Chapter 21. Chapter 21 Graphic Content

The purpose of this graph is to illustrate the relationship between the partial pressure of oxygen and the degree to which hemoglobin is saturated with oxygen. As the partial pressure of oxygen increases, more oxygen binds to hemoglobin. And as the partial pressure of oxygen decreases, oxygen bound to hemoglobin is released.

Red shading indicates a high hemoglobin saturation of oxygen, whereas blue shading indicates low hemoglobin saturation with oxygen. The color at any point along the S-shaped curve reflects the degree to which hemoglobin is saturated with oxygen at that partial pressure of oxygen.

In this graph there are no individual data points included. However, the curve appears to represent the general relationship between the partial pressure of oxygen and the oxygen saturation in hemoglobin. This relationship could be determined only through the measurement of the oxygen saturation in large amounts of hemoglobin across the range of oxygen partial pressure covered by the x-axis. We can assume, therefore, that the graph is based on data points that are not individually shown.

The curve’s S-shape reveals that there is not a linear relationship between the partial pressure of oxygen and hemoglobin’s saturation with oxygen. Rather, as the partial pressure decreases from 100 mm Hg, there is at first a very gradual decrease in the oxygen saturation in hemoglobin. But as the partial pressure is further reduced, to approximately 50 mm Hg, the rate at which oxygen saturation in hemoglobin drops is rapidly increasing. If the S-shaped curve were a straight line, organisms would release oxygen to tissue at a consistent rate as the partial pressure in the tissue decreased. With the S-shaped curve, hemoglobin stays relatively saturated with oxygen, even as the partial pressure is halved. But after that point, the rate at which oxygen is released from hemoglobin rapidly increases. Physical exertion, therefore, is accompanied by a rapid release of more oxygen than would occur if the curve were linear.

At a partial pressure of 60 mm Hg, the oxygen saturation of hemoglobin is approximately 90%. This would not make sense if we were considering just a single molecule of hemoglobin because the only possible states of saturation are 0%, 25%, 50%, 75%, or 100%, corresponding to 0, 1, 2, 3, or 4 oxygen molecules bound to the hemoglobin, which has just four binding sites. However, if numerous molecules of hemoglobin were analyzed for oxygen saturation, it is possible that some are at 75% saturation and others (most others) are at 100% saturation. When the average of these numbers is taken, it could be 90%.

When the curve is the steepest there is the greatest change in oxygen saturation for any change in the partial pressure of oxygen. From this graph, we can estimate this change to be approximately 25 - 45 mm Hg.

When people are at high elevation, the partial pressure of oxygen in the air they breathe is lower than it is at sea level. For this reason, their hemoglobin may not reach full saturation with each breath they take. As a consequence, they may not be able to deliver sufficient oxygen to their tissue, which may lead to shortness of breath and faster breathing. This oxygen deficiency can be exacerbated by the strenuous physical activity of climbing.