Exchange of Gases: Partial Pressures and Diffusion

Gas exchange in the human body is a study in quiet precision. Every breath draws a mixture of gases into the lungs, yet the body does not treat that mixture as a single substance. Instead, it responds to each gas individually, measuring the pressure that oxygen and carbon dioxide each contribute and moving them along gradients that never rest. This lesson examines that mechanism at its most fundamental level: the partial pressures that drive diffusion, the sites where exchange occurs, and the physical properties of the gases and membranes that determine how quickly the transfer proceeds.

The material builds directly on the pulmonary volumes and capacities used in clinical diagnosis, which describe how much air the lungs hold and move. Those measurements tell us about ventilation, the movement of air. What follows is the complementary question: once fresh air reaches the alveoli, how does oxygen leave the lung and enter the blood, and how does carbon dioxide make the reverse passage? The answer lies in simple diffusion, governed by pressure gradients and shaped by solubility and membrane thickness. These are the concepts that will carry the discussion forward into the structure of the diffusion membrane itself.

By the end of this lesson, you should be able to define partial pressure, read the standard values for ##pO_2## and ##pCO_2## at the atmosphere, alveoli, blood and tissues, and explain why the direction of diffusion differs for the two gases. You should also be able to state why carbon dioxide crosses the diffusion membrane far more readily than oxygen, and to identify the three layers that make up that membrane. Each of these points is drawn from the established treatment of respiratory physiology and will be reinforced with a short extension block at the close.

Alveoli as the Primary Sites of Gas Exchange

The exchange of gases is the functional purpose of the entire respiratory apparatus, and the alveoli are its primary sites. These small, thin-walled sacs cluster at the ends of the respiratory passages and present an enormous surface area to the air that reaches them. Exchange of gases also occurs between blood and tissues, where the same physical principles apply at the capillary level. In both locations, oxygen and carbon dioxide are exchanged by simple diffusion, a passive process that requires no energy expenditure by the cells involved and depends instead on the prevailing pressure or concentration gradients.

It is worth pausing on the word simple. Diffusion of this kind is the spontaneous movement of molecules from a region of higher concentration or pressure toward one of lower concentration or pressure. The body does not pump oxygen across the alveolar wall; it arranges conditions so that oxygen moves of its own accord. The same is true at the tissues, where carbon dioxide leaves the cells and enters the blood. This economy of design means that the respiratory system's task is essentially one of maintaining favourable gradients, a theme that recurs throughout the study of breathing.

Factors That Affect the Rate of Diffusion

Although the pressure or concentration gradient is the principal driving force, it is not the only factor that determines how rapidly a gas crosses from one compartment to another. The solubility of the gases matters a great deal, and so does the thickness of the membranes involved in diffusion. A gas that dissolves readily will cross a given membrane more quickly than one that dissolves poorly, and a thinner membrane permits faster transfer than a thicker one. These three considerations, gradient, solubility and membrane thickness, together govern the rate at which exchange proceeds in the lungs and tissues.

The material is careful to present these as factors that can affect the rate of diffusion rather than as independent mechanisms. They act together. A steep gradient across a thick membrane may deliver less gas than a modest gradient across a thin one, and a highly soluble gas may compensate for a smaller pressure difference. Understanding this interplay prepares the ground for the next lesson, which examines the diffusion membrane in detail, and it explains why the body's arrangements are described as favourable for exchange.

Exchange at the Alveoli and at the Tissues

At the alveoli, oxygen-rich air meets deoxygenated blood arriving from the tissues. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. At the tissues, the situation reverses: oxygenated blood delivers oxygen to cells that have been consuming it, and carbon dioxide produced by cellular activity passes from the tissues into the blood. In both settings the transfer is by simple diffusion along the prevailing gradients, and in both settings the same physical laws apply without modification.

This dual arrangement means that the blood acts as a shuttle between two exchange surfaces. It is loaded with oxygen at the lungs and unloaded at the tissues, while its relationship with carbon dioxide is exactly the opposite. The efficiency of this shuttle depends on the gradients being maintained at both ends, which in turn depends on continued ventilation of the alveoli and continued blood flow through the capillaries. The next sections quantify those gradients using partial pressures.

Partial Pressure and the Pressure Gradient

The pressure contributed by an individual gas in a mixture of gases is called its partial pressure. In the air we breathe, oxygen and carbon dioxide are present alongside nitrogen and other gases, and each contributes its own share to the total pressure. Partial pressure is represented as ##pO_2## for oxygen and ##pCO_2## for carbon dioxide. This concept is the key to understanding why gases move in particular directions, because diffusion responds to the partial pressure of each gas individually rather than to the total pressure of the mixture.

Once partial pressures are known at the relevant sites, the direction of diffusion follows directly. A gas moves from a region of higher partial pressure to one of lower partial pressure. The values in the standard table of partial pressures therefore do more than describe the composition of air and blood; they predict the movement of oxygen and carbon dioxide at every exchange surface in the body. Reading that table carefully is the central analytical skill of this lesson.

Partial Pressures in Atmosphere, Alveoli, Blood and Tissues

The standard values, expressed in millimetres of mercury, show how partial pressures change as air passes from the atmosphere into the body and as blood circulates between lungs and tissues. Atmospheric air carries oxygen at 159 mm Hg and carbon dioxide at only 0.3 mm Hg. In the alveoli, oxygen falls to 104 mm Hg while carbon dioxide rises to 40 mm Hg, reflecting the addition of carbon dioxide from the blood and the removal of oxygen into it. These figures establish the gradients that drive exchange at the lung surface.

In deoxygenated blood arriving at the lungs, oxygen stands at 40 mm Hg and carbon dioxide at 45 mm Hg. In oxygenated blood leaving the lungs, oxygen has risen to 95 mm Hg and carbon dioxide has fallen to 40 mm Hg. At the tissues, oxygen is 40 mm Hg and carbon dioxide is 45 mm Hg. The pattern is consistent: oxygen is highest in the alveoli and lowest in the tissues, while carbon dioxide is highest in the tissues and lowest in the alveoli.

Reading the Gradients for Oxygen and Carbon Dioxide

The data clearly indicate a concentration gradient for oxygen from alveoli to blood and from blood to tissues. Alveolar oxygen at 104 mm Hg exceeds the 40 mm Hg of deoxygenated blood, so oxygen diffuses into the blood. Oxygenated blood at 95 mm Hg then exceeds the 40 mm Hg of the tissues, so oxygen diffuses onward into the tissues. Each step follows the same rule: movement from higher to lower partial pressure, with no active transport required at either site.

For carbon dioxide, a gradient is present in the opposite direction, from tissues to blood and from blood to alveoli. Tissue carbon dioxide at 45 mm Hg exceeds that of oxygenated blood at 40 mm Hg, so carbon dioxide enters the blood. Deoxygenated blood at 45 mm Hg then exceeds alveolar carbon dioxide at 40 mm Hg, so carbon dioxide passes into the alveoli for exhalation. The two gases thus travel in counterflow, each driven by its own gradient.

Solubility and the Diffusion Membrane

Gradients alone do not tell the whole story. The solubility of the gases, as well as the thickness of the membranes involved in diffusion, are important factors that can affect the rate of diffusion. Carbon dioxide is 20 to 25 times more soluble than oxygen, and this difference has a striking consequence. The amount of carbon dioxide that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than the corresponding amount of oxygen. Solubility, in other words, amplifies the effect of a given gradient.

This asymmetry explains an apparent puzzle. The partial pressure gradient for carbon dioxide is far smaller than that for oxygen, yet carbon dioxide is cleared from the blood efficiently. Its greater solubility compensates for the modest gradient. Oxygen, by contrast, faces a much steeper gradient but crosses less readily per unit of pressure difference. The body's arrangements are therefore described as favourable for both gases, each accommodated according to its own physical character.

The Three Layers of the Diffusion Membrane

The diffusion membrane is made up of three major layers. The first is the thin squamous epithelium of the alveoli. The second is the endothelium of the alveolar capillaries. Between them lies the basement substance, composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer of endothelial cells of the capillaries. This three-layered arrangement is the structural basis of gas exchange, and its organisation places the air and the blood in extremely close proximity.

Despite comprising three layers, the total thickness of the diffusion membrane is much less than a millimetre. This thinness is a decisive advantage, because the rate of diffusion rises as the membrane becomes thinner. Combined with the large surface area of the alveoli and the favourable gradients already described, the thin membrane ensures that all the factors in the body are favourable for diffusion of oxygen from alveoli to tissues and of carbon dioxide from tissues to alveoli.

Why Carbon Dioxide Crosses More Readily Than Oxygen

The comparison between the two gases is one of the most instructive points in the study of exchange. Carbon dioxide is 20 to 25 times more soluble than oxygen, so the amount of carbon dioxide that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than that of oxygen. A small gradient is therefore sufficient to move a substantial quantity of carbon dioxide, whereas oxygen requires a much larger gradient to achieve a comparable transfer.

This has practical implications for how we interpret the partial pressure table. The modest 5 mm Hg difference in carbon dioxide between tissues and blood, and between blood and alveoli, is not a sign of weak exchange. It reflects the gas's high solubility, which allows efficient transfer at low gradients. Oxygen, less soluble, needs the steep fall from 104 to 40 mm Hg at the lungs and from 95 to 40 mm Hg at the tissues to sustain adequate delivery.

Connecting Exchange to Transport

Diffusion at the alveoli and tissues is only the beginning of the journey. Once oxygen enters the blood, it must be carried to the tissues, and once carbon dioxide enters the blood, it must be carried to the lungs. Blood is the medium of transport for both gases. About 97 per cent of oxygen is transported by red blood cells, with the remaining 3 per cent carried in a dissolved state through the plasma. Carbon dioxide follows a different distribution, which the next lesson's material will develop in full.

The link between exchange and transport is the partial pressure itself. Binding of oxygen with haemoglobin is primarily related to partial pressure of oxygen, and the binding of carbon dioxide is related to the partial pressure of carbon dioxide. The gradients established at the exchange surfaces therefore do double duty: they drive diffusion and they set the conditions for chemical combination with haemoglobin. This is why the partial pressure table is foundational rather than merely descriptive.

From Alveolar Gradient to Haemoglobin Binding

In the alveoli, where there is high ##pO_2##, low ##pCO_2##, lesser hydrogen ion concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin. In the tissues, where low ##pO_2##, high ##pCO_2##, high hydrogen ion concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from oxyhaemoglobin. Oxygen therefore binds to haemoglobin at the lung surface and dissociates at the tissues, a cycle repeated with every circulation of the blood.

For carbon dioxide, the relationship is complementary. When ##pCO_2## is high and ##pO_2## is low, as in the tissues, more binding of carbon dioxide occurs. When ##pCO_2## is low and ##pO_2## is high, as in the alveoli, dissociation occurs. The same gradients that drive simple diffusion thus also govern the reversible chemical associations that allow the blood to carry far more gas than plasma alone could dissolve.

Quantifying Exchange and Its Limits

The material provides a useful benchmark for the scale of oxygen delivery. Every 100 ml of oxygenated blood can deliver around 5 ml of oxygen to the tissues under normal physiological conditions. This figure connects the abstract partial pressures to a concrete physiological outcome, and it allows us to reason about the adequacy of exchange. If the gradients were reduced, or the membrane thickened, the volume delivered per 100 ml of blood would fall, with consequences for tissue function.

Such reasoning is the basis of clinical interpretation. The pulmonary capacities studied earlier describe the volume of air the lungs can move; the partial pressures and diffusion factors studied here describe how effectively that air refreshes the blood. Together they form a complete picture of respiratory function, one addressing ventilation and the other addressing exchange. Neither alone is sufficient to explain how the body sustains its oxygen supply.

Relating Partial Pressure Differences to Gas Transfer

The relationship between gradient and transfer can be expressed compactly. For a gas crossing a membrane, the rate of diffusion is proportional to the product of the surface area and the partial pressure difference, and inversely proportional to the membrane thickness. Written as a proportionality, this is:

Here ##A## is the surface area available for diffusion, ##\Delta p## is the partial pressure difference across the membrane, and ##T## is the membrane thickness. Solubility enters as an additional proportionality factor specific to each gas. This expression summarises why the alveoli, with their large area and thin walls, are such effective exchange surfaces, and why a fall in ##\Delta p## or a rise in ##T## impairs transfer.

Applying the Numbers to Oxygen and Carbon Dioxide

Using the standard values, the alveolar-to-blood gradient for oxygen is ##104 - 40 = 64## mm Hg, while the blood-to-tissue gradient is ##95 - 40 = 55## mm Hg. For carbon dioxide, the tissue-to-blood gradient is ##45 - 40 = 5## mm Hg, and the blood-to-alveolar gradient is ##45 - 40 = 5## mm Hg. The oxygen gradients are more than ten times larger than those for carbon dioxide, yet carbon dioxide still transfers efficiently because of its far greater solubility.

These simple subtractions make the counterflow pattern unmistakable. Oxygen descends in steps from atmosphere to alveoli to blood to tissues, while carbon dioxide ascends from tissues to blood to alveoli. The arithmetic also shows why the body cannot rely on gradients alone: if solubility were equal, the small carbon dioxide gradients would permit only sluggish transfer. Solubility is the property that makes the carbon dioxide system work at such modest pressure differences.

Going Further

Two background ideas help to place this material in a wider context. First, partial pressure is the pressure a single gas contributes to the total pressure of a gas mixture. In a mixture, each gas behaves as though it alone occupied the volume, and its partial pressure reflects its share of the molecules. This is why the oxygen in atmospheric air can be described by a single number, 159 mm Hg, even though air contains several gases. The concept allows us to speak precisely about the driving force for each gas independently.

Second, gases diffuse from a region of higher partial pressure to one of lower partial pressure. This is the general rule that underlies every gradient in the table. It applies equally to oxygen leaving the alveoli and to carbon dioxide entering them, and it applies at the tissues in the reverse directions. Once these two ideas are secure, the specific values and directions in respiratory physiology become a straightforward application of a general physical principle rather than a set of facts to be memorised.

Extending the Model to Clinical Reasoning

With the model in place, clinical reasoning becomes possible. A thickened diffusion membrane, whatever its cause, reduces the rate of transfer for both gases, and oxygen suffers more because it lacks carbon dioxide's solubility advantage. A reduced alveolar ##pO_2## lowers the gradient into the blood, and a reduced blood flow through the alveolar capillaries limits the quantity of blood that can be loaded. Each of these situations can be analysed using the same three factors: gradient, solubility and membrane thickness.

The next lesson examines the diffusion membrane in closer structural detail, building on the three layers introduced here. The present lesson has established the driving forces and the physical constraints; the next will show how the membrane's organisation supports the exchange described. Keeping the partial pressure values in mind will make that structural discussion easier to follow, because every layer of the membrane exists to serve the gradients we have just quantified.

Practice and Self-Assessment

Consolidating this lesson requires more than reading the table. A useful exercise is to reconstruct the partial pressure values from memory and then check each gradient by subtraction, confirming that oxygen falls and carbon dioxide rises in the expected directions. Another is to explain, in a few sentences, why carbon dioxide transfers efficiently despite its small gradients. If you can supply the solubility figure and connect it to the concept of transfer per unit pressure difference, you have grasped the central argument of the lesson.

A second exercise is to reason about a hypothetical change. Suppose the diffusion membrane became thicker while all partial pressures remained the same. Which gas would be affected more, and why? The answer follows from the factors discussed: both gases would transfer more slowly, but oxygen, lacking carbon dioxide's solubility advantage, would be more vulnerable. This kind of reasoning, grounded in the three factors of gradient, solubility and thickness, is what the material is designed to develop.

Common Misconceptions to Avoid

One frequent error is to assume that the larger oxygen gradient means oxygen is always the more rapidly exchanged gas. In fact, the amount of carbon dioxide that can diffuse per unit difference in partial pressure is much higher than that of oxygen, precisely because of its greater solubility. The gradients and the solubilities pull in opposite directions, and the net result is efficient exchange for both gases. Treating gradient size alone as a measure of transfer would misrepresent the physiology.

Another misconception is to imagine that diffusion requires active pumping by the lung. The material is explicit that exchange occurs by simple diffusion, a passive process. The body's contribution is to maintain the gradients through ventilation and blood flow, and to keep the diffusion membrane thin. Recognising this division of labour, passive transfer driven by maintained gradients, clarifies why respiratory failure can arise from problems with ventilation, with blood flow, or with the membrane itself.

Preparing for the Diffusion Membrane

The next lesson turns to the diffusion membrane as a structure. The three layers introduced here, the squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them, will be examined in greater detail. The key point to carry forward is that the total thickness is much less than a millimetre, a feature that, together with the large alveolar surface area, makes the membrane well suited to its task.

It is also worth carrying forward the idea that all the factors in the body are favourable for diffusion of oxygen from alveoli to tissues and of carbon dioxide from tissues to alveoli. This statement is not a vague reassurance but a summary of specific conditions: steep gradients, high solubility for carbon dioxide, and a thin membrane. The next lesson will show how the membrane's structure sustains these conditions, completing the picture of exchange that this lesson has begun.