| 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|
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
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
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.
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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