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 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

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.