Move the mouse pointer over the image to switch between the oxyHbA and deoxyHbA conformations of adult hemoglobin A their respective oxygen saturation levels in arterial (red) or venous (blue) blood.

Oxygen binding by Hb and its release are accompanied by noticeable conformational changes in the structure of Hb. 

As Hb is transported in red blood cells between the lungs and the capillaries, continuously oscillates between two (and probably more) conformational extremes.

In the "deoxyHb" conformation, which predominates in venous blood, hemoglobin's oxygen affinity is relatively low.

 In the "oxyHb" conformation, which predominates in arterial blood, hemoglobin's oxygen affinity is relatively high.

In effect, Hb is like an "oxygen sponge" when it oscillates between these conformation extremes.

As illustrated by the chart, the amount of oxygen bound to Hb, i.e., the saturation fraction, Ya, is measured a function of the pressure of oxygen gas in the immediate environment, pO₂.  

In arterial blood leaving the lungs, Hb has up to four oxygen molecules bound being nearly 100% saturated in this environment.  A high percentage of oxygen-saturated oxyHb molecules gives arterial blood its slightly pinkish coloration.  

In the capillaries or venous blood, Hb releases some its bound oxygen molecules, becoming desaturated of oxygen in this environment.  A high percentage of oxygen-depleted deoxyHb molecules gives venous blood  it is slightly bluish coloration.