Transport of Carbon Dioxide

Carbon dioxide is the quiet partner in respiratory physiology. Oxygen draws the attention because it sustains oxidative metabolism, yet the body must clear carbon dioxide with equal precision, and it does so through three distinct transport routes operating simultaneously in the blood. Roughly 20 to 25 per cent of CO₂ travels bound to haemoglobin as carbamino-haemoglobin, about 70 per cent is carried as bicarbonate, and approximately 7 per cent moves in a dissolved state through plasma. These proportions are not arbitrary; they reflect the chemistry of a gas that is far more soluble than oxygen and far more willing to react with water.

This lesson examines how that chemistry is exploited at the tissue and alveolar sites. It builds directly on the oxygen dissociation curve discussed previously, because the binding and release of carbon dioxide are governed by partial pressures that move in the opposite direction to those governing oxygen. Where tissues produce CO₂ through catabolism, the gas diffuses into blood and is converted, largely inside red blood cells, into bicarbonate and hydrogen ions. At the alveoli, where partial pressure of CO₂ is low, the same reaction runs in reverse and the gas is expelled. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

The material that follows stays close to the terminology and numerical values of the standard treatment, adding a short "Going further" block where established background helps the reader connect the mechanism to broader biochemistry. The aim is to make the three transport fractions feel inevitable rather than memorised, and to prepare the ground for the neural regulation of respiration that follows in the next lesson.

The Three Fractions of Carbon Dioxide Transport

Blood is the medium of transport for both oxygen and carbon dioxide, but the two gases use that medium very differently. About 97 per cent of oxygen is transported by red blood cells, with the remaining 3 per cent carried dissolved in plasma. Carbon dioxide distributes itself across three routes at once: a carbamino fraction bound to haemoglobin, a bicarbonate fraction that dominates the total, and a small dissolved fraction. Understanding this division is the foundation for everything else in the lesson, because each route responds to partial pressures in its own way.

Carbamino-haemoglobin and the Role of Partial Pressures

Carbon dioxide is carried by haemoglobin as carbamino-haemoglobin, accounting for about 20 to 25 per cent of the total. This binding is related to the partial pressure of CO₂, and the partial pressure of oxygen is a major factor that can affect it. When pCO₂ is high and pO₂ is low, as in the tissues, more binding of carbon dioxide occurs. When pCO₂ is low and pO₂ is high, as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place. The gas bound at the tissues is therefore delivered at the alveoli.

This reciprocal relationship is worth pausing over. The same haemoglobin molecule that picks up oxygen avidly in the lungs releases it in the tissues, and in doing so becomes more receptive to carbon dioxide. The reverse holds at the alveolar surface, where oxygen binding encourages the discharge of CO₂. The two gases are not competing for the same site so much as responding to the same shifting conditions of partial pressure, hydrogen ion concentration and temperature that the oxygen dissociation curve already made familiar.

The Dissolved Fraction in Plasma

About 7 per cent of carbon dioxide is carried in a dissolved state through plasma. This fraction is small in percentage terms but physiologically meaningful, because dissolved gas is the form that actually diffuses across membranes and the form whose partial pressure drives the entire system. The high solubility of CO₂ relative to oxygen means that a given difference in partial pressure moves far more CO₂ across the diffusion membrane than oxygen. This is why the dissolved fraction, though modest, remains the chemical signal to which the rest of the transport machinery responds.

The diffusion membrane itself is made up of three major layers: the thin squamous epithelium of the alveoli, the endothelium of alveolar capillaries, and the basement substance in between, composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer of endothelial cells. Its total thickness is much less than a millimetre, so all the factors in the body are favourable for diffusion of oxygen from alveoli to tissues and of carbon dioxide from tissues to alveoli.

Carbonic Anhydrase and the Bicarbonate Route

The bicarbonate route carries about 70 per cent of carbon dioxide, and it depends entirely on an enzyme. Red blood cells contain a very high concentration of carbonic anhydrase, and minute quantities of the same enzyme are present in the plasma too. This enzyme facilitates the reversible reaction between carbon dioxide and water on one side and carbonic acid, bicarbonate and hydrogen ions on the other. Without it, the reaction would proceed far too slowly to keep pace with the rate at which tissues produce CO₂ and the rate at which the lungs must clear it.

The Reversible Reaction at Tissue Level

At the tissue site, partial pressure of CO₂ is high because of catabolism. Carbon dioxide diffuses into blood, entering both red blood cells and plasma, and forms bicarbonate and hydrogen ions. The enzyme drives the reaction in the forward direction, and the resulting bicarbonate is carried in the plasma toward the lungs. The hydrogen ions produced along the way are buffered by haemoglobin, which is one reason the red cell is the natural home for this chemistry. The net effect is that a gas which is only slightly soluble in water is converted into a charged ion that dissolves readily.

The reaction can be written in the form the standard treatment uses, with the enzyme named above and below the arrow to indicate that it facilitates the conversion in both directions:

Read from left to right, this is the tissue-side reaction: carbon dioxide and water combine to form carbonic acid, which then dissociates into bicarbonate and a hydrogen ion. Read from right to left, it is the alveolar-side reaction. The double arrows matter, because the same enzyme catalyses both directions and the direction taken is decided entirely by the prevailing partial pressure of CO₂. At the tissues the high pCO₂ pushes the equilibrium to the right; at the alveoli the low pCO₂ pulls it back to the left.

Release at the Alveolar Site

At the alveolar site, partial pressure of CO₂ is low, so the reaction proceeds in the opposite direction, leading to the formation of carbon dioxide and water. Carbon dioxide trapped as bicarbonate at the tissue level and transported to the alveoli is thus released out as CO₂. This is the elegant part of the design: the gas is stored in a chemically stable form for the journey and regenerated only where it can be exhaled. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli, a figure that sits neatly beside the roughly 5 mL of oxygen that each 100 mL of oxygenated blood delivers to the tissues.

The symmetry is instructive. Oxygen is loaded at the lung surface and dissociated at the tissues; carbon dioxide is loaded at the tissues and discharged at the alveoli. The two gradients run in opposite directions, and the diffusion membrane is thin enough, well under a millimetre, for both to proceed without hindrance. Because the solubility of CO₂ is 20 to 25 times higher than that of oxygen, the amount of CO₂ that can diffuse through the membrane per unit difference in partial pressure is much higher than the corresponding figure for oxygen.

Partial Pressures at the Two Exchange Sites

The transport of carbon dioxide cannot be separated from the conditions that prevail at the two ends of the circuit. In the alveoli there is high pO₂, low pCO₂, lesser hydrogen ion concentration and lower temperature, and these factors are all favourable for the formation of oxyhaemoglobin. In the tissues, low pO₂, high pCO₂, high hydrogen ion concentration and higher temperature exist, and these conditions favour dissociation of oxygen from oxyhaemoglobin. The same set of conditions simultaneously governs the carbon dioxide side of the exchange, which is why the two gases are best studied together.

Conditions Favouring Binding and Release

When pCO₂ is high and pO₂ is low, as in the tissues, more binding of carbon dioxide to haemoglobin occurs. When pCO₂ is low and pO₂ is high, as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place. The oxygen dissociation curve, plotted as percentage saturation of haemoglobin with oxygen against pO₂, remains the reference tool for visualising how pCO₂ and hydrogen ion concentration interfere with oxygen binding. The carbon dioxide story mirrors it: the same variables that shift oxygen off haemoglobin in the tissues shift carbon dioxide onto it.

Temperature belongs in this list as well, and it is easy to overlook. Metabolically active tissue is warmer as well as more acidic and more charged with CO₂, and each of these conditions independently favours the unloading of oxygen and the loading of carbon dioxide. The body therefore does not need a separate control system for each gas; a single set of local conditions produced by metabolism accomplishes both adjustments at once. This is a recurring principle in respiratory physiology, and it explains why the two transport systems, though chemically distinct, behave as a coordinated pair.

The Diffusion Membrane and Solubility

The diffusion membrane is made up of three major layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between. Its total thickness is much less than a millimetre. Because the solubility of CO₂ is 20 to 25 times higher than that of oxygen, the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than that of oxygen. A gradient is present for CO₂ in the opposite direction to oxygen, running from tissues to blood and from blood to alveoli.

These structural facts explain why carbon dioxide exchange is rarely the limiting step in healthy lungs. The membrane is thin, the solubility is high, and the gradient is maintained by continuous production in the tissues and continuous removal at the alveoli. When the alveolar walls are damaged, as in emphysema, the respiratory surface decreases and this comfortable margin narrows. The consequences of that damage belong to a later discussion of respiratory disorders, but the connection to membrane surface area is worth noting here.

Quantitative Delivery and the Oxygen Comparison

Numbers anchor the lesson. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli, while every 100 mL of oxygenated blood can deliver around 5 mL of oxygen to the tissues under normal physiological conditions. These two figures are the practical output of the entire transport system, and they are close enough in magnitude to remind us that the body moves comparable volumes of both gases with every circulation. The carbon dioxide figure is the one to carry forward from this lesson.

Reading the Delivery Figures

The 4 mL figure refers to deoxygenated blood arriving at the lungs, and it represents the carbon dioxide that must be exhaled to keep the body in balance. If the tissues produce more CO₂ than this, as during vigorous exercise, the transport machinery has reserve capacity: more bicarbonate can be formed, more carbamino-haemoglobin can be loaded, and the respiratory rhythm can be adjusted. That adjustment is the subject of the next lesson, and the chemosensitive area sensitive to CO₂ and hydrogen ions will be introduced there.

It is worth noting how the three fractions cooperate to achieve the 4 mL total. The bicarbonate route carries the bulk, the carbamino route carries a substantial minority, and the dissolved route carries a small but essential remainder. No single route could handle the load alone, and the division of labour allows each to operate within its chemical limits. The dissolved fraction, small as it is, maintains the partial pressure that keeps the other two routes supplied with substrate.

Why the Comparison with Oxygen Matters

Placing the carbon dioxide figures beside the oxygen figures clarifies the design of the respiratory system. Oxygen is poorly soluble and must be carried almost entirely by haemoglobin, with only 3 per cent dissolved in plasma. Carbon dioxide is highly soluble and can afford to travel mostly as bicarbonate, a form that requires no binding site on haemoglobin at all. The body exploits solubility where it can and reserves the carrier protein for the gas that needs it most, which is a tidy economy of biological resources.

The comparison also explains why carbon dioxide is described as diffusing 20 to 25 times more readily than oxygen per unit difference in partial pressure. This does not mean the body is careless about CO₂; it means the physical chemistry gives the gas a head start, and the transport system is built to take advantage of that head start rather than to fight it. The result is a system in which oxygen delivery is the delicate operation and carbon dioxide removal is the robust one.

Going Further: Enzyme Kinetics and Bicarbonate Chemistry

The material above establishes what carbonic anhydrase does and why it matters. A short excursion into established background can deepen the picture without altering any of the numbers or terminology already given. Carbonic anhydrase is an enzyme that speeds up the conversion of carbon dioxide and water to bicarbonate and hydrogen ions, and its catalytic efficiency is among the highest known in biology. This is why the reaction can keep pace with metabolic demand despite the fact that uncatalysed hydration of CO₂ is comparatively slow.

The Enzyme in Context

Bicarbonate is the form in which most carbon dioxide travels in the blood, and the enzyme is the reason that form dominates. By accelerating both directions of the reaction, carbonic anhydrase allows the equilibrium to be reset almost instantly when blood reaches a region of different partial pressure. In the tissues the enzyme loads bicarbonate; in the alveoli it unloads it. The same molecule, present in high concentration inside red blood cells and in minute quantities in plasma, serves both ends of the circuit without modification.

A useful way to think about the enzyme is as a catalyst that removes a kinetic bottleneck rather than as a participant that changes the destination. The equilibrium position is still dictated by partial pressures and by the concentrations of the species involved; the enzyme only determines how quickly that position is reached. In a system where blood spends only a fraction of a second in the alveolar capillary, speed is not a luxury but a requirement, and the high concentration of the enzyme in red cells reflects that requirement.

Buffering and the Hydrogen Ion

Every bicarbonate ion formed at the tissues is accompanied by a hydrogen ion, and those ions must be managed. Haemoglobin buffers them, which is one reason the red cell is the natural site for the reaction. The buffering also links the carbon dioxide system to the oxygen system, because the same protein performs both functions. This interconnection is a theme that recurs throughout respiratory physiology, and it will surface again when the regulation of respiration is discussed in the next lesson.

The hydrogen ion concentration is itself one of the factors that interfere with the binding of oxygen to haemoglobin, alongside partial pressure of CO₂ and temperature. A rise in hydrogen ions in the tissues therefore does double duty: it encourages oxygen to dissociate and it accompanies the formation of bicarbonate from carbon dioxide. The body uses a single chemical signal, acidity, to coordinate two transport tasks, which is an economy that repays close study.

Preparing for the Regulation of Respiration

The transport of carbon dioxide is only half of a control loop. The gas must be carried, but the rate at which it is carried must also be adjusted to match production, and that adjustment is the work of the neural system. Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. A specialised centre in the medulla, the respiratory rhythm centre, is primarily responsible, and a pneumotaxic centre in the pons can moderate its functions by reducing the duration of inspiration.

From Transport to Control

A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO₂ and hydrogen ions, and an increase in these substances can activate it, which in turn signals the rhythm centre to make the necessary adjustments. Receptors associated with the aortic arch and carotid artery also recognise changes in CO₂ and hydrogen ion concentration and send signals to the rhythm centre. The role of oxygen in the regulation of respiratory rhythm is quite insignificant, which is a striking contrast with the central role that CO₂ plays.

This is why the present lesson matters beyond its own boundaries. The carbon dioxide that has been traced through carbamino-haemoglobin, bicarbonate and dissolved plasma is the very substance that the chemosensitive area monitors. Every adjustment of respiratory rhythm is, in effect, a response to the transport load described here. The next lesson will follow that signal into the brainstem, but the reader who has understood the three fractions and the reversible reaction will find the regulatory story far easier to follow.

Consolidating the Lesson

A final consolidation is worthwhile. Carbon dioxide travels as carbamino-haemoglobin at about 20 to 25 per cent, as bicarbonate at about 70 per cent, and dissolved in plasma at about 7 per cent. Carbonic anhydrase, present in high concentration in red blood cells and in minute quantities in plasma, facilitates the reversible reaction between CO₂ and water and between bicarbonate and hydrogen ions. At the tissues the reaction runs forward; at the alveoli it runs in reverse. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

These facts are best held together rather than memorised separately. The fractions, the enzyme, the reaction direction and the delivery figure all describe a single system responding to a single variable, the partial pressure of carbon dioxide. Once that variable is understood, the rest follows with a logic that makes the numbers easier to retain and the mechanism easier to explain. The lesson that follows will show how the body senses the same variable and adjusts the rhythm of breathing in response.