The Sponge-like Oxygen-Binding Action of Hemoglobin 

Oxygen Binding by Hemoglobin is Gauged by the Saturation Fraction, Ya

Move the mouse pointer over this image "squeezing" oxygen out of hemoglobin or "soaking" oxygen up as the  interconversion between oxyHbA and deoxyHbA takes place.

• By this sponge analogy, Ya is a measure of how much oxygen the Hb "sponge" has bound to it.  Since O₂ binding is a dynamic process, Hb is an "oxygen sponge" in that it continuously is either being soaked with oxygen (in the oxygen-rich environment of the lungs) or "squeezed" forcing it to release of some or all of its bound oxygen (in oxygen starved capillaries that penetrate oxygen-consuming tissues).  In other words, Hb is being forced to "expand" or "contract" between its oxy- and deoxy-conformations and in the process it binds or releases oxygen.  Just as a sponge can can hold only so much water, depending on its size and elasticity, Hb can bind only so much oxygen, 4 O₂ molecules for each Hb molecule to be exact.  

• The binding of oxygen to 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:  one conformer, deoxyHb, has relatively low affinity for oxygen and the other conformer, oxyHb, has a hundred-fold higher affinity for oxygen.  In effect, Hb is like an "oxygen sponge."

• As illustrated by the saturation plot above, the amount of oxygen bound to Hb, i.e., the saturation fraction, Ya, varies as a function of the partial pressure of oxygen gas, pO₂, in the hemoglobin's immediate environment. 

• 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 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.