Physics Made Easy

Gas Exchange

Respiratory surface: a surface over which gaseous exchange takes place (by diffusion). Efficient respiratory surfaces have the following properties (refer back to Cell Structure and Function for info on diffusion and Fick’s Law) :

  1. Large surface area
  2. High permeability
  3. Very thin (typical thickness 1 mm)
  4. Kept moist (O2 and CO2 diffuse more rapidly in solution)
  5. Associated with an efficient transport system that maintains a steep concentration gradient

Leaves are the main gas exchange surfaces of plants. Generally, they are broad and flat to maximise surface area; however, as this will also increase water loss.

Stomata (singular=stoma) are small holes found mainly on the underside of a leaf through which gases diffuse in and out of air spaces within the leaf. Stomata are numerous, and are placed at an optimum distance apart to fit in as many stomata as possible whilst ensuring their diffusions shells do not overlap.


Stomata can be open or closed, depending on conditions.

Guard cell mechanism controlling stomatal opening:


The entrance to each stoma is surrounded by guard cells. Usually, guard cells are turgid, and the stoma is open, allowing gas exchange and consequently water loss.

When water is low, however, the guard cells become plasmolysed and lose their rigid shape, thus closing the stomata and preventing further water loss (and gas exchange).

Gas exchange surface of locusts: small tubes or tracheoles penetrate through the entire body, and act as airways. This provides a large surface area, but also means that most insects are only a few mm in length- this is because the insect relies on direct diffusion through the tracheoles and diffusion only takes place over short distances.

Spiracles control the overall flow of air into and out of the tracheoles- they can open and close. Hairs around the opening of the spiracle reduce water loss and prevent the entrance of foreign bodies.

Condition of tracheoles in resting and active tissue:


Gas exchange in bony fish: water is drawn into the mouth and passed over the gills.


Counter-current flow mechanism: in the gills, the flow of blood is in the opposite direction to the flow of water across the gills. This means that blood is continually meeting fresh water with a higher percentage saturation of oxygen. In this way, the concentration gradient is maintained across the gill lamella and oxygen continues to diffuse into the blood.


If the blood and water flowed in the same direction (parallel flow), diffusion would only occur until equilibrium was achieved- this is less efficient as the blood would only be able to reach a maximum of 50% O2 saturation (some fish, however, do have a parallel instead of counter-current flow mechanism).

Gas exchange in mammals takes place in the lungs. Air is drawn into the trachea (windpipe), which divides into two smaller bronchi (singular: bronchus- one leads to each lung). In the lungs, the bronchi divide into even smaller bronchioles, small tubes which terminate at the alveoli (singular: alveolus), millions of small “bags” which are the main sites of gas exchange (a little gas exchange occurs in the bronchioles).

Alveoli are composed of thin flat squamous epithelium (gives large S.A.) and are attached to a basement membrane. Each alveolus is about 100 μm in diameter and is strengthened by elastic and collagen fibres. Alveoli are small and highly folded in order to fit a large surface into a comparatively small volume- for example, the total area of the gas exchange surface in the human lungs is about 70 m2!

Each alveolus is surrounded by a network of thin, narrow capillaries; red blood cells move slowly through these capillaries with much of their surface area in contact with the capillary wall, allowing for maximum diffusion from the alveolus to the cell.

Alveoli are also moist, allowing gases to diffuse in solution (more efficient than diffusing straight across as gases). The fluid in the alveoli contains surfactants to prevent their thin walls from collapsing under surface tension.

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