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Gas Laws in Respiration

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GAS LAWS
To understand the behavior of gases one should be able to explain 4 gas laws.
1. BOYLE’S GAS LAW
2. DALTON’S LAW OF PARTIAL PRESSURE
3. HENRY’S GAS LAW
4. GRAHAM’S LAW
BOYLE’S GAS LAW
P1V1 = P2V2
The relationship between the pressure and volume of gases is given by Boyle’s law. It states that
when the temperature is constant, the pressure of a gas varies inversely with its volume.
As volume increases, pressure decreases. As volume decreases, pressure will increase.
Boyle’s Law
• Inhalation:
Diaphragm contracts/flattens, rib muscles contract raising the rib cage, creating low pressure in
the chest cavity. The lungs respond by expanding, creating a low pressure in the lungs and the air
rushes from high partial pressure to low partial pressure into the lungs.
Exhalation:
• Diaphragm relaxes into dome-shape, rib muscles relax lowering the rib cage, creating high
pressure in the chest cavity. The lungs respond by reducing in size, creating a high pressure in the
lungs. The air rushes from high partial pressure to low partial pressure out the lungs.
DALTON’S LAW OF PARTIAL PRESSURE
The partial pressure of a gas in a gas mixture as the pressure that the gas would exert if it occupied
the total volume of the mixture in the absence of the other components, it means Dalton's law
follows directly from the Ideal Gas Law since it states that the pressure exerted by a gas is
proportional to the number of moles of that gas. Thus,

Total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by
each gas in the mixture. Ptotal = P1 + P2+ P3…

Since, by definition, PB is the total pressure of all gases in a mixture in contact with the
atmospheric pressure. For instance, at sea level, barometric pressure is 760 mm Hg, and oxygen
makes up 21% of dry air. Thus, the partial pressure of oxygen in dry air is PO2 = 0.21 × 760 mm Hg =
160 mm Hg.

Henry’s Law
When a liquid is put in contact with a gas phase of partial pressure, Pgas, gas will dissolve in the
liquid until equilibrium is reached between the two phases. The condition for equilibrium of a gas
between a gas phase and a liquid phase is that the partial pressures of the gas are equal in the two
phases, not the concentrations, as is the more familiar case for non-gaseous solutes. Thus, at
equilibrium, Pliquid = Pgas rather than Cliquid = Cgas. This is an important principle in
understanding gas exchange between the alveolar space (gas phase) and the pulmonary capillary
blood (liquid phase). In general, gases diffuse between sites where there is a difference in partial
pressure (e.g., red blood cell and plasma, ISF and cell cytoplasm), not in concentration. Thus, gas
exchange takes place between the two phases as long as there is a partial pressure difference
between them. Once equilibrium is reached, net gas exchange ceases.
A mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in to its partial
pressure and its solubility coefficient.
Graham’s Law
Graham’s Law can be applied to the diffusion rate of gasses in the alveoli. The relative rate of
diffusion of a gas is proportional to the solubility of that gas.
When gases are dissolved in liquids, the relative rate of diffusion of a given gas is proportional to
its solubility in the liquid and inversely proportional to the square root of its molecular mass.
Important in the transport of respiration gases is the relative diffusion rate of oxygen and carbon
dioxide in the plasma of the human body. Carbon dioxide has 22 times the solubility, but is more
massive (44 amu compared to 32 for oxygen). According to Graham's law, the relative rate of
diffusion is given by

Hyperphysics by Department of Physics and Astronomy in Georgia State University
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/henry.html
Respiratory System
How do your cells obtain oxygen and remove carbon dioxide?
http://www.haspi.org/curriculum-library/Med-Chemlessons/05%20Standard%204%20Gases%20and%20Their%20Properties/Medical%20Applications
%20and%20Resources/Med%20Chem%20Gas%20Laws%20and%20Respiration.pdf
Pittman RN.
San Rafael (CA): Morgan & Claypool Life Sciences; 2011. Chapter 3
The Respiratory System and Oxygen Transport
http://www.ncbi.nlm.nih.gov/books/NBK54114/
Muhammad Iqbal KMU, Integrated Sciences-II (Physics)
http://www.kmu.edu.pk/ins/sites/kmu.edu.pk.ins/files/Lectures/Application%20of%20gas%20law
s%20to%20resp%20system%20presentation.pdf

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