Gas Exchange

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Pulmonary Artery

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Pulmonary Vein

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Pulmonary Gas Exchange

lung anatomyWhen you take a breath of air, a pulmonary gas exchange is performed to accept oxygen. Pulmonary ventilation (breathing) brings the air to the alveoli (little sacs in the lungs) for this exchange. The alveolar introduces oxygen from your breath into your pulmonary capillaries, as well as expelling carbon dioxide through your exiting breath.

The Laws of Gas and Air Composition

When gas molecules apply force on a surface, it is called pressure. In the case of natural systems such as breathing, the gases are typically a mixture of different types of molecules. In the case of breathing, the basic type of molecules is going to be oxygen. However, the atmosphere also contains carbon dioxide, nitrogen, and other obscure molecules of gas. Partial pressure (Px) is the specific pressure of a single type of gas within a mixture of gases (example: the atmosphere). Oxygen exerts a partial pressure of 159 mmHg, while Nitrogen exerts a partial pressure of 593 mmHg, for a total of 752 mmHg combined. The total pressure of a mixture of gases is simply the sum of the partial pressures.

Understanding partial pressure is important in determining how the gas mixture will move. Gases equalize their pressure within two regions that connect. A gas with higher partial pressure moves into an area where it will have a lower partial pressure. Also, the movement of gases is faster when the difference in partial pressures is greater.

The Solubility of Gases within Liquids

Henry's Law states that the specific concentration of gas in a liquid is precisely proportionate to the solubility and partial pressure of the given gas. The higher mm HG of the gas, the more gas molecules that will dissolve into the liquid. The overall concentration of a specific gas in any given liquid is also reliant upon the solubility of the specific gas into the specific liquid. A good example is Nitrogen – while there is Nitrogen present in the atmosphere and your breath, very little Nitrogen actually dissolves into your blood. There is an exception, however, that occurs with scuba divers. Since the components of the compressed air mixture that divers breathe causes nitrogen to have a greater partial pressure than normal, it will dissolve in the blood at a higher rate as well. Having too much nitrogen in your bloodstream can result in a very serious condition that can be fatal if left untreated.

lungs-gas-exchangeThe components of the air you breathe and the air in the alveoli will differ. In both cases, the comparative concentration of gases is nitrogen, then oxygen, then water vapor, then carbon dioxide. You will find that the amount of water vapor in the alveolar air is greater than the amount of water vapor present in atmospheric air. Essentially, the respiratory system works to humidify the air as you breathe it in, causing the air in the alveoli to have a higher concentration of water vapor than the air in the atmosphere. Additionally, alveolar air will contain a higher amount of carbon dioxide and a less amount of oxygen than the air in the atmosphere. This is not surprising, though, considering the gas exchange removes the oxygen from the alveolar air and also adds carbon dioxide to the alveolar air. Both forced and deep breathing will cause the alveolar air components to change more rapidly than when breathing quietly and evenly. This results in a change of partial pressures of oxygen and carbon dioxide, ultimately affecting the overall diffusion process that move these molecules across the membrane. This will cause the oxygen molecules to enter the blood quicker and the carbon dioxide molecules to leave quicker.

Perfusion and Ventilation

Two of the most important aspects of pulmonary gas exchange in the lungs are perfusion and ventilation. Ventilation is air moving in and out of the lungs, more commonly called breathing. Perfusion is the blood flow in the pulmonary capillaries. For the gas exchange process to be truly efficient, the volumes involved in the ventilation and perfusion should be well-matched. Factors, including regional gravity, blocked alveolar ducts, or certain diseases, can cause an imbalance of perfusion and ventilation.


After you take in your breath, the process of the pulmonary gas exchange begins. Oxygen enters your bloodstream and carbon dioxide exits your blood stream through the little sacs in your lungs called alveoli. This process is possible thanks to the natural forces of partial pressures of individual gas molecules. As stated above, gases with a higher pressure will move to an area with lower pressure. The alveoli sacs in the lungs facilitate this movement.  The solubility of the gases allows the oxygen to diffuse across the membrane in the lungs. 

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