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An oxygen molecule would take about three years to diffuse from your lungs to your toes.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
An oxygen molecule would take about three years to diffuse from your lungs to your toes. Explain how oxygen really reaches your toes in a few seconds. Then explain how carbon dioxide from your toes reaches your lungs.
Need a reminder? The key ideaShow
Air passes through the nostrils, pharynx, larynx, trachea (held open by rings of cartilage), bronchi and bronchioles to the alveoli, where gases are exchanged with the blood. Breathing in, the ribs lift and the diaphragm flattens. Residual air stays in the lungs. Haemoglobin carries oxygen; most carbon dioxide travels dissolved in the plasma.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Too slow by diffusion. Over such a long distance, diffusion alone could never supply the cells.
Step 2. Oxygen. In the alveoli, oxygen enters the blood and joins haemoglobin, a red pigment in the red blood cells. It has a high affinity for oxygen. The heart pumps the blood to the toes, where the haemoglobin gives the oxygen up.
Step 3. Carbon dioxide. Carbon dioxide dissolves in water much more easily than oxygen. So most of it travels dissolved in the plasma, back to the lungs, where it diffuses into the alveoli and is breathed out.
Answer: haemoglobin in the red blood cells carries oxygen in the moving blood; carbon dioxide returns mostly dissolved in the plasma.
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