This chapter focuses on the transport of CO2 in the bloodstream, which is an issue at least as important as the transport of oxygen.
CO2 is transported by three major mechanisms:
- As bicarbonate (HCO3- )
- As carbamates, the conjugate bases of carbamino acids
- As dissolved CO2 gas
As it dissociates in the bloodstream, CO2 changes the pH. The acid-base consequences of acutely and chronically increased or decreased pCO2 are discussed in greater detail in the section dealing with acid-base disturbances, specifically in a chapter regarding the buffering of acute respiratory acid-base disorders. Extensive irrelevant digressions are indulged, particularly regarding the nature of carbonic acid and the precise mechanism via which CO2 changes the pH of a solution. In order to prevent those sorts of outbursts, this chapter will be limited only to whatever is relevant to CO2transport in the blood and body fluids. The most relevant references for this topic are Chapter 4.5 from "Acid-Base Physiology" by Brandis, as well as this excellent review article.
Whole-body CO2 stores
Much of one's CO2, tied up in non-dissolved form, can be viewed as a form of storage. Strange to think of it in that way (why would one possess a capacious reserve of a waste product?) but in fact the "stored" CO2 plays a major role in maintaining the acid-base balance.
How much CO2 do you have stored right now? To explore this topic in detail, an excellent article by Cherniack and Longobardo can be found in Physiology Reviews (1970) - but it is paywalled. Fortunately, the free preview at deepdyve.com offers a brief glimpse. The information afforded by this glimpse has been taken by the author and again represented as one of those disgraceful gamblegrams:
Thus, the adult 70kg male has about 1.8L/kg - up to 126L in total - of "potential" CO2 trapped inside them. A massive amount, about 1.6L/kg, is not readily accessible, as it is a part of the crystalline matrix of bone, or dissolved in the cytosol of tissues. The bony stores consist of about 30% bicarbonate, and 70% carbonate; they were revealed to researchers who heated bone and observed the release of CO2 (which was in considerable volume). This form of storage is largely irrelevant to the minute and second timescales of respiratory acid-base physiology, but becomes more important in chronic disorders, and plays a considerable role in the certain exotic circumstances-for example, in the demineralisation of bone associated with decompression sickness.
Of this massive bony storehouse, about 9L is gradually exchanged, and forms 15% of the total daily CO2 production; this exchange process probably plays little role in buffering because of its glacial slowness (its rate appears to be limited by blood flow to bone). The remainder is only about 3L - this is the CO2 in circulation - is involved in routine gas exchange processes and buffering; this is the CO2we are interested in when we discuss the compensation for acute acid-base disturbances.
So, it is possible to arrive at a volume of carbon dioxide which is available reasonably quickly. A confident-sounding radiolabelled bicarbonate study presents us with a figure of around 200 (+/- 42) ml/kg of CO2 for adults. The average adult male therefore has about 14 litres of CO2 on board which is fairly easy to access on the timescales involved in subacute acid-base disorders, and of these 14L about 3L are available for immediate use in buffering.
Of these 3 litres, the majority (80-90%) is stored in the form of bicarbonate anions (HCO3-) swimming around in extracellular fluid. About 5-10% is present as unchanged gas, dissolved in the water of extracellular fluid (predominantly in the blood). Another 5-10% or so is stored as carbamino compounds inside erythrocytes, and a minor fraction (perhaps 2-5%) is available as a free gas in the alveolar gas mixture. Lastly, the smallest fraction of all - measured probably in millilitres or microlitres- exists at any given time in the form of carbonic acid, and represents a transitional form; this is either CO2 in the process of being turned into bicarbonate, or bicarbonate in the process of being turned into CO2.
Proportions of different CO2 transport mechanisms
The proportions of each form depend considerably on the specifics of where the sample was taken (eg. veins of exercising muscle as compared to the radial artery).
The abovementioned review article by Geers and Gross is a goldmine of materal. Within it, Table 1 explores the changes in proportion of different species of transported CO2, depending on their environment. This table has been taken apart and crudely reassembled as a series of coloured cylinders, as can be seen below.
Thus, in the arterial bloodstream, the carbamate CO2-protein complexes and the dissolved CO2usually account for less than 10% of the total plasma CO2 carriage capacity. The rest is composed of HCO3-. Inside erythrocytes, where the haemoglobin concentration is aroung 330g/L, the amount of carbamate compounds is greatest, but bicarbonate remains the major player. As exercise increases the amount of CO2 produced by the tissues, the proportion of dissolved CO2 gas in both plasma and erythrocytes increases, but as these proportion cylinders show this increase in dissolved gas has a minor effect on the total blood CO2 content.
In short, in human whole blood CO2 is transported mainly by being converted into bicarbonate.
An even more effective way to represent this can be borrowed from Nunn's Respiratory Physiology.
In Chapter 10 (which is all about carbon dioxide transport) a diagram exists which illustrates the change in CO2 transport as plotted against CO2 partial pressure. It closely resembles this:
As one can plainly see, the bicarbonate concentration changes the most, the dissolved CO2 increases modestly in a linear fashion, and the carbamate content barely changes at all.
CO2 transport as a dissolved gas
CO2 happens to be one of the gases which enjoy a relatively good water solubility. For every 1 mmHg of CO2 about 0.03 mmol/L enters solution. This relationship is reasonably linear over a survivable range of pressures and temperatures.
In solution, CO2 will reversibly combine with water to form carbonic acid, but it does so with great reluctance. One can describe this reluctance mathematically, and express it as a hydration equilibrium constant (Henry's constant). This constant is derived by dividing the concentration of H2CO3 by the concentration of aqueous CO2. Thus, at 25°C Henry's constant for CO2 is only around 1.7 x10-3, which means that in any given jar of water, the majority of dissolved CO2 remains as a dissolved gas, and only a tiny fraction (0.17%) is present as H2CO3.
CO2 transport as bicarbonate
Conversion of CO2 to carbonic acid and bicarbonate
The combination of CO2 and H2O leads to the formation of H2CO3 by an entirely spontaneous uncatalysed manner. CO2 acts as a Lewis acid in this setting; having no protons to donate it instead acts as "something capable of forming a covalent bond with an electron pair".
The spontaneous reaction with water is actually rather sluggish. For clinically significant amounts of CO2 the uncatalysed dissociation reaction CO2 + H2O ⇌ H2CO3 can take about 3-4 seconds to half-equilibrate, and up to a minute to reach complete equilibrium (and that is in the steamy reaction-supporting body temperature, 37°C). At standard "lab" temperature, tyhe Obviously that is far too slow. There would not be enough time for this reaction to run to equilibrium during the rapid transit of red cells through the gas exchanging pulmonary capillaries, and CO2 removal would not occur.
Fortunately, the insides of erythrocytes (as well as the walls of blood vessles) are packed full of carbonic anhydrase (specifically CA type II) , which is perhaps the most rapidly acting enzyme in the human body, and with the aid of which this reaction progresses 25,000 times faster, reaching 95% completion within 2 milliseconds. In the macro-scales of ventilation, measured in seconds and minutes, this reaction can be viewed as essentially instantaneous. Because of this sort of catalytic support, the conversion of HCO3- and H+ back into CO2 and H2O can occur in the pulmonary capillaries during the very brief pulmonary capillary transit time.
In fact, so efficient is carbonic anhydrase, that relatively few enzyme molecules are required to sustain a completely normal level of CO2 exchange. It would appear that one requires perhaps as little as 2% of unblocked enzyme to carry on. Consider that this enzyme system is routinely blocked for therapeautic reasons by drugs such as acetazolamide. Indeed, massive overdoses of carbonic anhydrase inhibitors have been surprisingly survivable, in spite of the severe metabolic acidosis which occasionally ensues.
CO2 transport as carbamate compounds
Carbamate compounds are the dissociated conjugate bases of carbamino acids, which form in the spontaneous reaction of R-NH2 and CO2.
Thus, a "carbamate" is a conjugate base which acts as a storage device for CO2, and may contribute up to 14% of the total CO2 carrying capacity of the blood. The transitional "carbamino" stage depicted above is - like carbonic acid- a figure of speech rather than a proper macroscopic member of the bloodstream. At physiological pH it dissociates into carbamate and H+ so fast and so completely that its concentration is negligible, and for all internts and purposes it can be
The carbamate content of the plasma is rather poor because serum proteins (eg. globulins) have little interest in transporting CO2, and tend to form carbamates with the greatest reluctance. CO2 can really only bind to side chains of lysine and arginine, and any random plasma protein may only have one of these side chains exposed and available for CO2 binding. More binding sites are available within the red blood cells, where haemoglobin (especially deoxyhaemoglobin) is available in a high concentration.
This paucity of binding sites might make one think that carbamates dont play much of a role in the transport of CO2 in general, and specifically its transport in plasma. Indeed, the carbamates contribute relatively little to the overall CO2 carrying capacity of the human blood. Their importance becomes much greater if for whatever reason most of your carbonic anhydrase is disabled. In that case, suddenly this forgotten transport system becomes very relevant, as it can work with raw CO2and does not require H2CO3 as an intermediate.
Factors which influence the CO2 carrying capacity of proteins
CO2 transport as carbamates is influenced by the following factors:
- Oxygenation of haemoglobin (more deoxygenated= higher CO2 capacity)
- pH in the local environment (lower pH = higher CO2 capacity)
Inside the red cells, CO2 binds readily to amino acid side-chains of the haemoglobin molecule, particularly when it is deoxygenated. Indeed deoxyhaemoglobin has about 3.5 times the affinity for CO2 when compared to oxyhaemoglobin; this hypoxia-induced increase in CO2 carrying capacity is called the Haldane effect. This is exactly what you want in exercising tissue, which has just consumed all the oxygen in its local blood volume, and has produced a vast amount of CO2. Thus, the carbamate content of venous blood could potentially (at an SvO2 of 0%) be about triple the carbamate content of arterial blood. Of course in reality, the a-v difference is never that great, because venous blood is never that hypoxic.
The presence of carbamate groups in critical regions of the haemoglobin molecule tends to stabilise the deoxygenated form, which decreases the affinity of haemoglobin-oxygen binding. This effect (the Bohr effect) is discussed in greater detail together with the other factors which affect the affinity of haemoglobin for oxygen. Suffice to say the presence of a large amount of CO2 decreases this affinity and promotes the release of more oxygen from haemoglobin, which again is precisely what is needed in exercising tissue.