The gas laws in respiratory therapy

The gas laws in respiratory therapy

A good knowledge of the basic principles on which respiratory therapy works is essential as the subset of critical medical care cannot be overemphasized. The type of care that respiratory therapists give requires a in-and-out knowledge of the basic gas laws.


Boyle’s law demonstrates the relationship between the volume and density of a gaseous substance to the pressure. This is on the condition that the temperature of the gas is not varied.

The law states thus ‘the volume of a given mass of gas is inversely proportional to the pressure and the density of the gas is directly proportional to the pressure’

In respiratory therapy the implication of this law is that as the pressure around a patient is increased, the volume of gas in any gas filled space in the body increases; in this case, the lung is a good point of contact.

This is essentially important in the administration of hyperbaric oxygen to patients. as the pressure around the patient is increased, the volume of gas in the alveoli increase. Moreover, the greater the increase in pressure around the patient; the greater is the increase in the volume of oxygen in the alveoli. Conversely, the volume of each alveolus decreases as the pressure in the hyperbaric chamber is reduced.


Dalton’s law is used to explain the relationship of the pressure of individual gases in a mixture of gases. it is also called the Dalton’s law of partial pressure. in short, the total  pressure of a mixture of gases equals the partial pressures exerted by the weight of the individual constituents of the mixture.

A practical application of this is the relative percentages of the different fractions of gases that constitute the atmosphere. The atmospheric pressure in essence is a sum of all the fractional pressures exerted by the different gases namely: Oxygen, Nitrogen, Carbon Dioxide and the other rare gases.

With the principle of the Dalton’s law in mind, the partial pressure of Oxygen in a gas mixture can be increased. A 100% Oxygen given to the patient means that the pressure has been increased four times since the percentage of Oxygen in the atmosphere is 21%. This can go a long way in increasing the perfusion of Oxygen in the alveoli.

both the Boyle and Dalton’s laws can be utilized in hyperbaric Oxygen therapy. According to Dalton’s law, it is possible to increase the concentration of inspired Oxygen in the air if  a 100% oxygen is delivered to the lung alveoli and at the same time, the pressure of the surrounding air is increased.

Boyle’s law on the other hand can be interpreted to mean that as the pressure of the surrounding is increased, the volume of the alveoli decreases, but concomitantly increases the concentration of the gas molecules (this is the density)

In essence, a combination of these two laws deliver more gas molecules into the alveolus that can therefore diffuse from the alveolus into the blood to be transported in the circulatory system.


this law explains the relationship that exists between the pressure of a gas (i.e. its concentration) and the way the gas moves.

The states that “the rate of diffusion of a gas is inversely proportional to the square roof of their diffusion coefficients”

In the lungs, Oxygen must diffuse into the blood and Carbon Dioxide must diffuse out. This exchange occurs in the thin membrane of the alveolus of the terminal bronchioles which is a semi-permeable membrane.

In practical terms, this law can be exploited to deliver more oxygen molecules to the alveolar space just by increasing the density of the oxygen molecules. This is because the increase in the number of Oxygen molecules would interact more with the alveolar membrane. This would facilitate a better exchange of gases in the alveoli.

Henry’s Law

This law relates the amount of a gaseous substance that can be dissolved in a liquid to the pressure of the gas above the liquid.

The law states that “the solubility of a gas in a liquid is directly proportional to the pressure of the gas in contact with the liquid (Given a constant temperature and a state of equilibrium between the liquid and the gas)”

IN the body, the fluid can be taken as Blood. After oxygen is dissolved in the blood, it combines with Haemoglobin to become Oxy-Haemoglobin. This form of hemoglobin can easily release Oxygen to the tissues. However, the hemoglobin has a limited oxygen carrying capacity. Beyond this, no more Oxygen molecules can be attached to it. However, the plasma is capable of dissolving some amount of Oxygen. This serves as the alternative route of oxygen administration in hyperbaric Oxygen therapy. This is because by increasing the pressure of oxygen in the alveolus, more Oxygen molecules can actually bind with the plasma, this is after the Hemoglobin is already saturated with Oxygen. The Oxygen in the plasma can therefore be released when it gets to the tissue.

From the above laws it can be deduced that all the gas come into play in the dynamics of respiratory therapy. The effect of the individual laws cannot be isolated from one another. A combination of all the laws explain the actions that are performed in the respiratory system as regards the exchange or gases in the lungs, circulating round the body and delivered to the tissues.


K A. Wyka et al (2001). Foundations of Respiratory Care
L. O. Brown. (1992) Basics of Respiratory Medicine. Third Edition. Infinity Press, Australia.
Repiratory Medicine Online. The Gas Laws <http://resources.

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