A resin cast of the bronchial tree of a dog was studied down to branches of 0.5-mm diameter. The branches were ordered, and the number of branches, mean diameter, and mean length of branches in each order were determined. The model was developed from these data with the property that delta, the difference in order between the two daughter branches at a bifurcation, is identical for all parent branches of a given order. This property facilitates the use of the model for the calculation of physiological variables such as input impedance (Fredberg and Hoenig, ASME J. Biomech. Eng. 100: 57-66, 1978), and the comparison of such results with those obtained from symmetrical models (Sidell and Fredberg, ASME J. Biomech. Eng. 100: 131-138, 1978).

Bronchial tree casts were made from one or both lungs from nine children aged from five weeks to 17 years. The branches of the casts were ordered by the method of Strahler, and diameter ratio, length ratio, and branching ratio were determined. From five to 12 weeks the diameter ratio is about 1-35, but by 13 months it has risen to 1-45, indicating a faster rate of growth in the larger airways over the period; thereafter it stays at or above this level. The data show that the peripheral conducting airways are relatively large in diameter at 1-3 months, attaining their adult proportion after one year. In contrast, length ratio shows no trend with age. On the assumption of 25 000 terminal bronchioles, the mean number of orders between terminal bronchiole and trachea was estimated to be about 12.

Triple resin casts were made of the pulmonary arteries, veins, and airways from six dog lungs. The airways were cast at a pressure of 25 cm resin in all six. In the first three, both vessels were cast at a pressure of 30 cm resin, and in the second three, arteries were at 10 cm resin and veins at 5 cm resin. Measurements were made of luminal airway diameters down to 1 mm and of the luminal diameters of the corresponding segments of the vascular trees. The relation of one to the other was shown by calculating the regression lines for the corresponding diameters. Intrapulmonary arteries and veins are of approximately equal diameter when cast at 30 cm resin, while the veins are 20% larger than the arteries when cast at 5 cm and 10 cm of resin, respectively. Both vessel diameters are 75% of bronchial at the higher pressure, while at the lower pressures arteries are 59%, and veins 71% of bronchial. In any individual, vessel diameters are a relatively constant proportion of airway diameter. This constant of proportionality varies considerably between dogs, its value ranging from 0.48 to 1.03. It is concluded that in any individual all three trees have similar diameter ratios--that is, the ratio between mean diameters of branches in successive orders.

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