Hemoglobin’s affinity for O2 is variable

Various factors influence the O2-binding/dissociation properties of hemoglobin, thereby influencing O2 delivery to tissues. Three of these factors are the chemical composition of the hemoglobin, the blood pH, and the presence of 2,3-bisphosphoglycerate (BPG) in RBCs.

HEMOGLOBIN COMPOSITION There is more than one type of hemoglobin because the chemical composition of the polypeptide chains that form the hemoglobin molecule varies. The normal hemoglobin of adult humans has two each of two kinds of polypeptide chains—two α-globin chains and two β-globin chains. This normal adult hemoglobin has the O2-binding characteristics shown in Figure 48.13.

Before birth, the human fetus has a different form of hemoglobin, consisting of two α-globin and two γ-globin chains. The functional difference between fetal and adult hemoglobin is that fetal hemoglobin has a higher affinity for O2. Therefore the fetal hemoglobin–oxygen binding/dissociation curve is shifted to the left compared with the adult curve (see Figure 48.13). You can see from these curves that if both types of hemoglobin are at the same PO2 (as they are in the placenta), fetal hemoglobin will pick up O2 that the adult hemoglobin releases. This difference in O2 affinities enables the efficient transfer of O2 from the mother’s blood to the fetus’s blood.

HEMOGLOBIN AND pH The O2-binding properties of hemoglobin are also influenced by physiological conditions. The influence of pH on the function of hemoglobin is known as the Bohr effect. As blood passes through metabolically active tissue such as exercising muscle, it picks up acidic metabolites. As a result, blood pH falls. The excess H+ ions bind preferentially to deoxygenated hemoglobin and decrease its affinity for O2, and the O2-binding/dissociation curve of hemoglobin shifts to the right (see Figure 48.13). This shift means the hemoglobin will release more O2 in tissues where pH is low—another way that O2 is supplied where and when it is most needed.

2,3-BISPHOSPHOGLYCERATE 1,3-BPG is an intermediate in glycolosis (see p. 177), and therefore an important source of energy for cells. However, 1,3-BPG can be converted to 2,3-BPG by an enzyme in the RBC. 2,3-BPG is an important regulator of hemoglobin function. Like excess H+, 2,3-BPG reversibly combines with deoxygenated hemoglobin and lowers its affinity for O2. The result is that at any PO2, hemoglobin releases more of its bound O2 than it otherwise would. In other words, 2,3-BPG shifts the O2-binding/dissociation curve of mammalian hemoglobin to the right.

When humans go to high altitudes, or when they cease being sedentary and begin to exercise, the level of 2,3-BPG in their RBCs goes up, making it easier for hemoglobin to deliver more O2 to tissues. During pregnancy, a woman has about a 30 percent increase in 2,3-BPG, which makes more O2 available in the placenta to be picked up by the fetal hemoglobin. In addition, fetal hemoglobin has a left-shifted O2-binding/dissociation curve because its γ-globin chains have a lower affinity for 2,3-BPG than do the β-globin chains of adult hemoglobin.

1037

investigating life

Seals Are Champion Breath-Hold Divers

experiment

Original Paper: Castellini, M. A., G. L. Kooyman and P. J. Ponganis. 1992. Metabolic rates of freely diving Weddell seals: Correlations with oxygen stores, swim velocity, and diving duration. Journal of Experimental Biology 165: 181–194.

Weddell seals live near the edges of the ice in Antarctica where they feed by diving in the surrounding ocean. The ice-covered ocean presents a unique opportunity to study the diving and breathing behaviors of these seals if they have to return to the same breathing hole in the ice.

image

work with the data

How do the seal’s oxygen stores compare with its oxygen demands? To calculate the oxygen reserves of a Weddell seal, we have to assess the amount of oxygen in its blood and in the myoglobin in its muscles at the beginning of the dive. The amount of oxygen in its lungs is irrelevant since seals exhale before diving, so there is little oxygen available in its alveoli during the dive.

The problem requires calculating the amount of O2 in the arterial and venous blood and myoglobin at the beginning of the dive that is available to support the seal’s MR.

Assume the following:

  • Average body mass: 355 kg

  • Blood volume: 14.8% of mass (7% in humans)

  • Hematocrit: 58% (human averages 42%)

  • Hemoglobin content of red blood cells: 23.7%

  • O2 holding capacity of hemoglobin: 1.34 mL/g

  • Blood: 34% is arterial, 95% saturated at beginning of dive, can desaturate to 20%

Available O2 in arterial blood is calculated as follows:

Total arterial blood = 355 kg × 0.148 × 0.34 = 17.9 kg

Total arterial hemoglobin = 17.9 kg × 0.58 × 0.24 = 2.49 kg

Maximum O2 content = 3.34 L

Arterial O2 available = 3.34 × (95% – 20%) = 2.5 L

QUESTIONS

Question 1

Calculate the available O2 in the venous blood. Assume venous blood is 66% of total blood volume, and it is 90% saturated at the beginning of the dive and can desaturate to 0%.

O2 available in venous blood:
Total venous blood = 355 kg × 0.148 × 0.66 = 34.7 kg
Total venous hemoglobin = 34.7 kg × 0.58 × 0.24 = 4.8 kg
Total venous O2 available = 4.8 kg × 1.34 L/kg × 0 .9 = 5.8 L

Question 2

Calculate the available O2 in myoglobin. Assume 33% of body mass is muscle, myoglobin content of seal muscle is 45 g/kg (20–25 g/kg for humans), and O2 holding capacity of myoglobin is 1.34 mL O2/g.

O2 available in myoglobin:
Total amount of myoglobin: 355 kg × 0.33 × 44g/kg = 5.2 kg
O2 bound to myoglobin: 5.2 kg × 1.34 L/kg = 7 L

Question 3

What are the total O2 reserves at the beginning of the dive?

Total O2 reserves at beginning of dive:
2.5 L + 5.8 L + 7.0 L = 15.3 L

Question 4

Given the MR values found in the experiment, what do you predict would be the maximum duration of no breathing during sleep?

Maximum sleep episode = total O2 reserves divided by the diving metabolic rate. So, 15.3 L divided by the rate of O2 consumption which for a long dive would be 0.0035 L/kg min × 355 kg. So, 15.3L/ 1.24 L/min = 12.3 min, which corresponds well with the observed maximum sleep episode of 13.5 min.

Question 5

Using the table of MR values for all dives in the results portion of the experiment, predict the maximum duration of a Weddell seal dive.

Assuming the overall dive metabolic rate of 4.5 mL O2/min kg, the maximum dive time would be 15.31/(0.0045 mL O2/min × 355 kg) = 10 min, which is much less than the observed dive times.

A similar work with the data exercise may be assigned in LaunchPad.