Gas Exchange in the Lungs

Gas exchange between the alveolar air and the blood in pulmonary capillaries results in an increased oxygen concentration and a decreased carbon dioxide concentration in the blood leaving the lungs. This blood enters the systemic arteries,where blood gas measurements are taken to assess the effectiveness of lung function.

The atmosphere is an ocean of gas that exerts pressure on all objects within it. This pressure can be measured with a glass U-tube filled with fluid. One end of the U-tube is exposed to the atmosphere, while the other side is continuous with a sealed vacuum tube. Since the atmosphere presses on the open-ended side, but not on the side connected to the vacuum tube, atmospheric pressure pushes fluid in the U-tube up on the vacuum side to a height determined by the atmospheric pressure and the density of the fluid. Water, for example, will be pushed up to a height of 33.9 feet (10,332 mm) at sea level, whereas mercury (Hg)—which is more dense—will be raised to a height of 760 mm. As a matter of convenience, therefore, devices used to measure atmospheric pressure (barometers) use mercury rather than water. The atmospheric pressure at sea level is thus said to be equal to 760 mmHg (or 760 torr), which is also described as a pressure of one atmosphere (fig. 16.19).

According to Dalton's law, the total pressure of a gas mixture (such as air) is equal to the sum of the pressures that each gas in the mixture would exert independently. The pressure that a particular gas in a mixture exerts independently is the partial pressure of that gas, which is equal to the product of the total pressure and the fraction of that gas in the mixture.

The total pressure of the gas mixture is thus equal to the sum of the partial pressures of the constituent gases. Since oxygen constitutes about 21% of the atmosphere, for example, its partial pressure (abbreviated PO2) is 21% of 760, or about 159 mmHg. Since nitrogen constitutes about 78% of the atmosphere, its partial pressure is equal to 0.78 x 760 = 593 mmHg.

Test Yourself Before You Continue

1. Describe the actions of the diaphragm and external intercostal muscles during inspiration. How is quiet expiration produced?

2. Explain how forced inspiration and forced expiration are produced.

3. Define the terms tidal volume and vital capacity. Explain how the total minute volume is calculated and how this value is affected by exercise.

4. How are the vital capacity and the forced expiratory volume measurements affected by asthma and pulmonary fibrosis? Give the reasons for these effects.

Clinical Investigation Clues

Remember that Harry has a slightly low vital capacity and a significantly reduced FEV|.

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Essentials of Human Physiology

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  • fiyori
    How lungs exchange gases with atmosphere?
    8 years ago

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