BACKGROUND: Although pulmonary diffusion capacity (PDC) has been substantially proved in a declining tendency in age-dependent manner, the effects of such related factors as body mass,sex and height within an age range on pulmonary membrane conductance (Dm) and capillary blood volume(CBV),as well as the correlation of Dm with CBV and PDC deserve further investigation.OBJECTIVE:To study the correlation of Dm and CBV with PDC,age,body mass, height and sex in normaladults.DESIGN: With healthy adults as subjects, multiple factor linear correlation and regression analysis was used to elicit correlation coefficient and regression equation while F-test and Q-test were used for comparing the differences between groups.PARTICIPANTS:We randomly selected 100 healthy adults,50 males and 50 females, who came to the clinic of the General Hospital of Chinese PLA for routine physical examination between April and October 1997.The 100 volunteers were divided into five groups according to their age:20-29, 30-39, 40-49, 50-59 and 60-69 years old groups with 20 in each group, half males and half females.METHODS: PDC and Dm of the healthy adults were examined with Sensormedics 2200 pulmonary function detector. CBV was calculated with the following formula (1/PDC=I/ Dm +1/θCBV, θrefers to the combined velocity of CO with Hb,which was in inverse proportion to the inhaled partial oxygen pressure,Dm and CBV were unaffected by θ).Multiple factor linear correlation and progressive regression analysis was used to elicit correlation coefficient of PDC, Dm and CBV. Meanwhile, regression equation was used to reveal the correlation of age,body mass and height with pulmonary functions.MAIN OUTCOME MEASURES: Correlation ofDm and CBV with PDC and age, body mass, height and sex in normal adults.RESULTS:Data of the 100 healthy adults were processed according to the objective and statistically analyzed.① PDC and Dm were proved to decline in age-dependent manner from the onset of 40 years old in the 100 healthy adults (P＜ 0.05), and CBV also declined after 50 years old (P ＜ 0.05).②Relationship between women's age and pulmonary function:Age was proved to have significant negative correlation with PDC,Dm and CBV (r= -0.646 0 to -0.814 6, P＜ 0.01).By contrast,PDC was found to be positively correlated with Dm,CBV and alveolus ventilation volume (AVV) (r=0.949 7, 0.740 0, 0.735 6, P＜ 0.01), and AVV was found to be positively correlated with PDC, Dm and CBV (r=0.482 1-0.740 6,P＜ 0.01).③ Relationship between men's age and pulmonary function:Age had significant negative correlation with PDC, Dm and CBV (r=-0.712 to -0.830 5, P＜ 0.01). However, PDC was found to be moderately or highly positively correlated with Dm,CBV and AVV(r=0.585 2 to 0.946 0, P＜ 0.01), and AVV had moderately positive correlation with PDC, Dm and CBV (r=0.431 3-0.585 2, P＜ 0.01). ④ Regression analysis revealed that age and height were the main variables of the predicting equation of PDC,Dm and CBV.Body mass could not enter regression equation in females,and it was only proved to be correlated with PDC and CBV in males with the regression coefficient of 0.009 4 and 0.654 3, respectively.CONCLUSION:① PDC and Dm begin to decline from the onset of 40 years old in healthy adults (P＜ 0.05),and CBV declines after 50 years old.② PDC, Dm and CBV of healthy adults are proved to decline in age-dependent manner. ③ PDC increases with the increment of Dm and CBV. ④ Age and/or height are the main variables in the regression equation of PDC, Dm and CBV, but body mass is not the key predictor for Dm and CBV.