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All cells need oxygen. It is the essential fuel which is necessary to enable cells to stay alive and to carry out their various activities.

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Bringing oxygen to the cells requires the uptake of oxygen from the air in the lungs, its transportation in the blood, and its delivery to cells all over the body.

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The first step is the taking up of oxygen by blood flowing through fine capillaries in the walls of the lungs' air sacs, or alveoli.

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The oxygen molecules change from their state as a gas,

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freely circulating in the air,

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dissolving into a solution in the plasma within the capillaries of the alveoli.

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Once in the solution of the blood,

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98% of this dissolved oxygen is taken up by passing red cells,

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leaving just 2% remaining in the physical solution unattached.

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Red cells are particularly well-suited to transporting oxygen

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because they contain a special oxygen-binding protein known as hemoglobin.

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Each molecule of hemoglobin itself contains four molecules of heme,

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an iron-containing pigment which binds oxygen loosely and reversibly.

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Hemoglobin that is fully saturated with oxygen is bright red and is called oxyhemoglobin.

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On the other hand, hemoglobin that is not saturated with oxygen is purplish-blue in color

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and is called deoxyhemoglobin.

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It is heme which makes it possible for the red cells to pick up oxygen dissolved in the blood,

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transport it, combined with hemoglobin, and release it back into the blood as oxygen in solution,

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ready for delivery to the various cells of the body.

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Hemoglobin gives up its oxygen as red cells travel through capillaries in tissues

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where there is a low content or partial pressure of oxygen.

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The partial pressure of oxygen represents the level of dissolved oxygen in plasma.

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As oxygen is released and again is carried in solution, the partial pressure of oxygen in the capillaries becomes greater than the partial pressure of oxygen in the surrounding tissues.

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This causes oxygen to move out of the capillaries into the tissues and to finally reach the cells.

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This graph, the oxygen dissociation curve of hemoglobin, shows why hemoglobin is particularly suited to its role in transporting oxygen.

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The oxygen dissociation curve demonstrates the relationship between the oxygen carried in combination with hemoglobin,

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the O2 saturation, and the partial pressure of oxygen in the arterial blood.

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The sharp upstroke and the flat plateau illustrate how oxygen is released to the tissues over a wide range of conditions.

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Its shape means that although the partial pressure of oxygen in the blood returning from the lungs

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and being pumped out by the arteries may be reduced to only 50% of the normal value,

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say due to lung disease or high altitude, hemoglobin will still be 85% saturated with oxygen.