recap

48.4 recap

O2 is transported from the lungs to the body’s tissues in reversible combination with hemoglobin. Each hemoglobin molecule can reversibly combine with four O2 molecules; the percent saturation of the binding sites is a function of the PO2 in the hemoglobin’s environment. The hemoglobin–oxygen binding/dissociation curve is shifted by the concentration of H+ and the level of 2,3-BPG in the blood. CO2 is transported by the blood mostly in the form of HCO3–.

learning outcomes

You should be able to:

  • Explain the adaptive advantage of hemoglobin remaining 75 percent saturated with O2 in mixed venous blood.

  • Explain how 2,3-BPG and pH alter the O2 binding properties of hemoglobin, and discuss the functional significance of these changes.

  • Explain how the same enzyme, carbonic anhydrase, catalyzes both the loading of CO2 from cells to the blood and the off-loading of CO2 from the blood to the alveoli.

Question 1

The PO2 of atmospheric air at sea level is about 159 mm Hg. What is the functional significance of the fact that human hemoglobin fully saturates with O2 at a PO2 of 100 mm Hg and returns to the heart in venous blood with a PO2 of about 40 mm Hg, reflecting 75 percent saturation?

Because of mixing with dead-space air, the maximum PO2 in the alveoli is about 100 mm Hg, and hemoglobin can fully saturate at that PO2. The fact that mixed venous blood normally returns to the heart 75 percent saturated means that if the blood flows through any tissue that is very active metabolically, it has a reserve of O2 that it can off-load to satisfy that local high demand.

Question 2

When red blood cells are stored in the blood bank, they will eventually use their 1,3-BPG for energy. What will be the consequence for their ability to function after transfusion into a patient?

When the concentration of 1,3-BPG in stored red blood cells decreases, so does the concentration of 2,3-BPG that can be generated by conversion from 1,3-BPG. As a result, the hemoglobin-O2 binding/dissociation curve shifts to the left. Therefore the O2 is more tightly bound to the hemoglobin and is not released for use by the respiring cells.

Question 3

Why do HCO3– ions leave the red blood cells in systemic venous blood but enter red blood cells in the alveolar circulation?

The conversion of CO2 to H2CO3 and then to HCO3- is a reversible reaction depending on the concentrations of reactants and products. In the respiring tissues the the PCO2 is high and drives the reaction in the red blood cells toward HCO3-, which moves into the blood plasma in exchange for Cl-. In the alveoli, the PCO2 is low, so this entire suite of reactions is driven in reverse, meaning that the diffusion of CO2 out of the red blood cells causes HCO3- to be moved back into the red blood cells to be converted back to H2CO3 and then to CO2.

We must breathe every minute of our lives, but most of us usually don’t worry about it, or even think about it very often. In the next section we will examine how the regular breathing cycle is generated and controlled by the central nervous system.