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Gas exchange is the movement of a gas from an area of high concentration—an area where there’s a lot of it—to an area of low concentration, like the exchange of oxygen and carbon dioxide (CO2) when we breathe.

A gas is a substance that’s neither a solid nor a liquid—like oxygen, for example. Exchange refers to the process of a gas passing through a surface, moving from one area to another—like from water or blood to air.

Gas exchange commonly refers to the exchange of oxygen and carbon dioxide between a living thing and its environment. In animals, gas exchange happens during respiration (breathing). In plants, it happens during photosynthesis, which is the process by which plants make their own food.

It took a long time for scientists to understand the movement and interactions of gases that we now call gas exchange. The Ancient Greek philosopher and physician Galen developed a theory of respiration that was later challenged in the 1400s by physician Ibn al-Nafis and subsequently developed by various scientists through the centuries. It was in the 1600s that scientists started to understand the nature of gases, which paved the way for understanding gas exchange in the respiratory system. But it wasn’t until the early 1900s, with the work of August and Marie Krogh, that we began to fully understand the process.

In the respiratory system, gases (in our case, oxygen and carbon dioxide) diffuse, or flow, through the walls between the blood and lungs with the aid of a thin film, or membrane, of moisture. This lets oxygen into the blood, and carbon dioxide out. When we breathe in, there’s a lot of oxygen in the lungs, but not in the blood, and there’s a lot of carbon dioxide in the blood, but not in the lungs. When we breathe out, these two gases diffuse through the cell walls, swapping places so that our lungs are full of carbon dioxide and our blood is full of oxygen. In plants, the same process of exchange happens between water vapor and carbon dioxide. Water vapor and oxygen flow out through stomata, or pores, in the leaf surface, and carbon dioxide flows in the same way.

Gas exchange is an incredibly important process—it’s the main thing allowing us to breathe. Gas exchange is happening all around us, all the time, and knowing the fundamentals of the process can even help explain how rising carbon dioxide levels in the air make the level of carbon dioxide in the ocean rise, too.

The equations we can use to describe the diffusion of gases through a membrane are set forth in Fick’s laws, developed by German scientist Adolf Fick in the 1850s.

Gas exchange allows plants to absorb carbon dioxide and release oxygen.

My goldfish are doing very well this winter in their pond. Pond is deep enuf it won’t freeze solid. We place an aquarium bubbler inside to force fresh air into the water and a floating heater keeps an opening in any surface ice for gas exchange.

— DiamondLynne (@DiamondLynne1)

You can refrigerate an open can of soda and it will stay fizzy overnight. The cold slows down the gas exchange with atmosphere

— DaveinTexas (@DaveinTexas)

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• diffusion
• respiration
• photosynthesis
• atmosphere
• ventilation
• perfusion
• membrane

True or false?��

Gas exchange doesn’t occur in plants.