Oxygen dissociation curve.

 Oxygen is reversibly bound to hemoglobin in red blood cells. Each molecule of hemoglobin can carry a maximum of four molecules of O2. Because of positive cooperativity, the affinityof hemoglobin for O2 depends on the PO2 to which the hemoglobin is exposed. Therefore, hemoglobin picks up O2 as it flows
through respiratory exchange structures and gives up O2 in metabolically active tissues.As O2 diffuses into the red blood cells, it binds to hemoglobin.
Once O2 is bound, it cannot diffuse back across the red cell plasma membrane. By binding O2 molecules as they enter the red blood cells, hemoglobin maximizes the partial pressure gradient driving the diffusion of O2 into the cells. In addition, it enables the red blood cells to carry a large amount of O2 to the tissues of the body.
The ability of hemoglobin to pick up or release O2 depends on the Partial pressure of O2 of its environment. When the PO2 of the blood plasma is high, as it usually is in the lung capillaries, each molecule of hemoglobin can carry its maximum load of four molecules of O2. As the blood circulates through the rest of the body, it encounters lowerPO2 values. At these lower PO2 values, the hemoglobinreleases some of the O2 it is carrying The relation between PO2 and the amount of O2 bound to hemoglobin is not linear, but S-shaped (sigmoidal).

• The average PO2 of deoxygenated blood returning to the heart is 40 mm Hg.
• The PO2 of blood leaving the lungs is about 100 mm Hg.
• 25% of the O2 in arterial blood is released to tissues during rest or light exercise
• An oxygen reserve of 75% is held by the hemoglobin and can be released to tissues with a low PO2

The sigmoidal hemoglobin–O2 binding curve reflects interactions between the four subunits of the hemoglobin molecule. At low PO2 values, only one subunit will bind an O2 molecule. When it does so, the shape of that subunit changes, causing an alteration in the quaternary structure of the whole
hemoglobin molecule. That structural change makes it easier for the other subunits to bind a molecule of O2;
that is, their O2 affinity is increased. Therefore, a smaller increase in PO2 is necessary to get most of the hemoglobin molecules to bind two O2 molecules (that is, to become 50% saturated) than was necessary to get them to bind one O2 molecule (to become 25% saturated). This influence of the binding of O2 by one subunit on the O2 affinity of the other subunits is called positive cooperativity.
Once the third molecule of O2 is bound, the relationship seems to change, as a larger increase in PO2 is required for the hemoglobin to reach 100 percent saturation. This upper bend of the sigmoid curve is due to a probability phenomenon: The closer we get to having all subunits occupied, the less likely it is that any particular O2 molecule will find a place to bind. Therefore, it takes a relatively greater PO2 to achieve 100
percent saturation. The O2-binding properties of hemoglobin help get O2 to the tissues that need it most. In the lungs, where the PO2 isabout 100 mm Hg, hemoglobin is 100 percent saturated. The PO2 in blood returning to the heart from the body is usually about 40 mm Hg. At this PO2, the hemoglobin is still about 75 percent saturated. This means that as the blood circulates around the body, only about one in four of the O2 molecules it carries is released to the tissues. This system seems inefficient, but it is really quite adaptive, because the hemoglobin keeps 75 percent of its O2 in reserve to meet peak demands. When a tissue becomes starved of O2 and its local PO2 falls below 40 mm Hg, the hemoglobin flowing through that tissue
is on the steep portion of its sigmoid binding curve. That means that relatively small decreases in PO2 below 40 mm Hg will result in the release of lots of O2 to the tissue. Thus the positive cooperativity of O2 binding by hemoglobin is very effective in making O2 available to the tissues precisely when and where it is needed most.

 This curve  is highly useful in studying the effect of factors like pCO2, H+ concentration, etc., on binding of O2 with haemoglobin. In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration
and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO2, high pCO2, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. This clearly indicates that O2 gets bound to
haemoglobin in the lung surface and gets dissociated at the tissues.