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There are red blood cells filled with oxygen from the lungs to all parts of the body. There is a lot of oxygen in other red blood cells and very little in other body cells. There is more carbon dioxide in the body cells than in the blood cells. How does the carbon dioxide and oxygen move to where they need to go? Is it by osmosis, by diffusion or any other process?
Question Date: 2017-03-17
Answer 1:

Carbon dioxide and oxygen move by diffusion due to a pressure gradient. Since the pressure of oxygen in the blood due to the blood cells is so high, oxygen always moves out of the blood into the surrounding cells. In the reverse situation, since the pressure of carbon dioxide is always higher in the body cells, it diffuses into the blood. The carbon dioxide then usually turns into an acidic ion called bicarbonate and dissolves in the blood. The dissolved bicarbonate then is turned back into gaseous carbon dioxide by enzymes and breathed out in the lungs. The body carefully controls the pressure of these 2 gases in the lungs and blood to make sure oxygen reaches all cells that need it and carbon dioxide is eventually transported back to the lungs. The gases do not move by osmosis, since oxygen and carbon dioxide can freely move through cell membranes so it’s as if there are no physical barriers to these gases. Also, osmosis generally refers to the solvent moving due to concentration differences in a solute which cannot move through a cell membrane. Since these gases aren’t affected by any barriers, their concentration won’t affect the properties of the water in which they are dissolved.

Answer 2:

In complex animals (like us), the heart actively pumps the blood around, including the red blood cells and the oxygen that they contain, and including the carbonic acid that the carbon dioxide becomes.

For large animals, osmosis is too slow. This is why heart failure is fatal in something like a human. There are animals that are small enough that osmosis is fast enough to get the oxygen in and the carbon dioxide out. These animals do not need or have hearts.

Answer 3:

Carbon dioxide and oxygen move through the body by advection and by diffusion. Advection describes how these gases are carried along by the flow of blood, thanks to the pumping action from the heart. Diffusion is happening when a gas spreads out from areas of high concentration to areas of low concentration. When these two processes (advection and diffusion) happen at the same time, we call the overall process convection.


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