Page 901 - Fundamentals of anatomy physiology
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888 Unit 5 Environmental Exchange
the molecules release their oxygen reserves more readily, Hemoglobin and Temperature
so the slope of the hemoglobin saturation curve changes
(Figure 23–20a). In other words, as pH drops, the saturation At a given PCO2, hemoglobin releases additional oxygen if the
declines. At a tissue PO2 of 40 mm Hg, for example, you can see temperature increases. Changes in temperature affect the slope
in Figure 23–20a that a pH decrease from 7.4 to 7.2 changes of the hemoglobin saturation curve (Figure 23–20b). As the
hemoglobin saturation from 75 percent to 60 percent. This temperature increases, hemoglobin releases more oxygen. As the
means that hemoglobin molecules release 20 percent more temperature decreases, hemoglobin holds oxygen more tightly.
oxygen in peripheral tissues at a pH of 7.2 than they do at Temperature effects are significant only in active tissues that are
a pH of 7.4. This effect of pH on the hemoglobin saturation generating large amounts of heat. For example, active skeletal
curve is called the Bohr effect. muscles generate heat, and the heat warms blood that flows
through these organs. As the blood warms, the Hb molecules re-
Carbon dioxide is the primary compound responsible for lease more oxygen than can be used by the active muscle fibers.
the Bohr effect. When CO2 diffuses into the blood, it rapidly
diffuses into red blood cells. There, an enzyme called carbonic Hemoglobin and BPG
anhydrase catalyzes the reaction of CO2 with water molecules:
Red blood cells lack mitochondria. These cells produce ATP
CO2 + carbonic anhydrase ÷ H+ + HCO3− only by glycolysis. As a result, lactic acid is formed, as we saw
in Chapter 10. p. 344 The metabolic pathways involved in
H2O ÷ H2CO3 glycolysis in an RBC also generate the compound 2,3-bisphos-
phoglycerate (biz-fos-fo. -GLIS-er-a.t), or BPG. This compound
The product of this enzymatic reaction, H2CO3, is called has a direct effect on oxygen binding and release. For any partial
carbonic acid, because it dissociates into a hydrogen ion (H1) pressure of oxygen, the higher the concentration of BPG, the
and a bicarbonate ion (HCO3−). The rate of carbonic acid for- greater the release of oxygen by Hb molecules. Normal RBCs
mation depends on the amount of carbon dioxide in solution, always contain BPG.
which, as noted earlier, depends on the PCO2. When the PCO2 Both BPG synthesis and the Bohr effect improve oxygen deliv-
rises, the rate of carbonic acid formation accelerates and the ery when the pH changes: BPG levels rise when the pH increases,
and the Bohr effect appears when the pH decreases. The concentra-
reaction proceeds from left to right. The hydrogen ions that are tion of BPG can be increased by high blood pH, thyroid hormones,
growth hormone, epinephrine, and androgens. When plasma PO2
generated diffuse out of the RBCs, and the pH of the plasma
drops. When the PCO2 declines, hydrogen ions diffuse out of
the plasma and into the RBCs. As a result, the pH of the plasma
increases as the reaction proceeds from right to left.
Figure 23–20 The Effects of pH and Temperature on Hemoglobin Saturation. 10°C 20°C
100 100
80 80 38°C
60 60 43°C
7.6 7.4
Oxyhemoglobin (% saturation)7.2
Oxyhemoglobin (% saturation)
23
40 40
Normal blood pH range Normal blood temperature
20 7.35–7.45 20 38°C
0 20 40 60 80 100 0 20 40 60 80 100
PO2 (mm Hg) PO2 (mm Hg)
a Effect of pH. When the pH decreases below normal levels, b Effect of temperature. When the temperature increases,
more oxygen is released; the oxygen–hemoglobin more oxygen is released; the oxygen–hemoglobin
saturation curve shifts to the right. When the pH increases, saturation curve shifts to the right.
less oxygen is released; the curve shifts to the left.
