Boyle\'s Dalton\'s and Henry\'s Laws

¶ … relationship among Boyle's, Dalton's, and Henry's Laws and the physiology of the lung. Robert Boyle investigated the relationship between the volume of a dry ideal gas and its pressure. Since there are four variables that can be altered in a gas sample, in order to investigate how one variable will affect another, all other variables must be held constant or fixed. Boyle fixed the amount of gas and its temperature during his investigation. He found that when he manipulated the pressure that the volume responded in the opposite direction. For example, when Boyle increased the pressure on a gas sample, the volume would decrease.

A physiological example of Boyle's Law is the action of the diaphragm. This muscle is located just below the lungs. When one inhales, the diaphragm moves downward allowing the lungs an increased volume. Consequently, this decreases the pressure inside the lungs so that the pressure is less than the outer pressure. As a result, air is forced into the lungs. When one exhales, the diaphragm moves upward and decreases the volume of the lungs. This increases the pressure inside the lungs above the pressure on the outside of the lungs so that gases are forced out of the lungs. Pathologic conditions such as acute diaphragmatic injury may result in respiration difficulties due to incomplete ventilatory volumes.

John Dalton studied the effect of gases in a mixture. He observed that the total pressure of a gas mixture was the sum of the partial pressure of each gas as illustrated below:

Pressure (P) total = P1 + P2 + P3 + Pn

The partial pressure is defined as the pressure of a single gas in a mixture. Dalton maintained that since there was an enormous amount of space between gas molecules within a mixture, the gas molecules had no influence on the motion of other gas molecules. Therefore, the pressure of a gas sample would be the same whether it was the only gas in the container or if it were among other gases. However, lowering the temperature and/or compressing the gas will upset that assumption.

Henry's Law states that the amount of any gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in that particular liquid. That is, as the pressure of any gas increases, more of that gas will dissolve into any solution with which it is in free contact.

Taken together, Henry's and Dalton's laws predict two very important consequences:

When ambient pressure is lowered as at altitude, the partial pressure of oxygen and nitrogen in the body must fall, and there will be less molecules of each gas dissolved in the blood and tissues (Wagner, 1993). An example of this phenomenon is altitude sickness, which may occur at altitudes above 7000 feet.