Physical laws related to gases Gases diffuse from an area of Higher concentration gradient to an area of lower concentration gradient. The pressure with which a gas tends to come out of the gas mixture is called partial pressure. Total partial pressure of a gas mixture is the sum of partial pressures of the gases in the mixture e.g. atmospheric pressure is the sum of partial pressure of gases in atmosphere. Pressure is caused by multiple impacts of moving molecules against a surface. Partial pressure of a gas is directly proportional to the concentration of the gas molecules Concentration of dissolved gas = Pressure X solubility coefficient Or Partial Pressure of a gas = Concentration of dissolved gas/ solubility coefficient Concentration of dissolved gas = Pressure X solubility coefficient Or Partial Pressure of a gas = Concentration of dissolved gas/ solubility coefficient If the partial pressure is greater in the gas phase in the alveoli, as is normally true for oxygen, then more molecules will diffuse into the blood than in the other direction. If the partial pressure of the gas is greater in the dissolved state in the blood, which is normally true for carbon dioxide, then net diffusion will occur toward the gas phase in the alveoli At normal body temperature of 37°C the vapor pressure is 47 mm of Hg. The vapor pressure of water depends entirely on the temperature of the water. The greater the temperature, the greater the kinetic activity of the molecules and, therefore, the greater the likelihood that the water molecules will escape from the surface of the water into the gas phaseIt is defined as: “ It is a constant, which is the measure of a substance diffusing through the concentration gradient.”

1. PHYSICAL LAWS
RELATED TO GASES
Source: The Guyton and Hall physiology
Maryam Fida (o-1827)

2. DIFFUSION OF GASES
 Gases diffuse from an area of Higher
concentration gradient to an area of
lower concentration gradient.

3. PARTIAL PRESSURE OF
GASES
 The pressure with which a gas tends
to come out of the gas mixture is
called partial pressure.
 Total partial pressure of a gas mixture
is the sum of partial pressures of the
gases in the mixture e.g. atmospheric
pressure is the sum of partial pressure
of gases in atmosphere.

4. PARTIAL PRESSURE OF
GASES
 Pressure is caused by multiple
impacts of moving molecules against
a surface.
 Partial pressure of a gas is directly
proportional to the concentration of
the gas molecules

5. HENRY’s LAW OF DISSOLVED
GASES
 Concentration of dissolved gas =
Pressure X solubility coefficient
Or
 Partial Pressure of a gas =
Concentration of dissolved gas/
solubility coefficient

6. Solubility coefficients of Gases
 When partial pressure is expressed in atmospheres (1 atmosphere
pressure equals 760 mm Hg) and concentration is expressed in volume
of gas dissolved in each volume of water, the solubility coefficients for
important respiratory gases at body temperature are the as follows:

7. Diffusion of Gases Between the Gas Phase in the
Alveoli and the Dissolved Phase in the pulmonary
Blood
 If the partial pressure is greater in the gas phase in
the alveoli, as is normally true for oxygen, then
more molecules will diffuse into the blood than in
the other direction.
 If the partial pressure of the gas is greater in the
dissolved state in the blood, which is normally true
for carbon dioxide, then net diffusion will occur
toward the gas phase in the alveoli.

8. Vapor Pressure of Water
 At normal body temperature of 37°C the
vapor pressure is 47 mm of Hg.
 The vapor pressure of water depends
entirely on the temperature of the water.
 The greater the temperature, the greater
the kinetic activity of the molecules and,
therefore, the greater the likelihood that
the water molecules will escape from the
surface of the water into the gas phase.

9. DIFFUSION CAPACITY
 It is defined as:
“ The volume of gas that diffuses through the respiratory
membrane each minute for a pressure gradient of 1 mm Hg.”
For Oxygen it is 20-30 ml/min/mm of Hg at rest and during
exercise it is 60 ml/min/mm of Hg.
Diffusing capacity for CO2 is 20 times greater so it will be 400-
600 ml/min/mm of Hg at rest and 1300-1400 ml/min/mm of Hg
during exercise.
Diffusing capacity increases during exercise because of
opening of more alveoli.
Diffusing capacity decreases in pulmonary fibrosis and
sarcoidosis.

10. Factors affecting diffusion
capacity
 P = Difference of partial pressure of gas
 S = Solubility of the gas in the fluid
 A = Cross-sectional area of the fluid
 d = Distance through which the gas must
diffuse
 MW = Molecular weight of the gas
 Temperature of the fluid

11. Diffusion Coefficient of Gas
 It is defined as:
“ It is a constant, which is the measure
of a substance diffusing through the
concentration gradient.”
Diffusion coefficient =

13. Measurement of Diffusing
capacity of oxygen
 Carbon monoxide method

14. BOYLE’S LAW
 Volume of a gas is inversely
proportional to the pressure applied on
it.

15. CHARLE’S LAW
 At a constant pressure the volume of
gas is directly proportional to
temperature.

16. AVOGADRO’s LAW
 Equal volume of all gases at constant
pressure and temperature has equal
number of molecules.

17. IDEAL GAS LAW
Pressure = n RT/V
Where
 n is the number of molecules
 R is gas constant
 T is temperature
 V is volume

18. STRUCTURE OF
RESPIRATORY MEMBRANE

20. Six layers of respiratory membrane
from inside of alveolus to outside
 Thin layer of fluid containing surfactant
 Alveolar epithelium
 Basement membrane
 Narrow interstitial space
 Basement membrane of c capillary endothelium
 Endothelial cell layer of capillary endothelium

21.  Despite the large number of layers, the overall
thickness of the respiratory membrane in some
areas is as little as 0.2 micrometer, and it averages
about 0.6 micrometer, except where there are cell
nuclei.
 Total surface area of the RM is 70 to 100 square
meter