Gas exchange and transport: the oxygen-hemoglobin dissociation curve and CO2 transport as bicarbonate

Intuition Beginner

Oxygen cannot dissolve in blood in large enough amounts to keep you alive. Instead, a protein called hemoglobin inside your red blood cells carries oxygen from your lungs to every tissue in your body. Each hemoglobin molecule can carry up to four oxygen molecules at once.

The relationship between oxygen level and hemoglobin loading is not a straight line — it is an S-shaped curve. At high oxygen levels (in the lungs), hemoglobin fills up fast. At lower oxygen levels (in working muscles), hemoglobin releases oxygen where it is needed. This S-shape is a feature, not a flaw: it means hemoglobin loads efficiently in the lungs and unloads efficiently in tissues.

Carbon dioxide, the waste product of metabolism, travels back to the lungs mostly as bicarbonate, a dissolved ion. An enzyme called carbonic anhydrase inside red blood cells converts CO2 into bicarbonate rapidly. When the blood reaches the lungs, the reaction reverses and CO2 is exhaled.

Visual Beginner

The oxygen-hemoglobin dissociation curve plots oxygen saturation (vertical axis, 0 to 100%) against the partial pressure of oxygen (horizontal axis, 0 to 100 mmHg). The curve has a characteristic S-shape (sigmoid). The top portion (above 60 mmHg) is nearly flat: even if oxygen pressure drops somewhat, saturation stays high. The steep middle portion (20 to 60 mmHg) is where small changes in oxygen pressure cause large changes in saturation — this is the range where tissues extract oxygen.

When tissues produce more CO2 (and the blood becomes more acidic), the curve shifts to the right. This Bohr effect means hemoglobin releases even more oxygen exactly where metabolic activity is highest.

Worked example Beginner

A blood sample from a working muscle shows a local oxygen partial pressure of 30 mmHg. Using the oxygen-hemoglobin dissociation curve, estimate the hemoglobin saturation at this pressure.

Step 1. Find 30 mmHg on the horizontal axis.

Step 2. Move up to the curve. At 30 mmHg, the curve is in its steep middle region and saturation is approximately 57%.

Step 3. Compare with the lungs: at 100 mmHg, saturation is about 97.5%. The difference (97.5% - 57% = 40.5%) represents the fraction of oxygen unloaded to the tissues. This large unloading in the steep zone is exactly what the body needs during exercise.

Check your understanding Beginner

Formal definition Intermediate+

The oxygen-hemoglobin dissociation curve

The fraction of hemoglobin binding sites occupied by oxygen (saturation, $S_{O_2}$) depends on the partial pressure of oxygen ($P_{O_2}$) according to the Hill equation:

$$S_{O_2}=\frac{P_{O_2}^n}{P_{50}^n+P_{O_2}^n},$$

where $P_{50}$ is the partial pressure at which hemoglobin is 50% saturated ($\approx 26.6$ mmHg for adult human Hb A at pH 7.4, 37 degrees C) and $n$ is the Hill coefficient ($\approx 2.7$ for human Hb A, indicating positive cooperativity among the four binding sites). The sigmoidal shape arises because $n>1$: each successive oxygen molecule bound increases the affinity of the remaining sites.

The $P_{50}$ is a summary measure of hemoglobin's oxygen affinity. A higher $P_{50}$ means lower affinity (the curve shifts right); a lower $P_{50}$ means higher affinity (the curve shifts left).

Shifts of the dissociation curve

Several physiological variables shift the curve and change $P_{50}$:

• Bohr effect: Increased $P_{C{O}_2}$ or decreased pH shifts the curve rightward (increased $P_{50}$). The molecular mechanism involves protonation of histidine-146 on the beta chain and carbamino formation on the alpha-chain amino terminus, both stabilising the deoxy (T-state) conformation of hemoglobin.
• 2,3-BPG (2,3-bisphosphoglycerate): This organic phosphate, produced by red blood cells in the Rapoport-Luebering shunt of glycolysis, binds to deoxyhemoglobin in the central cavity between beta chains. Increased 2,3-BPG shifts the curve rightward. Chronic hypoxia (high altitude, anemia, chronic lung disease) raises 2,3-BPG levels over 24-36 hours.
• Temperature: Increased temperature shifts the curve rightward. Exercising muscles are warmer, promoting local oxygen unloading.
• Carbon monoxide: CO binds hemoglobin with ~240 times the affinity of O2, displacing oxygen and shifting the curve leftward for the remaining binding sites. This produces functional hypoxia: arterial PO2 may be normal but oxygen delivery is severely impaired.

CO2 transport

Carbon dioxide is transported in three forms:

1. Dissolved CO2 (7-10%): governed by Henry's law, $[{\text{CO}}_2{]}_{\text{dissolved}}=\alpha \times P_{C{O}_2}$, where $\alpha \approx 0.03$ mmol/(L $\cdot$ mmHg).

2. Carbaminohemoglobin (10-20%): CO2 binds to the terminal amino groups on hemoglobin globin chains, forming ${\text{Hb-NH-COO}}^{-}$.

3. Bicarbonate (70-80%): the dominant pathway. Inside red blood cells, the enzyme carbonic anhydrase catalyses:

$${\text{CO}}_2+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2{\text{CO}}_3\rightleftharpoons {\text{H}}^{+}+{\text{HCO}}_3^{-}.$$

The bicarbonate exits the red cell via the anion exchanger 1 (AE1, band-3 protein) in exchange for chloride entering the cell — the chloride shift (Hamburger phenomenon). This exchange maintains electroneutrality. In the lungs, the entire reaction reverses: bicarbonate re-enters the red cell, chloride exits, carbonic anhydrase regenerates CO2, and CO2 is exhaled.

The Haldane effect

Deoxygenated hemoglobin is a better buffer for H+ and a better carrier of CO2 (in carbamino form) than oxygenated hemoglobin. This means that in the tissues, as hemoglobin releases oxygen, its capacity to carry CO2 increases. In the lungs, as hemoglobin loads oxygen, CO2 is displaced and released. The Haldane effect accounts for approximately half of the CO2 exchange between tissues and lungs, making it the physiological partner of the Bohr effect.

The alveolar gas equation

The alveolar partial pressure of oxygen is predicted by:

$$P_{A{O}_2}=F_{I{O}_2}(P_B-P_{H_{2}O})-\frac{P_{AC{O}_2}}{R},$$

where $F_{I{O}_2}$ is the inspired oxygen fraction (0.21 in room air), $P_B$ is barometric pressure (760 mmHg at sea level), $P_{H_{2}O}=47$ mmHg at 37 degrees C, $P_{AC{O}_2}$ is alveolar (approximately equal to arterial) CO2 partial pressure, and $R$ is the respiratory quotient (${\dot{V}}_{C{O}_2}/{\dot{V}}_{O_2}$, typically 0.8-0.85).

The A-a gradient

The alveolar-arterial oxygen gradient ($A-a$ gradient) is:

$$A-a=P_{A{O}_2}-P_{a{O}_2}.$$

In healthy young adults breathing room air, $A-a\approx 5-10$ mmHg. It rises with age (approximately 2.5 mmHg per decade after age 20). A widened A-a gradient indicates V/Q mismatch, shunt, or diffusion limitation. A normal A-a gradient with hypoxemia suggests pure hypoventilation (both alveolar and arterial PO2 fall together).

Diffusion capacity (DLCO)

The lung diffusing capacity for carbon monoxide is:

$$D_{LCO}=\frac{{\dot{V}}_{CO}}{P_{ACO}-P_{cCO}}.$$

Because CO binds hemoglobin so tightly, capillary PCO is negligible and the equation simplifies to $D_{LCO}={\dot{V}}_{CO}/P_{ACO}$. The single-breath technique involves inhaling a gas mixture with a trace amount of CO, holding for 10 seconds, and measuring the rate of CO uptake. Normal $D_{LCO}$ is 20-30 mL/(min $\cdot$ mmHg). Reduced $D_{LCO}$ indicates loss of alveolar-capillary surface area (emphysema, pulmonary fibrosis, pulmonary vascular disease).

Key mechanism Intermediate+

Proposition (Bohr effect: pH-dependent rightward shift of the oxygen-hemoglobin dissociation curve). A decrease in blood pH of 0.1 units (from 7.4 to 7.3) increases the $P_{50}$ of adult hemoglobin by approximately 3 mmHg (from ~26.6 to ~30 mmHg), corresponding to a rightward shift of the sigmoidal saturation curve and enhanced oxygen unloading at any given tissue $P_{O_2}$.

Argument. The Bohr coefficient, defined as $\Delta \log P_{50}/\Delta \text{pH}$, is approximately $-0.48$ for human Hb A. For a pH decrease of 0.1:

$$\Delta \log P_{50}=-0.48\times (-0.1)=+\mathrm{0.048.}$$

The new $P_{50}$ satisfies $\log P_{50,\text{new}}=\log 26.6+0.048=1.425+0.048=1.473$, giving $P_{50,\text{new}}\approx 29.7$ mmHg. At a tissue $P_{O_2}$ of 40 mmHg, the original saturation (Hill equation with $n=2.7$, $P_{50}=26.6$) is:

$$S_{\text{pH 7.4}}=\frac{40^{2.7}}{26.6^{2.7}+40^{2.7}}\approx \mathrm{0.76.}$$

With the shifted curve ($P_{50}=29.7$):

$$S_{\text{pH 7.3}}=\frac{40^{2.7}}{29.7^{2.7}+40^{2.7}}\approx \mathrm{0.69.}$$

The Bohr effect unloads an additional 7% of hemoglobin's oxygen-carrying capacity at this tissue PO2. For a patient with [Hb] = 15 g/dL, this represents roughly $15\times 1.34\times 0.07\approx 1.4$ mL O2/dL of additional oxygen delivery per pass — a nontrivial contribution to tissue oxygenation during exercise or metabolic acidosis.

Bridge. The Bohr effect is the quantitative foundation for understanding why supplemental oxygen alone cannot compensate for severe acidosis: even with adequate arterial PO2, tissue acidosis impairs oxygen unloading. Clinical management of lactic acidosis must address both oxygen delivery and acid-base balance.

Exercises Intermediate+

Gas transport physiology — advanced topics Master

High-altitude acclimatization

Ascent to high altitude reduces barometric pressure and hence alveolar PO2. At 5,500 metres (approximately the elevation of Everest Base Camp), $P_B\approx 380$ mmHg and alveolar PO2 falls to approximately 45 mmHg. Acclimatization involves four time-scale-separated responses:

1. Hyperventilation (minutes to hours): Peripheral chemoreceptors sense the fall in arterial PO2 and increase minute ventilation. The resulting respiratory alkalosis (low PCO2) shifts the oxygen-hemoglobin curve leftward, partially opposing the benefit of hyperventilation. The kidneys compensate over 2-3 days by excreting bicarbonate, restoring CSF and blood pH toward normal.

2. Increased 2,3-BPG (24-48 hours): Red blood cells increase 2,3-BPG production via the Rapoport-Luebering shunt, shifting the curve rightward and improving tissue unloading. The trade-off is slightly reduced arterial saturation.

3. Erythropoiesis (days to weeks): Renal peritubular cells sense hypoxia via hypoxia-inducible factor (HIF) and increase erythropoietin (EPO) production. Red blood cell mass rises over 2-6 weeks, increasing hemoglobin concentration and total oxygen-carrying capacity. Polycythemia is the hallmark of chronic altitude acclimatization (hematocrit can exceed 60%).

4. Capillary density and mitochondrial changes (weeks to months): Tissue-level adaptations include increased capillary density (reducing diffusion distance) and increased mitochondrial efficiency. These changes are less well-quantified than the blood-level responses.

Fetal hemoglobin

Fetal hemoglobin (Hb F, ${\alpha }_2{\gamma }_2$) has two gamma subunits in place of the adult beta subunits. The gamma chain has a reduced positive charge in the central cavity, which decreases 2,3-BPG binding. The result is a leftward-shifted dissociation curve with $P_{50}\approx 19$ mmHg (compared to 26.6 mmHg for adult Hb A). At the maternal-fetal interface in the placenta, where maternal PO2 is approximately 40 mmHg, Hb F achieves ~80% saturation while Hb A achieves only ~75%. This affinity gradient drives oxygen transfer from maternal to fetal blood. After birth, gamma-chain production is silenced and beta-chain production increases over the first 6 months (the switch is regulated by BCL11A and other transcription factors), replacing Hb F with Hb A.

Carbon monoxide poisoning: quantitative analysis

The binding of CO to hemoglobin follows a competitive equilibrium. The ratio of carboxyhemoglobin to oxyhemoglobin depends on the ambient CO partial pressure:

$$\frac{[\text{HbCO}]}{[{\text{HbO}}_2]}=M\times \frac{P_{CO}}{P_{O_2}},$$

where $M\approx 240$ for human hemoglobin. Breathing room air ($P_{O_2}\approx 100$ mmHg) with only 0.1 mmHg CO:

$$\frac{[\text{HbCO}]}{[{\text{HbO}}_2]}=240\times \frac{0.1}{100}=\mathrm{0.24.}$$

So approximately 19% of hemoglobin is occupied by CO (0.24 / 1.24 = 0.194). The half-life of HbCO on room air is approximately 320 minutes. Breathing 100% oxygen reduces the half-life to approximately 80 minutes (by raising $P_{O_2}$ and thus driving the equilibrium toward HbO2). Hyperbaric oxygen at 3 atmospheres further reduces the half-life to approximately 23 minutes.

CO also shifts the dissociation curve leftward for the remaining binding sites (the Haldane-like effect of CO), increasing the affinity of unoccupied sites and further impairing tissue oxygen delivery. The combined effect of reduced capacity and impaired unloading makes CO poisoning a uniquely dangerous form of hypoxia.

Methemoglobinemia

In methemoglobinemia, the iron in the haem group is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state. Fe3+ cannot bind oxygen, reducing the oxygen-carrying capacity. Additionally, the presence of even one Fe3+ haem in a tetramer shifts the dissociation curve leftward for the remaining Fe2+ haems (an intramolecular cooperativity effect analogous to the leftward shift caused by CO), impairing oxygen unloading. The result is functional hypoxia disproportionate to the percentage of methemoglobin.

Methemoglobinemia is caused by oxidant drugs (dapsone, benzocaine, nitrates) or by genetic deficiency of cytochrome b5 reductase (the enzyme that normally reduces Fe3+ back to Fe2+). Treatment is methylene blue, which acts as an electron carrier to reduce Fe3+ back to Fe2+ via the NADPH-methemoglobin reductase pathway.

Exercise gas exchange

During maximal exercise, oxygen consumption can increase 20-fold (from ~250 mL/min at rest to ~5,000 mL/min). Gas exchange accommodates this through: increased cardiac output (5 to 25 L/min), increased ventilation (6 to 100+ L/min), increased oxygen extraction (A-V difference rises from ~5 to ~15-16 mL/dL), and rightward shifts of the dissociation curve from tissue acidosis, hypercapnia, hyperthermia, and increased 2,3-BPG. The A-a gradient increases modestly during exercise (from ~10 to ~25 mmHg at maximal effort) due to diffusion limitation: capillary transit time shortens as cardiac output rises, and some pulmonary capillaries may not fully equilibrate. In elite athletes at maximal exercise, arterial PO2 can fall to 65-75 mmHg — exercise-induced arterial hypoxemia — a consequence of diffusion limitation in the setting of very high cardiac outputs.

V/Q mismatch: diffusion limitation vs perfusion limitation

A gas is perfusion-limited if equilibration between alveolar and capillary partial pressure is complete well before the end of capillary transit. Under these conditions, the rate of gas transfer depends on blood flow, not on the diffusion properties of the membrane. Oxygen is normally perfusion-limited at rest (equilibration in ~0.25 s of a ~0.75 s transit).

A gas is diffusion-limited if equilibration is incomplete at the end of capillary transit. Transfer depends on the diffusion capacity of the membrane. CO is the prototypical diffusion-limited gas because hemoglobin binds CO so avidly that capillary PCO remains near zero throughout transit, maintaining a constant diffusion driving pressure.

Oxygen shifts from perfusion-limited to diffusion-limited under conditions that shorten transit time (exercise, high cardiac output), reduce driving pressure (high altitude), or impair membrane diffusion (pulmonary fibrosis, alveolar proteinosis). The $D_{LCO}$ measurement probes this diffusion capacity directly.

DLCO measurement and interpretation

The single-breath DLCO technique (Krogh and Krogh, 1909; Ogilvie et al., 1957) has the patient inhale a gas mixture containing ~0.3% CO and an inert tracer (helium or methane) to total lung capacity, hold for 10 seconds, and exhale. The initial and final alveolar CO concentrations are used to calculate the rate of CO uptake:

$$D_{LCO}=\frac{V_A}{t\times (P_B-47)}\times \ln \left(\frac{F_{ACO,\text{initial}}}{F_{ACO,\text{final}}}\right).$$

$D_{LCO}$ has two components in series: the membrane diffusing capacity ($D_M$) and the capillary blood volume-dependent component ($\theta \times V_c$, where $\theta$ is the rate of CO binding to hemoglobin per mL of blood and $V_c$ is the pulmonary capillary blood volume). The Roughton-Forster equation separates them:

$$\frac{1}{D_{LCO}}=\frac{1}{D_M}+\frac{1}{\theta \times V_c}.$$

By measuring DLCO at two different oxygen concentrations (which alter $\theta$), both $D_M$ and $V_c$ can be estimated. This separation is clinically useful: emphysema reduces $V_c$ (loss of capillary bed), while pulmonary fibrosis reduces $D_M$ (thickened membrane).

DLCO must be corrected for hemoglobin concentration (low Hb reduces $\theta$ and falsely lowers DLCO) and altitude. The normal value is 20-30 mL/(min $\cdot$ mmHg) and declines with age.

Pulse oximetry

Pulse oximetry estimates arterial oxygen saturation ($S_p{O}_2$) noninvasively using two wavelengths of light (660 nm, where deoxyhemoglobin absorbs more; and 940 nm, where oxyhemoglobin absorbs more). The ratio of absorbances at these wavelengths is empirically calibrated to arterial saturation. Because pulse oximetry measures functional saturation (oxyhemoglobin as a fraction of oxyhemoglobin plus deoxyhemoglobin), it does not detect carboxyhemoglobin or methemoglobin. CO-oximetry (multi-wavelength spectrophotometry, typically using 4-7 wavelengths) is required to detect these dyshemoglobins.

The sigmoidal shape of the oxygen-hemoglobin curve has a direct clinical implication for pulse oximetry: the curve is nearly flat above 60 mmHg, so a saturation of 90% corresponds to a PO2 of only about 60 mmHg. A small further drop in saturation (from 90% to 85%) represents a large drop in PO2 (from ~60 to ~50 mmHg). Pulse oximetry is an early-warning monitor because the saturation changes relatively slowly across the flat upper portion of the curve, giving clinicians time to respond before the steep portion is reached.

Connections Master

1. 18.03.01 Respiratory physiology — gas exchange. The oxygen-hemoglobin dissociation curve and the alveolar gas equation are the quantitative backbone of the gas-exchange framework introduced in 18.03.01. The A-a gradient derived here is the diagnostic bridge between predicted alveolar PO2 and measured arterial PO2.

2. 18.02.01 Cardiovascular physiology. Cardiac output determines pulmonary blood flow and capillary transit time. During exercise, increased cardiac output shortens transit time and can produce diffusion limitation. The Fick equation (${\dot{V}}_{O_2}=CO\times (C_a-C_v)$) links cardiac output directly to the arteriovenous oxygen difference derived from the dissociation curve.

3. 18.02.03 pending Hemodynamics. The chloride shift is an ion-exchange process across the red-cell membrane governed by the same electrochemical driving forces that determine ion flux across vascular endothelium. The AE1 (band-3) exchanger is a membrane transport protein operating under principles described in the hemodynamics unit.

4. 18.08.01 Renal physiology. The Henderson-Hasselbalch equation links bicarbonate (the CO2 transport product) to acid-base balance. Chronic CO2 retention (COPD) produces respiratory acidosis; the renal compensation (bicarbonate retention, H+ excretion) is governed by renal tubular mechanisms. The bicarbonate buffer system is shared territory between respiratory and renal physiology.

5. 18.07.01 Endocrine physiology. Erythropoietin (EPO), produced by renal peritubular interstitial cells in response to hypoxia via HIF, is the hormonal mediator of the erythropoietic response to altitude and chronic hypoxemia. The endocrine regulation of red blood cell mass directly determines hemoglobin concentration and hence oxygen-carrying capacity.

6. 18.05.01 Nervous system. Peripheral chemoreceptors in the carotid and aortic bodies sense arterial PO2, PCO2, and pH, driving the ventilatory response to hypoxia and hypercapnia. The Bohr effect and the Henderson-Hasselbalch equation provide the blood-chemistry inputs that the nervous system monitors.

Historical notes Master

Christian Bohr published his observations on the effect of CO2 on hemoglobin oxygen affinity in 1904, in a paper with Hasselbalch and Krogh titled "Ueber den Einfluss des Kohlensaeuregehalts des Blutes auf die Sauerstoffaufnahme" in the Skandinavisches Archiv fur Physiologie. Bohr's data showed that increasing CO2 concentration shifted the oxygen dissociation curve rightward. His co-author August Krogh went on to develop the Krogh cylinder model of tissue oxygenation and received the Nobel Prize in 1920 for his work on capillary regulation. Ironically, Bohr and Krogh later disagreed on the mechanism: Bohr initially believed the lung actively secreted oxygen against its partial-pressure gradient (the "secretion theory"), while Krogh's measurements supported passive diffusion. The diffusion theory prevailed.

John Scott Haldane, working at Oxford, established the quantitative basis of CO2 transport and the bicarbonate system in a series of papers and his monograph Respiration (1922, with J. G. Priestley). Haldane developed the first practical method for measuring blood gases and demonstrated that CO2 transport depends on hemoglobin's oxygenation state — the Haldane effect. His gas-analysis methods were the foundation of respiratory physiology for decades.

Archibald Hill applied the law of mass action to hemoglobin-oxygen binding in 1910, deriving the equation $y=K{x}^n/(1+K{x}^n)$ that bears his name. Hill's approach was phenomenological — he treated hemoglobin as having $n$ interacting binding sites without knowing the molecular structure. The Hill coefficient $n$ remains the standard clinical summary of cooperative binding, even though the underlying molecular mechanism (the MWC model) was not proposed until 1965 by Monod, Wyman, and Changeux. Hill received the Nobel Prize in 1922 for his work on muscle heat production, not for the hemoglobin equation.

Max Perutz solved the crystal structure of hemoglobin in 1960 after 22 years of effort (work begun in 1937), providing the molecular basis for cooperative binding. Perutz's structures showed the T (tense) and R (relaxed) conformations and identified the specific residues responsible for the Bohr effect (His-146 beta, the alpha-chain amino termini) and 2,3-BPG binding (the central cavity between beta chains). He received the Nobel Prize in Chemistry in 1962, shared with John Kendrew (myoglobin structure).

The Rapoport-Luebering shunt, a bypass in red-cell glycolysis that produces 2,3-BPG, was described by David Rapoport and his colleagues in the 1970s, establishing the metabolic basis for the adaptive regulation of hemoglobin oxygen affinity. The clinical importance of 2,3-BPG became apparent when banked blood (which loses 2,3-BPG during storage) was found to have impaired oxygen-unloading capacity — a finding that transformed blood banking practice.

The single-breath DLCO technique was introduced by Ogilvie, Forster, Blakemore, and Morton in 1957, building on the conceptual framework of Roughton and Forster (1957), who separated membrane and blood components of diffusing capacity. John West's application of the multiple inert gas elimination technique (MIGET) to V/Q distributions in the 1970s provided the first comprehensive maps of gas-exchange efficiency across the lung, work built on the theoretical foundation laid by Rahn and Fenn in the 1950s.

Bibliography Master

1. Bohr, C., Hasselbalch, K., and Krogh, A. (1904). Ueber den Einfluss des Kohlensaeuregehalts des Blutes auf die Sauerstoffaufnahme. Skandinavisches Archiv fur Physiologie, 16, 402-412.

2. Hill, A. V. (1910). The possible effects of the aggregation of the molecules of haemoglobin on its dissociation curves. Journal of Physiology, 40, iv-vii.

3. Haldane, J. S. and Priestley, J. G. (1922). Respiration. Yale University Press.

4. Monod, J., Wyman, J., and Changeux, J.-P. (1965). On the nature of allosteric transitions: a plausible model. Journal of Molecular Biology, 12, 88-118.

5. Perutz, M. F. (1970). Stereochemistry of cooperative effects in haemoglobin. Nature, 228, 726-739.

6. Roughton, F. J. W. and Forster, R. E. (1957). Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lung. Journal of Applied Physiology, 11, 290-302.

7. Ogilvie, C. M., Forster, R. E., Blakemore, W. S., and Morton, J. W. (1957). A standardized breath holding technique for the clinical measurement of the diffusing capacity of the lung for carbon monoxide. Journal of Clinical Investigation, 36, 1-17.

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12. Rapoport, S. and Luebering, J. (1950). The formation of 2,3-diphosphoglycerate in rabbit erythrocytes. Journal of Biological Chemistry, 183, 507-516.

13. Bunn, H. F. and Forget, B. G. (1986). Hemoglobin: Molecular, Genetic and Clinical Aspects. W. B. Saunders.