Factors That Shift the Oxygen-Hemoglobin Dissociation Curve - Their Importance for Oxygen Transport
The oxygen-hemoglobin dissociation curves of Figures 40–8 and 40–9 are for normal, average blood. However, a number of factors can displace the disso-ciation curve in one direction or the other in the manner shown in Figure 40–10. This figure shows that when the blood becomes slightly acidic, with the pH decreasing from the normal value of 7.4 to 7.2, the oxygen-hemoglobin dissociation curve shifts, on average, about 15 per cent to the right. Conversely, an increase in pH from the normal 7.4 to 7.6 shifts the curve a similar amount to the left.
In addition to pH changes, several other factors are known to shift the curve. Three of these, all of which shift the curve to the right,are (1) increased carbon dioxide concentration, (2) increased blood tempera-ture, and (3) increased 2,3-biphosphoglycerate (BPG), a metabolically important phosphate compound present in the blood in different concentrations under different metabolic conditions.
Increased Delivery of Oxygen to the Tissues When Carbon Dioxide and Hydrogen Ions Shift the Oxygen-Hemoglobin Disso-ciation Curve—The Bohr Effect. A shift of the oxygen-hemoglobin dissociation curve to the right in response to increases in blood carbon dioxide and hydrogen ions has a significant effect by enhancing the release of oxygen from the blood in the tissues and enhancing oxygenation of the blood in the lungs. This is called the Bohr effect, which can be explained as follows: As the blood passes through the tissues, carbon dioxide dif-fuses from the tissue cells into the blood.This increases the blood PO2, which in turn raises the blood H2CO3 (carbonic acid) and the hydrogen ion concentration. These effects shift the oxygen-hemoglobin dissocia-tion curve to the right and downward, as shown in Figure 40–10, forcing oxygen away from the hemoglo-bin and therefore delivering increased amounts of oxygen to the tissues.
Exactly the opposite effects occur in the lungs, where carbon dioxide diffuses from the blood into the alveoli. This reduces the blood PCO2 and decreases the hydrogen ion concentration, shifting the oxygen-hemoglobin dissociation curve to the left and upward. Therefore, the quantity of oxygen that binds with the hemoglobin at any given alveolar PO2 becomes considerably increased, thus allowing greater oxygen transport to the tissues.
Effect of BPG to Shift the Oxygen-Hemoglobin Dissociation Curve. The normal BPG in the blood keeps theoxygen-hemoglobin dissociation curve shifted slightly to the right all the time. In hypoxic conditions that last longer than a few hours, the quantity of BPG in the blood increases considerably, thus shifting the oxygen-hemoglobin dissociation curve even farther to the right. This causes oxygen to be released to the tissues at as much as 10 mm Hg higher tissue oxygen pressure than would be the case without this increased BPG. Therefore, under some conditions, the BPG mechanism can be important for adaptation to hypoxia, especially to hypoxia caused by poor tissue blood flow.
Shift of the Dissociation Curve During Exercise. During exer-cise, several factors shift the dissociation curve con-siderably to the right, thus delivering extra amounts of oxygen to the active, exercising muscle fibers. The exercising muscles, in turn, release large quantities of carbon dioxide; this and several other acids released by the muscles increase the hydrogen ion concentra-tion in the muscle capillary blood. In addition, the tem-perature of the muscle often rises 2° to 3°C, which can increase oxygen delivery to the muscle fibers even more.All these factors act together to shift the oxygen-hemoglobin dissociation curve of the muscle capillaryblood considerably to the right. This right-hand shiftof the curve forces oxygen to be released from the blood hemoglobin to the muscle at PO2 levels as great as 40 mm Hg, even when 70 per cent of the oxygen has already been removed from the hemoglobin. Then, in the lungs, the shift occurs in the opposite direction, allowing the pickup of extra amounts of oxygen from the alveoli.
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