Pulmonary alveolus

An alveolus (plural:alveoli), is an anatomical structure that has the form of a hollow cavity. In the lung, the alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.

Alveolar gas pressures

Normal alveolar partial pressures for O2 and CO2 are 105 mmHg and 40 mmHg respectively. For dry air at sea level, the partial pressures for O2 and CO2 are 160mmHg and 0.3mmHg respectively. The alveolar oxygen pressure is lower than the atmospheric O2 partial pressure for two reasons. Firstly, as the air enters the lungs, it is humidified by the upper airway, thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg. The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO2 out of the capillaries into the alveoli.

Related Topics:
Partial pressure - MmHg - Pulmonary - Capillaries

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The factors that determine the values for alveolar PO2 and PCO2 are:

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• The pressure of outside air
• The partial pressures of inspired oxygen and carbon dioxide
• The rates of total body oxygen consumption and carbon dioxide production
• The rates of alveolar ventilation and perfusion

The "ideal" alveolar pO2 is given by the Alveolar Gas Equation:

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paO2 = pIO2 - pACO2/RQ + F

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where:

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RQ is the Respiratory Quotient(normally about 0.8)

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paO2 is the Alveolar pO2

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pI O2 is the Inspired pO2 (760-47) mmHg

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pAO2 is the Arterial pO2

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F is a correction factor (usually less than 2 mmHg)

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Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases. Hyperventilation exists when the same ratio decreases.

Related Topics:
Hypoventilation - Hyperventilation

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