In this article we will discuss about:- 1. Meaning of Diffusion 2. Factors Controlling Diffusion 3. Diffusing Capacity for O2 and CO2.
Meaning of Diffusion:
Diffusion means movement of a substance from an area of high concentration to an area of low concentration. In the present context the diffusion of O2 from alveoli to pulmonary capillaries and of CO2 in the reverse direction is to be considered. N2 being metabolically inert may be left out of discussion.
The following points are to be noted in this connection:
i. Gases in the alveoli are dissolved in small quantity of alveolar fluid and are in equilibrium with partial pressure of the respective gases in alveolar air.
ii. Gases in the blood of pulmonary capillaries are also dissolved in water of the plasma where these exert a tension.
The average values of tension of O2 and CO2 in these two areas are given in Table 8.5.
Diffusion, therefore, takes place in the direction shown by the arrows through the alveolo-capillary membrane (Fig. 8.21) which consists of:
1. Alveolar epithelium-thin epithelial cells together with its basement membrane.
2. Thin interstitial space between (1) above and (3) below.
3. Capillary endothelium together with its membrane.
The alveolo-capillary membrane consisting of all these layers is very thin-the average thickness being about 0.5 µm. This membrane is freely permeable to respiratory gases and thus ensure rapid diffusion of O2 and CO2 through them in the direction shown by the arrows from the point of high pressure to the point of low pressure.
The total surface area of the alveolo-capillary membrane is approximately 70 sq. metres in an adult, indicating the large area available for diffusion. The total amount of blood in the lungs at any moment is about 150 ml. This small quantity of blood spread over this large area naturally facilitates diffusion.
Further the diameter of the pulmonary capillaries is about 8 µm – the red blood cell membrane, therefore, touches the capillary wall during their passage through the lungs and the gas molecules get almost direct access to the RBC without passing through significant layer of plasma-a fact which also accelerates the process.
Factors Controlling Diffusion:
Rates of diffusion of a gas between two given points depends not only on the tension gradient between the points but also on the distance between the two points, the surface area available for diffusion, the temperature and the characteristic properties of the gas itself.
The most important properties of a particular gas which determines its rate of diffusion are:
(a) Solubility, and
(b) Molecular weight.
Solubility of a gas in water is indicated by its solubility coefficient which may be defined as ml of gas (STPD) dissolved in 1 ml of water per atmosphere of pressure. When a liquid is exposed to a mixture of gases at a particular temperature-the pressure exerted by an individual component of gas will be related to its percentage composition (law of partial pressure). And the quantity dissolved in the fluid will be regulated by it.
The pressure of the dissolved gas in the fluid will be same as the partial pressure of the same gas in the atmosphere to which it is exposed and with which it is in tension equilibrium. The solubility coefficients of important gases at 0°C and 1 atmosphere of pressure are listed in Table 8.6.
This means that N2 is half as soluble in water as O2 under standardised condition of temperature and pressure. Undoubtedly, CO2 is most highly soluble of all the respiratory gases.
(b) Molecular Weight:
The second important property which determines the rate of diffusion is the molecular weight of the gas. The rate of diffusion of a gas at a particular temperature and pressure gradient is inversely related to the molecular weight (MW) of the gas and directly related to other factors, such as temperature.
Tension gradient remaining constant the rate of diffusion of a gas = S/√MW where S is the solubility coefficient and MW is the molecular weight. The diffusion coefficient of a gas has been defined as the volume in ml of a gas which will diffuse 0.001 mm distance over a sq. cm of surface per minute at a pressure of 1 atmosphere.
A more practical way of consideration of the problem is to assume the diffusion coefficient of O2 to be 1 and compare it with those of other gases of respiratory importance as given below:
Pulmonary Diffusing Capacity for O2 and CO2:
DO2 is the ratio of O2 transfer in millilitres per minute (VO2) from the alveolar gas to the capillaries divided by the pressure in mmHg required for this transfer.
DO2 = VO2/(PAO2 – PcO2)
PO2 indicates the average pressure of O2 in the pulmonary capillary which changes from moment to moment due to gas transfer in a non-linear manner and to obtain an accurate figure involves mathematical calculation and measurements of complicated nature.
For all practical purposes, therefore, CO diffusing capacity is measured and the result is multiplied by 1.23. The affinity of haemoglobin for CO is so great that with small concentration all the CO remains combined with haemoglobin so that the PCO of plasma is practically ‘zero’.
The above equation, therefore, is simplified to:
DCO = VCO/PACO and these can be measured with a CO analyser.
In a normal person DO2 at rest is approximately 15 to 20 ml of O2/minute/mm Hg pressure difference between alveolar air and mean capillary PO2.
This may be increased from 3 to 4 times during muscular exercise due to increase in the surface area for diffusion from opening up of new capillaries and alveoli and dilatation of the existing capillaries.
In various diseases, e g., interstitial fibrosis, pulmonary collagenosis there occur an increase in resistance to diffusion of O2 from alveoli to the capillaries. These diseases together constitutes alveolo-capillary block syndrome.
Diffusion of CO2:
CO2 is much more soluble than O2 and its diffusibility is at least 20 times higher than that of O2. An alveolo-arterial PCO2 difference never occurs and in alveolo-capillary block syndrome retention of CO2 in the blood does not occur but hypoxia is always prominent.