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Pittman RN. Regulation of tissue Oxygenation. San Rafael (CA): Morgan & Claypool Life Sciences; 2011.


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Some general comments about gas exchange and diffusion will be made, complied with by a summary of how oxygen is carried in the blood. The binding the oxygen come hemoglobin will certainly be discussed, including the oxygen saturation (or dissociation) curve and also factors (allosteric effectors) which reason it to shift. Next, a discussion of the effects of carbon monoxide top top oxygen binding will certainly be presented. Finally, a summary of fabricated oxygen carriers will certainly be presented. Many of this topics room covered in conventional textbooks <6,10,54,113> and also monographs ~ above oxygen carry <112>.


Overall Gas Exchange

Table 2 provides the partial pressures of the 4 respiratory gases in dried air, moist tracheal air, alveoli and also arterial and venous blood.


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The ingredient of alveolar gas relies upon the composition of motivated gas, ingredient of gas in the practical residual capacity (FRC), minus the O2 taken increase by the blood plus the CO2 added from blood. Details of exactly how the provided composition arises are discussed in conventional monographs of respiratory physiology top top the object of ventilation/perfusion defects <6,10,54,113>.


Diffusion

Diffusion takes ar in the gas phase by the random movement of gas molecules.

Graham"s regulation of diffusion (1833) says that the rate of diffusion that a gas is inversely proportional to the square root of its molecular weight (D∼MW−½). Thus, the relative prices of diffusion the CO2 and O2 are equal to √(32/44) or 0.85. Diffusion coefficients in the gas phase space Dgas ≈ 10−1 cm2/sec. In the fluid phase, diffusion prices of gases are typically 10,000 times smaller than those in gas environments due to the much much shorter mean complimentary path between collisions with various other molecules (e.g., the solvent); thus, Dliquid ≈ 10−5 cm2/s <55>. This is no a significant handicap, however, because the ranges over which gas transfer must take location in the fluid phase space generally quick (about 100 times shorter than the in the gas phase).


Fick"s legislation of Diffusion

Fick"s an initial law says that the lot of gas moved per unit time (ΔN/Δt) across a membrane that thickness Δx is proportional come the area (A) obtainable for exchange and also the partial pressure difference (ΔP) that the gas throughout the membrane. The continuous of proportionality (K) is referred to as Krogh"s diffusion coefficient (see below) to identify it native D:

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For gas exchange across an alveolus in the lung, A and also Δx room the same for all gases; different transfer rates an outcome from distinctions in K and also ΔP. Because that the lung, Δx is around 0.5 μm—a an extremely thin barrier; and also A is about 70 m2—a very large surface area. Krogh"s diffusion coefficient (K = aD) is equal to the diffusion coefficient (D) times the solubility (a) that a gas in the fluid through which the gas diffuses. Because that example, CO2 is 24 times much more soluble than O2 in water. Thus, the rate of CO2 diffusion is 0.85 × 24 = 20 times as rapid as that for O2 provided the same partial push difference.


Summary of Diffusion Properties

Table 3 summarizes the solubilities and diffusion coefficients for typical respiratory gases loved one to those determinants for oxygen (i.e., the worths for a given gas are separated by the value of the variable for oxygen), where α is the solubility (Henry"s regulation coefficient), D is the diffusion coefficient (gas phase), and also K = αD is Krogh"s diffusion coefficient (liquid phase).


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Table 3

Solubility, diffusion coefficient, and also Krogh"s diffusion coefficient for various gases relative to worths for oxygen.


Note that all the D values are about the same due to the fact that the molecular weights of these gases room similar. Thus, distinctions in diffusion with the fluid phase are identified primarily by the solubility coefficient.


Gas Exchange restricted by Diffusion and Perfusion

A quantitative summary of the gas exchange attributes of the lungs leads to the conclusion that the exchange of most gases is restricted by perfusion (i.e., blood flow). This is the situation for oxygen, so that the blood flowing with the pulmonary capillaries comes right into equilibrium with the PO2 in the alveolar gas after traversing around one-third the length of the pulmonary capillaries. One can bring out an analysis of gas exchange by making use of Fick"s very first law to determine the gas move that bring away place in between an alveolus and also a tiny volume of blood together it traverses the gas exchange an ar of the lung. A similar analysis is carried out because that oxygen in the peripheral circulation in chapter 8. For simplicity, gases that just exist in the physical dissolved type (e.g., He, Ar, N2O) room usually considered, so that one go not have to address the complications added by binding to proteins in the blood or carriage in a chemically modified form. This evaluation can be discovered in many monographs on respiratory physiology, and the much more involved instance of oxygen exchange wherein oxygen binding come hemoglobin within the red blood cells need to be taken right into account <47,113>.


Blood: Plasma and Red Blood Cells

For objectives of pointing out oxygen transport by the blood, us will take into consideration blood to be written of two phases: plasma and red blood cells (RBCs). The spring volume the blood inhabited by RBCs is dubbed the hematocrit, and its worth is a little less 보다 50% in person adults (∼40% for females and also ∼45% because that males). Oxygen is brought in the blood in 2 forms: (1) dissolved in plasma and RBC water (about 2% the the total) and also (2) reversibly bound to hemoglobin (about 98% that the total).

At physiological PO2 (40 < PO2 < 100 mm Hg), just a little amount that oxygen is dissolved in plasma because oxygen has actually such a low solubility. In ~ elevated PO2 (breathing 100 % oxygen or during hyperbaric oxygenation), however, the physically dissolved kind of oxygen can come to be significant. Henry"s legislation states the the lot of oxygen dissolved in plasma is straight proportional to PO2: = α PO2, wherein α = 0.003 ml O2 (100 ml plasma)−1 mm Hg−1. Thus, in ~ a PO2 that 100 mm Hg (typical worth for arterial blood), 100 ml that plasma contains 0.3 ml O2 (or 0.3 vol%). Oxygen is carried in two forms inside RBCs: the is dissolved in RBC water (about 70% that RBC volume is water) in accordance with Henry"s law, and also a much bigger amount of oxygen is reversibly bound come the hemoglobin contained within the RBCs.


Hemoglobin (Heme + Globin)

The protein hemoglobin is a molecule i m sorry is responsible for transporting almost every one of the oxygen in the blood. It is created of four subunits, each v a heme team plus a globin chain. The heme team is composed of a porphyrin ring which has an iron (Fe) atom in the center. Normally, the Fe is in the +2 redox state (ferrous) and can reversibly tie oxygen. There room at least six gene that regulate globin synthetic in humans, leading to the development of six structurally various polypeptide chain that space designated α, β, γ, δ, ξ, and also ς chains. Every normal and also most abnormal hemoglobin molecules are tetramers consists of two different pairs the polypeptide chains, every chain developing a monomeric subunit.

The blood of a normal adult human contains at the very least six different species of hemoglobin molecules, all of which have actually the same primary structure and also function. Hemoglobin A (A for adult) provides up 92% the the full hemoglobin concentration in a regular adult human. To date, around 200 structurally different human hemoglobin variants have actually been reported. This abnormal hemoglobins (relative come hemoglobin A) frequently have different oxygen-binding properties.

Hemoglobin A (HbA) is written of two α chains and two β chains, symbolically created as α2 β2. Its molecular weight is 64,400 Da. Each a chain has actually 141 amino acids, and each β chain has actually 146. The concentration the Hb inside red blood cells is 330 g/l (= 33 g%). In ~ a hematocrit the 45%, this yields a blood that 150 g/1 or 15 g%. The framework of the Hb molecule has actually been elucidated by the x-ray crystallographic work-related of Perutz and also his co-workers <14,30>. The α and β chains room arranged in αβ pairs, and also any conformational adjust in one polypeptide chain istransfer come the rather in the molecule. There space two various arrangements that the subunits in ~ the tetramer that are much much more stable 보다 all others. One of these two quaternary conformations predominates once the stole atoms space saturated v oxygen (oxy structure), and also the other predominates when these binding sites are vacant (deoxy structure). The deoxy framework is characterized by the visibility of inter- and intrasubunit salt bridges which give it a constrained or taut (T) configuration. The oxy configuration is acquired when the salt bond are broken so as to give the tetramer a peaceful (R) quaternary structure.

When the stole atom in the heme group becomes oxidized (loses an electron), that valence state changes from +2 (ferrous) to +3 (ferric). The hemoglobin is then dubbed methemoglobin (metHb) or ferrihemoglobin (Fe+3 will certainly not bind oxygen). Ordinarily, around 1% of the hemoglobin in a red blood cabinet is in this form. The level the metHb is preserved at this short level mostly by the enzyme NADH-methemoglobin reductase. It is important that the level of metHb be retained low due to the fact that it will certainly not reversibly tie oxygen and thus cannot bring oxygen.


Binding of Oxygen come Hemoglobin: Oxygen Saturation (Dissociation) Curve

The hemoglobin molecule has 4 binding sites because that oxygen molecules: the iron atom in the 4 heme groups. Thus, every Hb tetramer deserve to bind four oxygen molecules. Native the molecular weight of Hb, one deserve to calculate that 1 g of Hb can integrate with 1.39 ml the oxygen. Actually, several of the Hb typically in red blood cells cannot tie oxygen (it is one of two people metHb or HbCO), and also the empirically figured out oxygen-binding volume of hemoglobin (CHb) is 1.34 ml O2 every gram Hb. In 100 ml that blood, over there is around 15 g that Hb, so that 100 ml the blood has actually the capacity to bind 20.1 ml that oxygen. This amount is called the oxygen-binding capacity of blood (CB). Note that CB is proportional come the hematocrit the the blood.

If one starts with a deoxygenated sample that blood and permits it come equilibrate in measures with gas mixture of increasing PO2, the binding sites for oxygen will end up being progressively lived in until, at a high enough PO2, all of them will contain oxygen. A curve representing the equilibrium binding that O2 come blood is displayed in figure 2. The curve is recognized as the oxygen saturation curve or the oxygen dissociation curve and expresses the relationship between PO2 and also the tied oxygen content.


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FIGURE 2

Oxygen dissociation curve-relating oxygen bound to hemoglobin (oxygen saturation, SO2) together a role of partial pressure of oxygen (PO2). From CC Michel, The deliver of oxygen and carbon dioxide by the blood, in respiratory Physiology, eds. AC Guyton, (more...)


P50 is defined as the PO2 in ~ which oxygen saturation is 50%. The standard conditions under i m sorry oxygen binding is measured are T = 37 °C, pH = 7.4 and PCO2 = 40 mm Hg. The fountain oxygen saturation of Hb is the amount of oxygen merged with Hb split by the oxygen-binding volume of the blood (20.1 vol% at common Hct). The bound oxygen content is proportional to hematocrit:
4.2
where is blood hemoglobin concentration and also is regarded hematocrit (Hct) by
4.3
where RBC is the median hemoglobin concentration the a single RBC. That is crucial to identify the difference between oxygen content and oxygen saturation:

The oxygen dissociation curve is stated to have a sigmoid shape, which reflects the participating nature that oxygen binding to Hb. This curve is very nonlinear in the normal physiological variety of PO2 (i.e., 40 to 100 mm Hg). The middle section of the curve (20–80% saturation) is steeper 보다 the short PO2 and high PO2 segments. The affinity the Hb for oxygen increases steadily together oxygen saturation goes indigenous 0% to 100% for a provided oxygen dissociation curve. For various oxygen dissociation curves, the affinity that Hb because that oxygen rises with to decrease P50. A straightforward quantitative summary of the oxygen dissociation is express by Hill"s equation <3,30>:
where n is Hill"s coefficient and is around 2.7 for human adult hemoglobin and is regarded the level of cooperativity the oxygen binding to hemoglobin. The oxygen-binding properties of myoglobin, a connected protein v one heme group that reversibly binds oxygen in striated muscle cells, can likewise be described by Hill"s equation through n = 1.


Allosteric Effectors that Oxygen Binding to Hemoglobin

Several determinants influence the binding the oxygen to hemoglobin: temperature, pH, PCO2 and 2,3 diphosphoglycerate (2,3 DPG). Increasing the temperature that Hb lowers the affinity because that O2 and also shifts the oxygen dissociation curve to the right, as displayed in figure 3. This has actually physiological importance during exercise because the temperature the muscle organization is higher than 37°C, and oxygen deserve to be unloaded indigenous Hb an ext easily in ~ the greater temperature (lowered oxygen affinity).


FIGURE 3

Shifts in the oxygen dissociation curve early out to changes in temperature. Native CC Michel, The carry of oxygen and also carbon dioxide by the blood, in respiratory Physiology, ed. AC Guyton, JG Widdicombe, Baltimore: university Park Press, pp. 67–104, (more...)


As checked out in figure 4, boosted H+ task (decreased pH) also lowers the affinity that Hb for O2. This was originally noticed by Bohr (Bohr effect) and also his partner (1904) as an result of raised PCO2, however it has been presented to be mainly an effect of pH within the red blood cell. CO2 through itself, at consistent pH, likewise affects the oxygen dissociation curve together that increased PCO2 shifts the curve come the appropriate (i.e., lowers the affinity of Hb because that oxygen).


FIGURE 4

Shifts in the oxygen dissociation curve due to changes in pH. Native CC Michel, The deliver of oxygen and carbon dioxide through the blood, in respiratory tract Physiology, eds. AC Guyton, JG Widdicombe, Baltimore: university Park Press, pp. 67–104, 1974. (more...)


2,3 DPG is a glycolytic intermediate developed within the RBC that affects the affinity the Hb for oxygen. Increases in RBC cause decreases in <2,3 DPG>, and also decreases in RBC cause increases in <2,3 DPG>. 2,3 DPG is a fee ion that cannot pass through the RBC membrane. Rises in that is concentration change the oxygen dissociation curve to the right. 2,3 DPG is important throughout respiratory compensation seen in acclimatization come altitude, by which the hypoxic hyperventilation the high altitude causes PCO2 and also H+ to decrease (left change of oxygen dissociation curve), leading to rise in 2,3 DPG which move the curve back to the right. 2,3 DPG binds to the terminal amino groups of the beta chains and competes v CO2 for binding at those sites.

Shifts in the oxygen dissociation curve can be summarized as follows. A right change in the oxygen dissociation curve (↑P50 or ↓ Hb-O2 affinity) have the right to be created by boosts in any type of of the following: T, PCO2, (↓ pH) or <2,3 DPG>. A left transition in the oxygen dissociation curve (↓ P50 or Hb-O2 affinity) have the right to be produced by to reduce in any of the following: T, PCO2, ( pH) or <2,3 DPG>. Ordinarily, the Bohr impact is not necessary except in exercise. In this situation, the oxygen dissociation curve is shifted to the appropriate to permit easier unloading of oxygen from Hb in the tissues. The rightward change in the oxygen dissociation curve is much more important at reduced PO2. Return the rightward change interferes through oxygen loading in the lungs, this never causes a problem in oxygen transport.


Overall Oxygen Transport

Figure 5 mentions the normal partial pressures and also contents (or concentrations) because that oxygen. It likewise describes the all at once transfer that gas between the lungs and also blood and also between the blood and also tissue.


FIGURE 5

Overall oxygen transport. The partial press of oxygen is presented in the dried air, humidified tracheal waiting (PIO2) and also the alveolar compartment (PAO2). After ~ pulmonary gas exchange takes place, the composition of arterial blood is shown (PaO2, a and (more...)


Carboxyhemoglobin

Carbon monoxide has a an extremely high affinity for Hb (200–300 times the of oxygen in common adults). In instances where over there is simultaneously enough oxygen and also carbon monoxide to totally saturate the hemoglobin, these two ligands contend for the same binding sites, and the relative amount of every bound come Hb is provided by Haldane"s an initial law:
where M lies in the range 220–270 for regular adult hemoglobin. For example, if PCO = 0.08 mm Hg and also PO2 = 80 mm Hg, around 20% that the Hb is bound up v CO (if M = 250). This represents 20% that the Hb that cannot carry oxygen (i.e., over there is one component of HbCO to 4 parts of HbO2). Figure 6 illustrates the effect of CO top top the oxygen dissociation curve.


FIGURE 6

Effect of carbon monoxide ~ above the oxygen dissociation curve. Indigenous CC Michel, The transfer of oxygen and carbon dioxide through the blood, in respiratory Physiology, eds. AC Guyton, JG Widdicombe, Baltimore: university Park Press, pp. 67–104, 1974. (more...)


The curve labeled “50% Anaemia” to represent a sample the blood whose oxygen-binding volume is one-half the regular value, and also none the the hemoglobin is combined with CO. Compare this curve with the one labeling “50% HbCO” just over it (HbCO = 50%) that likewise has one oxygen-binding capacity of one-half the normal value and note that the blood with HbCO has actually a left-shifted oxygen dissociation curve.

Carbon monoxide is danger for number of reasons. As soon as CO binding to among the binding sites on hemoglobin, the raised affinity the the various other binding sites because that oxygen leads to a left transition of the oxygen dissociation curve and also interferes with unloading the oxygen in the tissues. The visibility of CO stays clear of loading of oxygen as result of competition because that the very same binding sites. Carbon monoxide binds tightly to hemoglobin (high affinity because that hemoglobin), and also the cumulative effect of the binding as much as the limit given over by Haldane"s an initial law shows that really low partial pressure of CO (<1 mm Hg) can properly block a large fraction of the heme-binding sites from oxygen. Thus, the oxygen contents of blood in the presence of carbon monoxide is much reduced than normal. Blood remains red because the absorb spectrum for HbCO is similar to the of HbO2, other than that the is shifted slightly to higher wavelengths contrasted with HbO2. Finally, there space no obvious physical signs of carbon monoxide poisoning because carbon monoxide is colorless, odorless and also tasteless; the does not produce respiratory reflexes prefer coughing or sneezing; over there is no rise in ventilation (thus, PaO2 is normal); and also no feeling of challenge in breathing.


ARTIFICIAL OXYGEN CARRIERS

In cases where there is a far-ranging (∼30–40%) loss of blood volume (i.e., hemorrhage, view Chapter 9), it is essential to restore blood volume as shortly as possible, so the the volume of the blood to carry oxygen to the organization is not seriously compromised. The natural fluid for such transfusions is totality blood since it contains all the biologically relevant contents normally existing in blood. Since of concerns around the extent and also safety of the blood supply, including adverse transfusion reactions and also inadvertent infection of transmittable diseases, there has been a good effort to produce synthetic oxygen carriers that have the right to act together substitutes for whole blood transfusions. Two varieties of fluids have been emerged for this purpose, hemoglobin-based oxygen carriers and perfluorocarbon emulsions. Their attributes in regard come oxygen transport and also regulation of tissue oxygenation will currently be presented.


Hemoglobin-Based Oxygen Carriers

Since plasma cannot lug much oxygen, because of its short solubility because that oxygen, and also hemoglobin is the oxygen carrier in ~ RBCs, it is organic to consider hemoglobin once formulating an synthetic oxygen carrier. Number of hemoglobin-based oxygen carriers (HBOCs) space in various stages of breakthrough for the function of treating hemorrhagic and hypovolemic shock in trauma patients and other circumstances where over there is a jeopardized oxygen it is provided <9,12,116,117>. HBOCs room made from expired person blood or new bovine blood which experience numerous adjustments to do them safe and effective oxygen carrier <12>. The RBCs are very first lysed to release their hemoglobin, and also then the stroma is removed by a range of methods, including centrifugation, filtration and chemical exploit <50>. The stroma-free hemoglobin is then purified and undergoes adjustments to cross-link, polymerize or conjugate the to other compounds. Without these modifications, the oxygen affinity the the stroma-free hemoglobin is too an excellent to facilitate oxygen relax in the tissues as result of the reduction in 2,3 DPG. As soon as it is external the RBC, hemoglobin promptly dissociates right into 32 kDa αβ dimers and also 16 kDa α or β monomers, both of i beg your pardon are quickly filtered in the kidney and also can precipitate in the loop the Henle, causing severe renal toxicity <36>. Because that this reason, 4 different types of HBOCs have been considered: cross-linked hemoglobins, cross-linked and polymerized hemoglobins, hemoglobins conjugated come macromolecules and encapsulated hemoglobins <12>.

The many notable result following administration of HBOCs is a pressor effect, boost of typical arterial blood push (MAP) through as lot as 10–35% within 15–30 minute following administration <36>. The press usually returns to baseline within 2 hours following administration in most pet studies. The dimension of the HBOC appears to play a role in this pressor effect. Smaller sized HBOCs, such together cross-linked tetramers, develop a bigger rise in MAP 보다 do larger polymerized versions, possibly due to extravasation of smaller materials of HBOCs indigenous the microcirculation. There are number of theories regarding why the pressor effect occurs <9,50,89>, yet the most favored possibility has actually something to perform with the interaction of hemoglobin with nitric oxide (NO). Nitric oxide released from endothelial cells relaxes the smooth muscle in the blood vessel walls. Normally, that is believed that the RBC membrane stays clear of NO from communicating with hemoglobin. However, the cell-free hemoglobin that HBOCs reacts openly with NO to develop HbNO from deoxyhemoglobin and methemoglobin and nitrate native oxyhemoglobin, bring about a palliation in NO and also leading to an unopposed vasoconstriction <50>. One more mechanism that has been proposed to account for the arteriolar vasoconstriction and elevated MAP considers the the HBOC increases the oxygen-carrying capacity of blood and also produces one oversupply that oxygen come the tissues, leading to a compensatory autoregulatory vasoconstriction <89,116,117>. However, a pressor result has been observed in situations where little amounts that HBOC have been infused, too small to have actually an result on oxygen supply.

In the visibility of an HBOC, the total oxygen concentration that blood is given by:
where D is the dissolved oxygen provided by αO2 PO2, RBC is SO2RBC Hct RBC CHb and HBOC is SO2HBOC (1-Hct) HBOC CHb. RBC is the average concentration the hemoglobin in one RBC, and HBOC is the concentration the HBOC in the plasma. The assumption is made the the oxygen-binding capacity of hemoglobin is the very same for the hemoglobin in the RBC and that comprising the HBOC. This assumption can be serene if the oxygen-binding capacities are known to be different. Because the oxygen dissociation curves because that RBC hemoglobin and also the HBOC will usually be different (i.e., different P50s and also Hill coefficients, watch Eq. 4.5), one would expect the SO2 worths to it is in different, also assuming that equilibrium exists for PO2 between the RBCs and also the HBOC.


Perfluorocarbon Emulsions

Perfluorocarbon-based emulsions (PFCs) space mixtures the fluorocarbons and also emulsifying agents that differ greatly in structure and also mechanism of action from HBOCs. Fluorocarbons differ in shape and also size, however share countless of the same general chemical characteristics. This molecules room hydrocarbon chains the are highly substituted through fluorine atoms. The carbon-fluorine bonds give them their distinct chemical and biological inertness. Fluorocarbon molecules used in potential man-made oxygen carriers have the right to be linear or cyclic, although it has actually been shown that straight molecules dissolve greater quantities of oxygen <85,89>. Fluorocarbons have a high gas-dissolving capacity and also low viscosity yet are extremely insoluble in aqueous solutions; they must be emulsified in order to travel through the circulatory system. Surfactants, or surface-active agents, are provided as emulsifiers to type small droplets that are 0.1 come 0.3 μm in diameter. Castle are qualified of dissolving huge quantities that gases, such as oxygen and carbon dioxide; however, they bring less oxygen 보다 hemoglobin itself <68,95>. Castle are capable of carrying oxygen come the organization passively and can bring an lot of oxygen proportional to the ambient PO2 without having actually to depend on the red blood cells <68>.

PFCs have actually a half-life clearance from the body of around 2–4 hours and are eliminated unmetabolized v the lungs after gift taken increase by the reticuloendothelial mechanism (RES) <89>. This brief half-life could potentially limit clinical offers in traumatic injury and also hemorrhagic shock. The most vital limitation of PFCs is the direct relationship between dissolved oxygen concentration and also PO2. Number of side impacts have been reported because that PFCs <89,96>, and tiny is known about the long-term effects of PFC retention.

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Unlike hemoglobin molecules, PFCs do not bind oxygen, so the oxygen is carried by PFCs only in the dissolved form. Thus, the concentration the oxygen carried by PFCs is given by Henry"s regulation <85,97>:
where αPFC is the solubility that oxygen in the PFC emulsion. The relationship in between oxygen concentration and PO2 for hemoglobin is sigmoidal in shape, whereas that is linear for PFCs. In comparison to the oxygen bound to hemoglobin, oxygen liquified in PFCs is not impacted by pH, 2,3 DPG or other physicochemical factors <97>.