Oxyhemoglobin dissociation curve.
This graph shows how changes in the partial pressure of oxygen (PO2) influence oxygen (O2) binding to, and dissociation from, hemoglobin (Hb). PO2 is plotted on the x-axis from 0 to 100 mm Hg (millimeters of mercury). The upper range of 100 mm Hg is chosen because it represents the value of PO2 in the alveoli of the lungs under resting conditions (resting respiratory rates) (red arrow). The PO2 in the lung alveoli drives the PO2 of arterial blood because as blood travels through the pulmonary capillaries surrounding the alveoli, it comes into full equilibrium with the alveoli with respect to PO2 (O2 diffuses out of the alveoli and into the lung capillaries). Thus, the PO2 of blood leaving the lung capillaries is 100 mm Hg. Once this blood returns to the left heart, it is pumped by the left ventricle into the arterial circulation. Therefore, arterial blood has a PO2 of 100 mm Hg, similar in value to the PO2 of lung alveoli. On the other hand, PO2 within metabolically-active tissues is 40 mm Hg (light blue arrow). When arterial blood enters tissue capillaries, it comes into full equilibrium with the tissues with respect to PO2 (O2 diffuses out of the tissue capillaries and into the tissues, where O2 is used in cellular respiration).Thus, as arterial blood passes through the tissue capillaries, it gives up O2 lowering its PO2 to 40 mm Hg. This is why venous blood has a PO2 of 40 mm Hg.
The left y-axis of the graph shows the percent O2 saturation of hemoglobin as a function of PO2. The values range from 0 to 100%. As PO2 is increased, Hb % O2 saturation exhibits a sigmodal relationship, suggesting that there is cooperativity with respect to O2 binding to hemoglobin. At PO2 = 0 mm Hg, Hb % O2 saturation is zero, indicating that no hemoglobin molecule has any O2 bound to any of its four heme groups (i.e., all four O2 binding sites are vacant). At PO2 = 100 mm Hg (arterial blood), Hb % O2 saturation is nearly 100%, indicating that nearly all hemoglobin molecules have all of their four O2 binding sites occupied by O2. Another way of looking at this is that at PO2 = 100 mm Hg, the probability that any given Hb molecule will have a total of four O2 molecules bound is nearly 1. That is to say that there is one O2 molecule bound to each of the O2 binding sites, leading to a fully O2-loaded Hb molecule with a total of four O2 molecules bound. At PO2 = 40 mm Hg (venous blood), approximately 75% of the binding sites are occupied by O2. Another way of looking at this is that in venous blood, on average, every hemoglobin molecule has O2 bound to three of its four binding sites.
It is often useful to refer to the partial pressure of O2 that leads to 50% saturation of Hb. This is referred to as the O2 P50 which, for human hemoglobin, has a value of 28 mm Hg under normal physiological conditions. Several factors influence the value of P50 such as temperature, pH, partial pressure of carbon dioxide (PCO2), etc.
The right y-axis of the graph shows the O2 content of whole blood as a function of PO2. The values range from 0 to 20 mL%. At PO2 = 100 mm Hg, O2 content of whole blood is 20 mL%, which means that 100 mL of whole blood contains 20 mL of O2. Thus, O2 content of arterial blood is 20 mL%. At PO2 = 40 mm Hg, O2 content of whole blood is approximately 15 mL%, which means that 100 mL of whole blood contains 15 mL of O2. Thus, O2 content of venous blood is 15 mL%. It is important to see that while venous blood is often referred to as "deoxygenated blood", it still contains a significant amount of O2 (75% of that in arterial blood).
When examining the O2 content of whole blood, it is clear from the graph that most of the O2 contained within blood is bound to hemoglobin molecules inside red blood cells (erythrocytes). Only a small amount of O2 is simply dissolved in the fluid portion of blood (i.e., plasma). For arterial blood, 19.7 mL% (98.5% of total) is bound to hemoglobin (blue sigmoidal line at 100 mm Hg PO2) and 0.3% (1.5% of total) is dissolved in plasma (gray line at 100 mm Hg PO2).
For a healthy individual at rest and at sea level, the PO2 of blood fluctuates between 100 mm Hg and 40 mm Hg as blood travels through the lung and tissue capillaries, respectively (double-headed arrow). The PO2 of the alveoli determines arterial blood PO2 (100 mm Hg), and the PO2 of tissues determines venous blood PO2 (40 mm Hg). Arterial blood contains 20 mL of O2 for every 100 mL of whole blood (20 mL%), meaning that nearly 100% of the hemoglobin O2 binding sites are occupied by O2. On the other hand, venous blood contains 15 mL of O2 for every 100 mL of whole blood (15 mL%), meaning that approximately 75% of the binding sites are occupied by O2.