Carrying Oxygen and Carbon Dioxide

A litre of your arterial blood carries about 200 mL of oxygen. If that blood were plasma alone, it would carry about 3 mL.

Oxygen dissolves poorly in water, and plasma is mostly water. This lesson is about how blood makes up the difference: how a carrier picks up oxygen in the lungs and releases it in tissues, what makes it release more, how different animals solve the same problem, and how carbon dioxide gets back the other way. It builds on gas exchange, where oxygen crosses into the blood, and on blood and vessels and the heart, which move it.

The idea: a carrier with an S-shaped curve

Gases move by diffusion from where their partial pressure is high to where it is low. Partial pressure is the share of a gas mixture’s pressure due to one gas, written PO2 for oxygen and measured here in mmHg (1 Torr is essentially the same unit). In an adult human at rest, arterial PO2 is about 100 mmHg and the PO2 of blood returning from the body is about 40 mmHg.

The amount of oxygen that dissolves is proportional to PO2: about 0.003 mL per dL of blood for each mmHg. At 100 mmHg that is 0.3 mL per dL, far too little. In mammals, almost all the oxygen rides on hemoglobin, an iron-containing protein packed inside red blood cells. Each human hemoglobin molecule has four chains, two alpha and two beta in the adult form, and four oxygen-binding sites.

The saturation of hemoglobin is the fraction of its binding sites that hold oxygen. Plot saturation against PO2 and you get the oxygen dissociation curve. For adult human blood it is S-shaped, or sigmoid. Above about 60 mmHg it is nearly flat: blood in the lungs loads almost fully, and a modest fall in lung PO2 costs little. Between about 10 and 40 mmHg it is steep: a small fall in tissue PO2 releases a large amount of oxygen.

The P50 is the PO2 at which hemoglobin is half saturated. It measures affinity, how tightly the carrier holds oxygen, in reverse: a higher P50 means lower affinity. In a standard model of human blood at 37 degrees C, pH 7.4 and a carbon dioxide pressure of 40 mmHg, P50 is 26.8 mmHg.

The S shape comes from cooperative binding: oxygen binding at one site makes the remaining sites of the same molecule bind more readily. How the four chains pass that change between them is told in protein binding and allostery. Think of a bus that fills slowly while it is empty and quickly once a few passengers are on, and empties the same way.

Worked problem: how much oxygen does the body use? Take an illustrative adult human at rest. Assume hemoglobin 15 g per dL, 1.34 mL of oxygen per g of fully saturated hemoglobin (a common working value; the theoretical figure is near 1.39), dissolved oxygen 0.003 mL per dL per mmHg, a steady state and a cardiac output of 5.0 L per min.

oxygen content = (1.34 x hemoglobin x saturation) + (0.003 x PO2) oxygen use = cardiac output x (arterial content - venous content)

1. Arterial blood (saturation 98%, PO2 100 mmHg): bound = 1.34 x 15 x 0.98 = 19.70 mL per dL; dissolved = 0.003 x 100 = 0.30 mL per dL; total 20.00 mL per dL.
2. Mixed venous blood (saturation 75%, PO2 40 mmHg): bound = 1.34 x 15 x 0.75 = 15.08 mL per dL; dissolved = 0.12 mL per dL; total 15.20 mL per dL.
3. Difference: 20.00 - 15.20 = 4.80 mL per dL. The tissues took 4.80 / 20.00 = 24% of the oxygen offered.
4. Fick principle: 5.0 L per min is 50 dL per min. Delivery is 50 x 20.00 = 1,000 mL per min; use is 50 x 4.80 = 240 mL per min.

Only 0.30 of 20.00 mL per dL, 1.5%, is dissolved. Notice too that three-quarters of the oxygen comes back unused. That venous reserve sits on the steep part of the curve, so tissues can draw on it in exercise, a point taken up in muscle and movement.

Shifting the curve: Bohr’s dog blood, temperature and BPG

In Copenhagen in 1903, Christian Bohr, working with Karl Hasselbalch and August Krogh, took fresh dog blood and exposed samples to gas at set oxygen pressures, measuring how much oxygen the blood bound. Then they raised the carbon dioxide in the gas and measured again at the same oxygen pressure. If carbon dioxide simply travelled alongside oxygen, the bound oxygen should not have changed. Instead, more carbon dioxide lowered oxygen binding, and the effect was largest at low oxygen pressures and small near full saturation.

That result showed a large effect of carbon dioxide on oxygen binding in dog blood, and it acts where it matters: in respiring tissue, carbon dioxide is high and PO2 is low. It did not say how carbon dioxide acts, since carbon dioxide also makes blood more acidic.

Today the Bohr effect names the fall in hemoglobin’s oxygen affinity when pH falls or carbon dioxide rises. Both shift the curve to the right, raising P50, so at the same PO2 the blood holds less oxygen and has released more. Two other changes also shift the curve right, and they are separate effects, not the Bohr effect:

• Higher temperature lowers affinity. Working muscle is warmer than resting muscle.
• More 2,3-bisphosphoglycerate (BPG), a small molecule made in red cells from a side branch of glycolysis, lowers affinity. Ruth and Reinhold Benesch reported this in 1967, as did Alfred Chanutin and Richard Curnish the same year. BPG binds preferentially to the deoxygenated form of hemoglobin.

Reverse any of these, with higher pH, less carbon dioxide, lower temperature or less BPG, and the curve shifts left, toward higher affinity.

Fetus, muscle, body size and icefish

The same problem of loading and unloading oxygen is solved differently in different places and animals.

The human fetus. Fetal blood picks up oxygen from the mother’s blood in the placenta (see animal reproduction), and it has higher affinity than adult blood: its curve sits to the left. Fetal hemoglobin has gamma chains in place of the adult beta chains. In 1970 H. Franklin Bunn and Robin Briehl showed that, with BPG removed and then added back at the same concentration, adult hemoglobin’s P50 rose about twofold while fetal hemoglobin’s rose less. Fetal hemoglobin still binds BPG; it responds less to it. Whole cord blood also had higher affinity than adult blood in a 1972 comparison by Enoki and colleagues.

Myoglobin. Myoglobin is a single-chain oxygen-binding protein in muscle. With one site there is no cooperation, and its curve is hyperbolic, rising steeply from zero and flattening early. Horse myoglobin in solution at 37 degrees C and pH 7.0 has a P50 of 2.39 Torr, roughly a tenth of the 26.8 mmHg modelled for human blood. Because myoglobin has such a low P50, it stays largely loaded until the PO2 inside muscle fibres is very low, so it can take up oxygen from blood and give it up when the PO2 falls, as in hard work. Keep in mind that one number comes from purified horse protein and the other from a model of human blood.

Body size. Among mammals, small species tend to have higher P50 values than large ones, a pattern described by Knut Schmidt-Nielsen and J. L. Larimer in 1958, though the trend is loose and differs among lineages. A mouse uses more oxygen per gram of tissue than an elephant (see size and scaling), and lower affinity favours unloading. Other animals have shifts linked to altitude, such as the bar-headed goose and the llama, whose hemoglobins are described in protein binding and allostery.

Other carriers, and none. Horseshoe crabs and octopuses carry oxygen on hemocyanin, a copper-containing protein dissolved in their blood rather than packed in cells; the fluid is called hemolymph in animals with an open circulation, such as horseshoe crabs, while octopuses have a closed circulation. It is colourless without oxygen and blue with it. Adult Antarctic icefish, family Channichthyidae, have no red cells and no working hemoglobin, as Johan Ruud reported in 1954. Their oxygen travels dissolved in plasma, in water near freezing that holds more dissolved gas than warm water.

Carrying carbon dioxide back

Carbon dioxide made in the tissues returns to the lungs in three forms. Textbooks give rough splits that differ between sources; one common one for an adult human puts about 5% dissolved, about 70% as bicarbonate and about 25% bound to hemoglobin. These fractions are approximations: they depend on conditions and differ along the path from tissue to lung.

Bicarbonate. In red cells, the enzyme carbonic anhydrase speeds the reaction of carbon dioxide with water (see enzyme action):

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

The bicarbonate leaves the red cell in exchange for a chloride ion through a membrane exchanger (see membrane transport), and it travels in plasma. The hydrogen ions are largely buffered by hemoglobin, and they are the source of part of the Bohr effect in tissue capillaries. In the lungs, where carbon dioxide is low, the reactions run backwards and carbon dioxide diffuses out to be exhaled.

Carbamino compounds. Some carbon dioxide binds directly to the end amino groups of hemoglobin’s chains, forming carbamino compounds. It does not bind the iron where oxygen sits. Experiments that blocked those amino groups in horse hemoglobin removed the oxygen-linked part of this binding.

The Haldane effect is the other side of Bohr’s linkage. At the same carbon dioxide pressure, deoxygenated blood carries more carbon dioxide than oxygenated blood; Christiansen, Douglas and Haldane described it in 1914. Deoxygenated hemoglobin buffers hydrogen ions better and forms more carbamino compounds. In the lungs, oxygenation releases hydrogen ions, which push bicarbonate back to carbon dioxide for exhalation. The Bohr effect is carbon dioxide and pH acting on oxygen binding; the Haldane effect is oxygen acting on carbon dioxide carriage. Bicarbonate also links to blood pH, which the kidney helps regulate.

Design problem

Human fetal blood binds oxygen more tightly than adult blood. Is that higher affinity built into fetal hemoglobin itself, or does it come from how fetal hemoglobin responds to BPG? Design an experiment to tell.