Comprehensive Study Notes on Alveolo-Capillary Diffusion

Definition and Principles of Alveolo-Capillary Diffusion

Alveolo-capillary diffusion is defined as the passive movement of gas molecules from areas of high partial pressure toward areas of lower partial pressure. This process represents the transfer of gas across the air-blood barrier, also known as the alveolo-capillary membrane. In the context of the lung, this mechanism allows oxygen to enter the blood and carbon dioxide to leave it, following their respective pressure gradients.

The surface of the blood-gas barrier in the lung is considerable, measuring between $50\,m^2$ and $100\,m^2$. Its thickness is notably minute, being less than $0.5\,\mu$. These specific dimensions—a vast surface area combined with an extremely thin barrier—are considered ideal for the process of diffusion. Furthermore, gas transfer depends on the solubility properties of the gas within the membrane. Specifically, carbon dioxide ($CO_2$) is 20 times more diffusible than oxygen ($O_2$) due to its significantly greater solubility.

The Laws of Diffusion

Diffusion within the lungs is described by Fick's Laws. The law states that the transfer of gas across a permeable membrane is proportional to the surface area of that membrane and the difference in partial pressure on either side of the membrane, while being inversely proportional to the thickness of the tissue. This relationship is expressed by the following formula:

$V_{gaz} = D \times S \frac{(PA_g - Pc_g)}{E}$

In this equation, $V_{gaz}$ represents the quantity of gas transported per unit of time. The variable $D$ stands for the diffusion constant, which depends on the properties of the gas and the membrane. $S$ corresponds to the surface area available for diffusion, while $E$ represents the thickness of the barrier. The term $(PA_g - Pc_g)$ reflects the partial pressure gradient, where $PA_g$ is the alveolar partial pressure of the gas and $Pc_g$ is the capillary partial pressure of the gas.

Oxygen Transfer and Capillary Transit Time

The passage of red blood cells (RBCs) through the pulmonary capillaries typically lasts approximately $3/4$ of a second. When a red blood cell enters a capillary, the initial partial pressure of oxygen ($PO_2$) is approximately $40\,mmHg$. On the other side of the blood-gas barrier, which is less than $0.5\,\mu$ away, the alveolar partial pressure of oxygen ($PAO_2$) is approximately $100\,mmHg$. Oxygen follows this significant pressure gradient, causing the $PO_2$ within the red blood cell to rise rapidly.

The internal $PO_2$ of the red blood cell reaches nearly the same level as the alveolar gas $PO_2$ by the time the cell has traversed only $1/3$ of the capillary length. Under normal circumstances, the difference between alveolar oxygen partial pressure and end-capillary oxygen partial pressure is extremely small, often consisting of only a simple fraction of a $mmHg$.

Factors Affecting Diffusion: Exercise, Altitude, and Pathology

During intensive muscular exercise, pulmonary blood flow is greatly increased. Consequently, the time normally spent by a red blood cell in the capillary—approximately $3/4\,s$—may be reduced to as little as $1/3$ of its normal value. In normal subjects breathing room air, there is still no appreciable decrease in end-capillary $PO_2$, even with this reduction in available oxygenation time.

However, if the blood-gas barrier is significantly thickened, the diffusion of $O_2$ is perturbed. In such cases, the rate of increase of $PO_2$ within the red blood cells becomes slow. The blood $PO_2$ may fail to reach the level of the alveolar gas before the oxygenation period in the capillary expires, resulting in a notable difference between alveolar $PO_2$ and end-capillary $PO_2$.

Diffusion properties can also be disturbed by lowering the alveolar $PO_2$. Suppose the $PAO_2$ is reduced to $50\,mmHg$, either through ascent to high altitude or by breathing a gaseous mixture poor in $O_2$. Under these conditions, the $PO_2$ in the red blood cell at the start of the capillary might be approximately $20\,mmHg$. The partial pressure difference responsible for driving oxygen across the barrier is reduced from $60\,mmHg$ to $30\,mmHg$. Consequently, oxygen crosses the barrier more slowly, the rise in capillary $PO_2$ is sluggish, and it is more likely that the blood will be unable to reach the level of alveolar $PO_2$. Thus, exhausting muscular exercise at very high altitude is one of the rare circumstances where clear oxygen diffusion disorders appear in normal subjects. Similarly, a patient with a thickened blood-gas barrier will more easily develop diffusion disorders if they breathe an oxygen-poor mixture, particularly during exercise.

Measurement of Lung Diffusion Capacity (DLCO)

The transfer of carbon monoxide ($CO$) is limited by diffusion, which is why this gas is employed to measure the diffusion properties of the lung. Applying Fick's Law to the complex structure of the lung, where it is impossible to measure exact surface area and thickness during life, the equation is modified. The expression $D \times \frac{S}{E}$ is replaced by a single term, $DL$, yielding:

$V_{gaz} = DL \times (PA_g - Pc_g)$

In this equation, $DL$ represents the "Diffusion Capacity of the Lung." This symbol accounts for the surface area, thickness, and properties of both the tissue layer and the gas. The diffusion capacity for oxygen ($DLO_2$) is defined as the quantity of $O_2$ crossing the membrane per unit of time for a transmembrane pressure gradient of $1\,mmHg$. For carbon monoxide, the formula is:

$DL_{CO} = \frac{V_{CO}}{PA_{CO} - Pc_{CO}}$

Here, $PA_{CO}$ and $Pc_{CO}$ represent the alveolar and capillary partial pressures of $CO$, respectively. Because the partial pressure of $CO$ in the capillary blood is so low it can be neglected, the formula is simplified to:

$DL_{CO} = \frac{V_{CO}}{PA_{CO}}$

$DL_{CO}$ is expressed in $ml/min/mmHg$ of alveolar partial pressure. The normal value of $DL_{CO}$ at rest is approximately $25\,ml/min/mmHg$. This value increases by $2$ to $3$ times during muscular exercise.

Resistance and Reaction Kinetics with Hemoglobin

Respiratory gases diffuse from the alveoli to the blood in three distinct stages. The first stage is passive physical diffusion through the alveolar space to the alveolo-capillary membrane. The second stage involves passive diffusion across the alveolo-capillary membrane, the plasma, and the wall of the red blood cells. The third stage is the chemical combination of $O_2$ (or $CO$) with hemoglobin ($Hb$) and the corresponding release of $CO_2$.

The inverse of diffusion capacity ($1/DL$) represents the quotient of the pressure difference by the volume of gas transported, which is analogous to electrical resistance. The total resistance to diffusion is the sum of the resistance of the blood-gas barrier ($1/DM$, where $M$ stands for membrane) and the resistance related to the reaction rate with hemoglobin. Let $\theta$ be the rate of the reaction of $O_2$ with $Hb$ (measured in $mlO_2/min/mmHg/ml$ of blood) and $Vc$ be the volume of capillary blood. The effective diffusion capacity of the reaction rate is $\theta \times Vc$, and its resistance is $\frac{1}{(\theta \times Vc)}$. The total resistance equation is written as:

$\frac{1}{DL} = \frac{1}{DM} + \frac{1}{(\theta \times Vc)}$

In practice, the resistances offered by the membrane and by the blood components are approximately equal. Therefore, a pathological decrease in capillary blood volume can decrease the overall diffusion capacity of the lung.

Carbon Dioxide Diffusion Dynamics

For carbon dioxide ($CO_2$), the capillary blood partial pressure ($PcCO_2$) is approximately $45\,mmHg$, while the alveolar gas partial pressure ($PACO_2$) is approximately $40\,mmHg$. Under normal conditions, the time required for $PcCO_2$ to reach equilibrium with $PACO_2$ is similar to that required for oxygen; the blood almost reaches alveolar levels by the time the red blood cell has traveled $1/3$ of the capillary length. However, when the diffusion capacity of the membrane is reduced to $1/4$ of its normal value, a small but detectable difference remains between the end-capillary blood and the alveolar gas, demonstrating the influence of a thickened blood-gas barrier on $CO_2$ elimination.