Amount of Oxygen in the Blood

Partial Pressures

When determining the amount of oxygen in the blood, the first important point to establish is the partial pressure. This term is easy to grasp when used with a gas mixture, such as that in an alveolus. But what does it mean when applied to blood? When this term is used with blood, it refers to the gas with which the blood is in equilibrium.

As we have discussed in class, in a healthy person, there is plenty of time for equilibration between the gas in an alveolus and the blood flowing through a pulmonary capillary. Even at the peak of exercise, the blood spends enough time in a pulmonary capillary to come to equilibrium with the gas in the alveolus (except rarely in exceptional athletes with powerful hearts). Thus, in healthy people the partial pressures of oxygen and carbon dioxide in systemic arterial blood are the same as those in the alveoli. (But of course this breaks down in certain respiratory disorders, such as those that cause fluid accumulation in the alveoli.)

But while the partial pressures alone determine the direction a gas will diffuse, additional factors determine the amount of a gas in the blood. Oxygen, for example, is found in two forms in the blood:

The figure to the right shows these two forms.

Oxygen in Solution

Oxygen in solution is easy to calculate because it is directly proportional to the partial pressure. Thus, to determine the amount of oxygen in solution, one only has to multiply the partial pressure times the solubility, which at 37 degrees is equal to 0.03 ml O2/(liter blood x mm Hg).

amount O2 in solution = 0.03 ml 02/(liter blood x mm Hg) X Partial Pressure

Since the partial pressure of oxygen at sea level in normal systemic arterial blood is 100 mm Hg, the amount in solution is equal to 3 ml O2/liter blood.

But if one takes a sample of systemic arterial blood and determines the amount of oxygen present, one finds that the amount is actually about 200 ml O2/ liter blood. Obviously, hemoglobin accounts for very nearly all of the oxygen in the blood. This fact has many physiological ramifications.

Oxygen Bound to Hemoglobin

To determine the amount of oxygen bound to hemoglobin, it is first necessary to determine the percent saturation of the hemoglobin. This is done using an oxygen dissociation curve for hemoglobin, such as is found on your handout. This curve was determined experimentally by equilibrating hemoglobin with various partial pressures of oxygen.

Look at your handout and observe that the percent saturation at 100 mm Hg is 97.5%. Now all we need is the amount of oxygen hemoglobin holds when completely saturated. The constant here is 1.39 ml O2/ gram hemoglobin. Assume that is a person's blood has 150 grams of hemoglobin per liter, and verify that the amount bound to hemoglobin in systemic arterial blood comes out to be 203.3 ml per liter of blood.

Total Amount of Oxygen

Thus, the total amount of oxygen in this person's systemic arterial blood is 206.3 ml O2/liter blood, since 3 ml O2/liter blood are in solution and 203.3 ml O2/liter blood are bound to hemoglobin.

Factors Affecting the Binding of Oxygen to Hemoglobin

A number of substances bind to hemoglobin and alter the relative affinity of hemoglobin for oxygen. Notably, carbon dioxide, H+, and DPG (same as BPG) all bind to the protein portion of hemoglobin and shift the curve to the right. Thus, do these substances increase or decrease the affinity of hemoglobin for oxygen? Also, how is the curve for fetal hemoglobin different from the adult and why is this helpful?

Carbon Dioxide in the Blood

The amount of carbon dioxide in the blood depends on three factors:

The latter reaction is facilitated by carbonic anhydrase, which is found in red blood cells. Beyond this, however, we will not be attempting to calculate the actual amounts of carbon dioxide in solution.

Quick Quiz

QUESTION: What is the percent saturation of hemoglobin if the partial pressure of oxygen is 40 mm Hg? (Determine using curve on handout, ignoring any effect of carbon dioxide on this curve)

QUESTION: Does the affinity of hemoglobin for oxygen increase or decrease if the carbon dioxide increases?

QUESTION: Why would such a changed affinity be helpful?

QUESTION: How does the oxygen dissociation curve for fetal hemoglobin differ from that for adult hemoglobin?

QUESTION: Why would this be helpful?