Transcript 4.3 Exchange of gases in the lungs

4.3 Exchange of gases in the
lungs
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
What are the essential features of
exchange surfaces?
Feature
Reason
Large surface area to Speed up the rate of exchange
volume ratio
Very thin
Keep the diffusion pathway
short
Partially permeable
To allow selected materials to
diffuse easily
Movement of external To maintain a diffusion gradient
medium e.g. air
Movement of internal To maintain a diffusion gradient
medium e.g. blood
Fick’s Law of Diffusion
Diffusion is proportional to:
surface area x difference in concentration
--------------------------------------------------------Length of diffusion path
Not only have I come up with an
undisputed scientific law, I also
have an extraordinarily awesome
moustache which I curl daily.
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.
Alveoli structure
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
Gas exchange at the Alveolus
Task
• Complete summary Questions 1 and 2 on
page 79.