Why oxygen does diffuses into the blood capillaries in the lungs, but out of the blood capillaries in the tissues.

Dear Student

In lungs, partial pressure of oxygen is higher in the alveoli of lungs than in capillaries. So, oxygen diffuses down the pressure gradient (oxygen moves out of the alveoli into the capillaries). In capillaries oxygen binds to haemoglobin. In tissues, the partial pressure of oxygen is less and its is higher in capillaries so oxygen diffuses from blood capillaries into the tissues.

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