PaO2, O2 content and O2 saturation The haemoglobin dissociation curve Myoglobin 2,3-Diphosphogylerate (2,3-DPG)CyanosisShifting the dissociation curve

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Although O2 is dissolved in plasma, the vast majority of O2 transport is carrier-mediated by haemoglobin (Hb), a process that can become saturated. Haemoglobin is close to 100% saturated at an arterial O2 partial pressure (PaO2) of 10 kPa or more when O2 content (total O2 in blood) is around 200 mL/L. In anaemia, PaO2 and Hb saturation can be normal, but the O2 content of blood will be reduced, explaining many of the symptoms. Understanding the relationships between PaO2, O2 content and Hb saturation is important when interpreting disturbed O2 transport.

PaO2, O2 content and O2 saturation

The partial pressure of O2 in blood (PaO2) indicates the amount of blood dissolved in plasma. If you left plasma (i.e. having removed blood cells) in an open vessel it would equilibrate with room air and have a PO2 of 21 kPa. How much O2 is this though?


That’s about 0.5 mL dissolved in each 100 mL of plasma (O2 content).   However, blood equilibrates with alveolar air, which has a lower fraction of O2 (PAO2 = 14 kPa usually) and this is reflected in the lower PaO2 of around 12-14 kPa.  So, the amount usually dissolved in plasma is about 0.3 mL O2/100 mL.  By contrast, the usual O2 content of whole blood is around 19 mL/100 mL (see below).  This huge difference illustrates the importance of the haemoglobin (Hb) in red blood cells as a carrier for oxygen transport.

The relationship between PaO2 and O2 content is fairly predictable, if the number of red cells and their content of haemoglobin are in the usual range (see below).  So, a PaO2 reading tells us what the O2 content should be under normal circumstances, but if something goes awry, the PaO2 can look normal when everything else doesn’t.  Anaemia is the classical example of this, where a given volume of blood doesn’t contain the usual number of red cells, and hence haemoglobin.

There should be about 14 g of Hb per 100 mL of blood (this is a number worth remembering and it’s a bit lower in females than males) and each gram binds 1.34 ml of O2.  What would happen if this level was halved?  We’ll assume that the usual 0.3 mL of dissolved O2 is present, and that there’s adequate ventilation to saturate Hb:


In this example the PaO2 and O2 saturation could be identical in these patients; only the O2 content will differ.

If we took a venous (deoxygenated) blood sample and bubbled pure O2 though it we would soon occupy all the sites for O2 and the haemoglobin would become saturated (14 x 1.34 = 18.8 mL per 100 mL).  If we measured the percentage Hb saturation (%), PaO2 (kPa) and O2 content (mL/L) of sample during this procedure we would find that different things happen to each variable, as the Hb dissociation curve is not a simple, straight line.  We’d also find that pure oxygen isn’t required for this experiment.  In fact there’s more than enough O2 in room air (21% O2) to saturate Hb; even 10% O2 would suffice.


The oxyhaemoglobin dissociation curve

Oxygen binds to haemoglobin (to form oxyhaemoglobin) and unbinds from it (to form deoxyhaemoglobin) depending on the PO2 of plasma.  The haemoglobin (Hb) dissociation curve has two features that are of clinical relevance.  Firstly, the curve is close to linear from a PaO2 value of 10 kPa or more, meaning that in the typical arterial range (10-13 kPa), haemoglobin is close to fully saturated.  Secondly, between a typical partial pressure of O2 in venous blood (PvO2) of 5 kPa and the lower end of the PaO2 the curve shows that a large drop in O2 saturation occurs over a narrow PaO2 range, allowing O2 to be delivered to tissues.  As blood enters a tissue with low PO2, Hb gives up O2 more readily.

oxyhaemoglobin dissociation

Figure 1: The oxygen-Hb dissociation curve, describing the O2 content of normal blood. A typical PvO2 and the normal range for PaO2 are shown. Note that for a wide range of PaO2, Hb is almost completely saturated. Below 10 kPa, Hb gives up O2 more readily to supply O2 to the tissues.

The shape of the dissociation curve is the product of the complex way that O2 binds to Hb.  Each molecule of Hb can bind four molecules of O2, but the kinetics of each binding event differ.  Binding of the first O2 molecule is the hardest step; subsequently each O2 molecule binds more readily.  The point is that four different chemical reactions account for O2 binding to Hb.



Myoglobin is an intracellular protein found in muscle cells and is the protein that gives meat its red colour.  It is an intracellular storage depot for O2. If you place a breast of chicken next to the same cut of meat from a duck, the former will be pale pink, while the latter is a deep red.  This difference reflects the different way this muscle is used by these related animals.  A duck flies and requires very high performance muscles to do so, a chicken doesn’t.  Myoglobin doesn’t release O2 into the blood, since it has a higher affinity for O2 than Hb.  Furthermore, the PO2 in mitochondria is much lower than blood (0.4 kPa), so that when myoglobin does give up its O2, the concentration gradient favours delivery to mitochondria rather than blood.


2,3-Diphosphogylerate (2,3-DPG)

Red blood cells contain no mitochondria (leaving room for more haemoglobin) and rely on anaerobic metabolism. Hence, red blood cells do not consume the O2 that they transport.  2-3-diphosphoglycerate is a product of such metabolism and has an important effect on the haemoglobin dissociation curve (see below). Chronic hypoxia (such as may be caused by moving to high altitude or a chronic respiratory diseases such as COPD) increase the concentration of 2,3-DPG in red blood cells. 



The colour of blood through skin appears purple/blue when it is deoxygenated as deoxy-Hb has a different colour spectrum.  In fact, venous blood is just a darker red colour – the blue/purple phenomenon is due to light scattering in the skin.  Once blood contains about 5 g/100 mL deoxy-Hb the colour difference is noticeable.

Shifting the dissociation curve

The oxygen dissociation curve is influenced by temperature, pH, 2,3-DPG levels and PaO2. By shifting the curve, what we really mean is changing the affinity of Hb for O2. It’s pretty easy to remember how the location of the curve is affected by remembering that the curve shifts to meet increased oxygen delivery to organs with increased metabolism. It’s physiology that makes sense. Consider the heart during exercise: as its rate increases the myocardium will produce more CO2 (and hence increase H+, decreasing pH). This shifts the curve to the right (this effect on the oxygen dissociation curve is called the Bohr effect) so that Hb gives up O2 more readily. Similarly, the myocardium will become warmer as it converts more energy into contractions. This increase in temperature will also shift the curve to the right. Finally, increased temperature will increase red blood cell metabolism, causing an increase in 2,3-DPG, which also causes a rightward shift of the curve. As all of these changes result in the oxygen dissociation curve shifting to the right, at a given and PaO2, Hb more readily releases oxygen for the myocardium to utilise (Figure 2).

effect oxyhaemoglobin curve

Figure 2: The effect of changes in pH, 2,3-DPG concentrations and temperature on the oxyhaemoglobin dissociation curve. In tissues where metabolism is high, Hb will release more O2 than usual as the curve is shifted to the right. The opposite changes in these variables will cause a leftward shift, meaning that for a given PaO2, Hb retains more O2 than usual.

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