Gases (oxygen, nitrogen and carbon dioxide) flow from high concentrations to lower concentrations. Oxygen deplete blood takes up (higher concentrated) oxygen from the lungs while giving up (higher concentrated) carbon dioxide from the blood to the lungs. The red blood cells are capable of carrying oxygen and carbon dioxide but must pass through tiny capillaries (one red blood cell wide) in the lungs and tissues to efficiently exchange gas. Blood also carries nitrogen in proportion to its pressure although not used up in metabolism. If a deep sea diver surfaces too quickly from a great depth, the dissolved nitrogen comes out of solution as bubbles (like opening a can of soda) causing great pain (the bends) and possibly death. Blood serum also carries some oxygen (perhaps 16% vs 21% in the atmosphere) and about 10% of needed oxygen can enter the body through skin (the largest body organ).
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