B3.1 Gas exchange

Whether you are a person, a fish or an oak tree, you face the same problem: oxygen and carbon dioxide must move between your cells and the environment, but they can only travel the final distance by diffusion, which is hopelessly slow over more than a fraction of a millimetre. Evolution’s answer is the gas exchange surface — a specialised interface, thin and vast, where gases cross efficiently. In B3.1 you study what makes such surfaces effective and how lungs, leaves and gills all embody the same design principles despite looking nothing alike. The recurring idea is maximising the rate of diffusion.

Properties of an efficient gas exchange surface

Gases cross exchange surfaces by passive diffusion, so the structures are adapted to make diffusion work in the cell’s favour. The rate of diffusion is increased by a large surface area, a short diffusion distance and a steep concentration gradient. Effective surfaces therefore share these features:

• Large surface area — more area means more diffusion can occur at once.
• Thin (short diffusion distance) — often a single layer of cells, so gases cross quickly.
• Permeable to the relevant gases.
• Moist — gases dissolve before diffusing across cell membranes.
• Maintained concentration gradients — a transport system (such as blood) and ventilation keep fresh supply on one side and remove gases on the other.

A good answer always ties an observed feature to one of these principles rather than just describing it.

The mammalian lung and ventilation

In humans, gas exchange occurs in millions of tiny air sacs called alveoli, which together provide an enormous surface area. Each alveolus has a wall just one cell thick and is wrapped in a dense network of capillaries, giving a very short diffusion distance. The surfaces are moist, and a thin film of fluid containing surfactant reduces surface tension so the alveoli do not collapse. Oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses the other way.

Ventilation (breathing) maintains the concentration gradients by continually refreshing the air. It depends on antagonistic muscle action:

• Inhalation: the diaphragm contracts and flattens and the external intercostal muscles contract, raising the ribs; thoracic volume increases, pressure falls, and air flows in.
• Exhalation: these muscles relax (and, when forced, the internal intercostals contract); thoracic volume decreases, pressure rises, and air flows out.

Gas exchange in plants and in water

In a leaf, gas exchange supports photosynthesis and respiration. Gases enter and leave through pores called stomata, usually on the lower epidermis, whose opening is controlled by guard cells. Inside, the loosely packed spongy mesophyll has many air spaces that give a large internal surface area, and the moist cell walls allow gases to dissolve and diffuse. During the day net carbon dioxide moves in and oxygen out as photosynthesis exceeds respiration.

Water holds much less dissolved oxygen than air, so fish need especially efficient surfaces. Gills have many thin filaments covered in tiny lamellae, giving a huge surface area and short diffusion distance. Many fish use a countercurrent arrangement in which water and blood flow in opposite directions, so a diffusion gradient is maintained along the whole length of the lamella, extracting far more oxygen than if the flows ran together.

Measuring and interpreting exchange

The syllabus expects you to handle data about ventilation and exchange. Ventilation rate is the number of breaths per minute, and tidal volume is the volume of one normal breath; their product gives the volume of air moved per minute. A spirometer traces lung volumes over time and lets you read these values, as well as showing how they rise with exercise.

When interpreting such data, connect changes back to demand: during exercise, muscles respire faster, so carbon dioxide rises and oxygen falls in the blood; the body responds by increasing both the depth and rate of breathing to steepen the concentration gradients at the alveoli. Always justify trends with the underlying need to maintain diffusion.