Pulmonary ventilation is the movement of air into and out of the lungs. Pulmonary ventilation is influenced by arterial partial pressure of carbon dioxide (pCO2) and oxygen (pO2) as well as blood pH.
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The respiratory system increases or decreases pulmonary ventilation to maintain blood carbon dioxide and oxygen concentrations within normal limits. The volume of air inhaled and exhaled per minute is a measurement called minute ventilation (MV in L/min). Minute ventilation (L/min) is the product of tidal volume and breathing rate and is normally about 5-6 liters/min.
Carbon dioxide, released as a by-product of metabolism, has a greater influence on pulmonary ventilation than oxygen. The partial pressure of carbon dioxide in arterial blood is normally 40 mm Hg. Even a slight increase above 40 mm Hg is termed hypercapnia.
Hypercapnia is an increase in the arterial partial pressure of carbon dioxide (pCO2). As arterial pCO2 increases, more carbonic acid, bicarbonate ions, and hydrogen ions are formed. The pH of blood drops below the normal range (7.35-7.45) as the hydrogen ion concentration increases. Increased hydrogen ions and pCO2 stimulate central and peripheral chemoreceptors. This results in an increase in breathing rate and tidal volume, allowing the lungs to exhale more CO2 gas, bringing levels back to normal.
Under normal situations, stimulation of pulmonary ventilation is less sensitive to lowered arterial pO2 levels than to increased pCO2 levels. The partial pressure of oxygen in arterial blood is normally about 100 mm Hg. A decrease in arterial oxygen is called hypoxemia. Hypoxia is decreased oxygen in the tissues.
If blood pO2 levels decrease, peripheral chemoreceptors are stimulated. This results in an increased breathing rate and tidal volume allowing the lungs to inhale more O2 gas to bring levels back to normal.
Minute ventilation is measured directly with a spirometer. Arterial blood samples are needed to measure blood gas levels. Blood oxygen levels (pO2) are measured by oxygen polarography. Oxygen from the sample binds to an electrode causing the generation of an electric current (measured in amps). The more electric current generated, the more oxygen is present in the blood sample.
Blood carbon dioxide levels (pCO2) are measured with CO2 potentiometry and pH is measured by pH potentiometry. In CO2 potentiometry, electric potentials (measured in volts) result from differences between the concentration of carbon dioxide in the experimental (blood) sample and a standard solution (solution containing a known concentration of carbon dioxide). The greater the potential difference, the more carbon dioxide present in the blood sample.
In pH potentiometry, electric potentials result from differences between the concentration of hydrogen ions in the experimental (blood) sample and a standard solution (solution containing a known concentration of hydrogen ions). The greater the potential difference, the more hydrogen ions present in the blood sample and the lower the pH.
