Exchange Surfaces

Gas exchange is really important so that we take oxygen for aerobic respiration and get rid of the carbon dioxide so that it doesn’t accumulate inside of us. That’s why exchange surfaces, like the lungs, are specially adapted to let gases pass through them as speedily as possible…

 
 

Adaptations of exchange surfaces

For respiration, organisms need to take oxygen into their bodies and remove carbon dioxide. These gases diffuse across exchange surfaces, such as the lungs, so exchange surfaces are adapted to make diffusion as efficient as possible. Most gas exchange surfaces are extremely thin (sometimes just one cell thick), ensuring a short diffusion pathway across the exchange surface. They will also have a large surface area to volume ratio which provides more space for the diffusion of gases. The organism itself will also have features which maximise the concentration gradient of gases across the exchange surface.

Adaptations of the lungs

Each alveolus are adapted to make gas exchange as efficient as possible:

Large surface area: there are approximately 700 million alveoli in our lungs with a combined surface area of 70 square meters.

Good blood supply: lots of capillaries surround each alveolus

Short diffusion distance: the walls of both the alveoli and capillaries are just one cell thick

Moist surfaces: the liquid on the surface of alveoli dissolves gases and facilitates diffusion

Inhalation and exhalation: breathing in and out replaces the air in the alveoli, maintaining a steep concentration gradient.

Location in the centre of the body: the lungs are right in the middle of our body, inside our thorax. The core of our body is the warmest, giving the gas molecules more heat energy and making them move around faster (so quicker diffusion).

Fick’s Law

Fick’s Law describes how the rate of diffusion is affected by the surface area, concentration gradient of gases and the thickness of the exchange surface. It is summarised in the equation below:

 
 

Since the rate of diffusion is in a proportional relationship to the three factors above, this means that if the rate of diffusion doubles if:

  • Surface area doubles

  • Concentration gradient doubles

  • Diffusion distance halves

Did you know..

The lungs are the only organ of the human body which can float on water. Medical examiners use the ‘lung float test’ to determine whether a child was stillborn or died after childbirth. If the lungs float then it means the baby died after they were born, since there is still gas in the lungs.