveoli of humans?
What are the essential features of exchange surfaces?
Feature Reason
Large surface area to volume ratio
Speed up the rate of exchange
Very thin Keep the diffusion pathway short
Partially permeable To allow selected materials to diffuse easily
Movement of external medium e.g. air
To maintain a diffusion gradient
Movement of internal medium e.g. blood
To maintain a diffusion gradient
Fick’s Law of Diffusion
Diffusion is proportional to:
surface area x difference in concentration--------------------------------------------------------
-Length of diffusion path
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Being thin, specialised gas exchange surfaces are easily damaged, so they are often positioned inside the organism.
The Alveoli (the facts)• There are 300
million in each
lung.
• Total surface
area is around
the size of half
a tennis court.
The Alveoli (the facts)
• Each alveolus is lined with a single layer of flattened epithelial cells.
• Around each alveolus is a network of pulmonary capillaries lined with a single layer of endothelial cells.
• The capillaries are narrow so that rbc are flattened and squeezed through.
• The capillaries have thin walls (one layer of cells thick).
Diffusion is rapid because:• Rbc are slowed as they pass through pulmonary
capillaries.• The distance between the alveolar air and rbc is reduced
as the rbc are flattened against the capillary wall.• The walls of the alveoli + capillaries are v.thin short
diffusion distance.• Sa of alveoli (due to folds) and capillaries (because there
are many) is large• Breathing (ventilaton) and blood movement (due to the
heart) ensure a concentration gradient is maintained