In general, a healthy individual has a value of between 75% and 125% of the average. However, individuals vary according to age, sex, height and a variety of other parameters. For this reason, reference values have been published, based on populations of healthy subjects as well as measurements made at altitude, for children and some specific population groups. Blood CO levels may not be negligible In heavy smokers, blood CO is great enough to influence the measurement of , and requires an adjustment of the calculation when COHb is greater than 2% of the whole. The two components of While is of great practical importance, being the overall measure of gas transport, the interpretation of this measurement is complicated by the fact that it does not measure any one part of a multi-step process. So as a conceptual aid in interpreting the results of this test, the time needed to transfer CO from the air to the blood can be divided into two parts. First CO crosses the alveolar capillary membrane (represented by ) and then CO combines with the hemoglobin in capillary red blood cells at a rate times the volume of capillary blood present (). Since the steps are in series, the conductances add as the sum of the reciprocals: Any change in alters The volume of blood in the lung capillaries, , changes appreciably during ordinary activities such as exercise. Simply breathing in brings some additional blood into the lung because of the negative intrathoracic pressure required for inspiration. At the extreme, inspiring against a closed glottis, the Müller's maneuver, pulls blood into the chest. The opposite is also true, as exhaling increases the pressure within the thorax and so tends to push blood out; the Valsalva maneuver is an exhalation against a closed airway which can move blood out of the lung. So breathing hard during exercise will bring extra blood into the lung during inspiration and push blood out during expiration. But during exercise (or more rarely when there is a structural defect in the heart that allows blood to be shunted from the high pressure, systemic circulation to the low pressure, pulmonary circulation) there is also increased blood flow throughout the body, and the lung adapts by recruiting extra capillaries to carry the increased output of the heart, further increasing the quantity of blood in the lung. Thus will appear to increase when the subject is not at rest, particularly during inspiration. In disease, hemorrhage into the lung will increase the number of haemoglobin molecules in contact with air, and so measured will increase. In this case, the carbon monoxide used in the test will bind to haemoglobin that has bled into the lung. This does not reflect an increase in<unk>  of the lung to transfer oxygen to the systemic circulation. Finally, is increased in obesity and when the subject lies down, both of which increase the blood in the lung by compression and by gravity and thus both increase . Reasons why varies The rate of CO uptake into the blood, , depends on the concentration of hemoglobin in that blood, abbreviated Hb in the CBC (Complete Blood Count). More hemoglobin is present in polycythemia, and so is elevated. In anemia, the opposite is true. In environments with high levels of CO in the inhaled air (such as smoking), a fraction of the blood's hemoglobin is rendered ineffective by its tight binding to CO, and so is analogous to anemia. It is recommended that be adjusted when blood CO is high. The lung blood volume is also reduced when blood flow is interrupted by blood clots (pulmonary emboli) or reduced by bone deformities of the thorax, for instance scoliosis and kyphosis. Varying the ambient concentration of oxygen also alters . At high altitude, inspired oxygen is low and more of the blood's hemoglobin is free to bind CO; thus is increased and appears to be increased. Conversely, supplemental oxygen increases Hb saturation, decreasing and . Lung diseases that reduce and Diseases that alter lung tissue reduce both and to a variable extent, and so decrease . # Loss of lung parenchyma in diseases like emphysema. # Diseases that scar the lung (the interstitial lung disease), such as idiopathic pulmonary fibrosis, or sarcoidosis # Swelling of lung tissue (pulmonary edema) due to heart failure, or due to an acute inflammatory response to allergens (acute interstitial pneumonitis). # Diseases of the blood vessels in the lung, either inflammatory (pulmonary vasculitis) or hypertrophic (pulmonary hypertension). Lung conditions that increase . # Alveolar hemorrhage Goodpasture's syndrome, polycythemia, left to right intracardiac shunts, due increase in volume of blood exposed to inspired gas. # Asthma due to better perfusion of apices of lung. This is caused by increase in pulmonary arterial pressure and/or due to more negative pleural pressure generated during inspiration due to bronchial narrowing.
Diffusing capacity
Gas exchange
Closing capacity
Blood–gas partition coefficient
Airway resistance
Vascular resistance
Ventilatory threshold
Renal threshold