Gas Exchange

Gas exchange is a biological process through which different gases are transferred in opposite directions across a specialised respiratory surface.

Features of a Gas Exchange Surface

Gas exchange surfaces in humans (the lungs) and in plants (the leaves) usually have a large surface area to increase the rate of exchange.
These surfaces are often very thin to shorten the distance gases have to diffuse across.
They are typically moist as gases mainly diffuse in solution.
They also benefit from a good blood supply (in animals) or being close to the transport system (in plants) to maintain a steep concentration gradient.

Gas Exchange in Humans

In humans, oxygen is breathed in through the mouth and nose, and travels down the trachea which splits into two bronchi, one leading to each lung.
Inside the lungs, the bronchi branch out into bronchioles which then end in tiny air sacs known as alveoli.
These alveoli provide a large, moist surface area, thin walls, and a rich blood supply for gas exchange. Oxygen diffuses across the alveolar wall and into the red blood cells in the capillaries while carbon dioxide diffuses out of the blood and into the alveoli to be breathed out.

Gas Exchange in Plants

In plants, gas exchange occurs in the leaf, which provides a large surface area.
The stomata (microscopic pores in the leaf surface) allow gas exchange between air spaces in the leaf and the outside atmosphere.
Guard cells can open and close these stomata to control the rate of gas exchange.
Carbon dioxide is absorbed by stomata from the air and used in photosynthesis, whilst oxygen, the by-product of photosynthesis, is released into the air.

Gas Exchange in Fish

Fish extract dissolved oxygen from water by passing it over gills, which are rich in blood vessels.
The structure of the gills provides a large surface area, thin walls, and a good blood supply for efficient gas exchange. Water containing dissolved oxygen enters the fish through its mouth, passes over the gills, and exits through the gill slits.
Here, oxygen in the water diffuses into the blood in the gill capillaries, while carbon dioxide in the blood diffuses into the water.

It’s important to remember that effective gas exchange can only occur if the concentration gradient of gases across the respiratory surface is maintained.