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Fractal geometry (FG) is a branch of mathematics that instructively characterizes structural complexity. Branched structures are ubiquitous in both the physical and the biological realms. Fractility has therefore been termed nature's design. The fractal properties of the bronchial (airway) system, the pulmonary artery and the pulmonary vein of the human lung generates large respiratory surface area that is crammed in the lung. Also, it permits the inhaled air to intimately approximate the pulmonary capillary blood across a very thin blood-gas barrier through which gas exchange to occur by diffusion. Here, the bronchial (airway) and vascular systems were simultaneously cast with latex rubber. After corrosion, the bronchial and vascular system casts were physically separated and cleared to expose the branches. The morphogenetic (Weibel's) ordering method was used to categorize the branches on which the diameters and the lengths, as well as the angles of bifurcation, were measured. The fractal dimensions ( D F ) were determined by plotting the total branch measurements against the mean branch diameters on double logarithmic coordinates (axes). The diameter-determined D F values were 2.714 for the bronchial system, 2.882 for the pulmonary artery and 2.334 for the pulmonary vein while the respective values from lengths were 3.098, 3.916 and 4.041. The diameters yielded D F values that were consistent with the properties of fractal structures (i.e. self-similarity and space-filling). The data obtained here compellingly suggest that the design of the bronchial system, the pulmonary artery and the pulmonary vein of the human lung functionally comply with the Hess-Murray law or 'the principle of minimum work'.

The Ostrich, Struthio camelus is the largest extant bird. The arrangement of the airway and the vascular components of the parabronchus of its lung were investigated by 3D serial section reconstruction. Modestly developed atrial muscles, shallow atria, paucity of infundibulae with preponderant origination of the air capillaries (ACs) from the atria and lack of interparabronchial septa, structural features that epitomize lungs of most highly derived metabolically active volant birds were observed. Intertwined very closely, the ACs and the blood capillaries (BCs) are not straight, blind-ended tubules that run in contact, counter and parallel to each other as has been claimed and/or modeled. Crosscurrent (perpendicular = orthogonal) orientation between the centripetal (inward) flow of the venous blood (VB) from the periphery of the parabronchus and the flow of air in the parabronchial lumen occur. Also, a countercurrent-like arrangement between the ACs which convey air centrifugally (outwards = radially) and the BCs that transport venous blood centripetally (inwards) was identified. The VB is conveyed to the parabronchus by the interparabronchial arteries and delivered to the exchange tissue by the intraparabronchial arterioles: it is then arterialized at the infinitely many points where the ACs and the BCs contact. Functionally, the crosscurrent arrangement grants a multicapillary serial arterialization arrangement which extends the time that the respiratory media, air and blood, are exposed to each other. The contribution that the countercurrent-like arrangement makes to the gas exchange process remains obscure.

In the embryo of the domestic fowl, Gallus gallus variant domesticus, the lung buds become evident on day 3 of development. After fusing on the ventral midline, the single entity divides into left and right primordial lungs that elongate caudally while diverging and shifting towards the dorsolateral aspects of the coelomic cavity. On reaching their definitive topographical locations, the lungs rotate along a longitudinal axis, attach, and begin to slide into the ribs. First appearing as a solid cord of epithelial cells that runs in the proximal-distal axis of the developing lung, progressively, the intrapulmonary primary bronchus begins to canalize. In quick succession, secondary bronchi sprout from it in a craniocaudal sequence and radiate outwards. On reaching the periphery of the lung, parabronchi (tertiary bronchi) bud from the secondary bronchi and project into the surrounding mesenchymal cell mass. The parabronchi canalize, lengthen, increase in diameter, anastomose, and ultimately connect the secondary bronchi. The luminal aspect of the formative parabronchi is initially lined by a composite epithelium of which the peripheral cells attach onto the basement membrane while the apical ones project prominently into the lumen. The epithelium transforms to a simple columnar type in which the cells connect through arm-like extensions and prominently large intercellular spaces form. The atria are conspicuous on day 15, the infundibulae on day 16, and air capillaries on day 18. At hatching (day 21), the air and blood capillaries have anastomosed profusely and the blood-gas barrier become remarkably thin. The lung is well developed and potentially functionally competent at the end of the embryonic life. Thereafter, at least upto day 26, no further consequential structures form. The mechanisms by which the airways in the avian lung develop fundamentally differ from those that occur in the mammalian one. Compared with the blind-ended bronchial system that inaugurates in the mammalian lung, an elaborate, continuous system of air conduits develops in the avian one. Further studies are necessary to underpin the specific molecular factors and genetic processes that direct the morphogenesis of an exceptionally complex and efficient respiratory organ.

The lung buds were first conspicuous on day 3 of embryogenesis. They fused on day 4 and the common growth divided into left and right primordial lungs on day 5. Progressively, the lungs elongated, diverged, and advanced towards the respective dorsolateral aspects of the body wall, reaching their definitive topographical locations in the coelomic cavity on day 6. On day 7, they rotated, attached onto the ribs, gradually started to slide into them, and were deeply inserted by day 8. The primary bronchus (PB) first appeared as a solid cord of epithelial cells (day 4) that successively canalized as it invaded the surrounding mesenchyme, extending along the proximal-distal axis of the lung. From day 8, the secondary bronchi (SB) begun to sprout from the PB in a craniocaudal sequence. On day 9, the parabronchi (PR) started to bud from the SB, projecting into the adjacent mesenchyme. They commenced to canalize on day 10 and greatly increased in length, number, and diameter. By day 13, the PR had anastomosed profusely and totally masked the SB. The luminal surface of the PR was lined by a columnar epithelium from which the atria (day 15), infundibulae (day 16), and air capillaries (ACs) (day 18) developed. At hatching (day 21), the ACs were well developed and had anastomosed profusely with the blood capillaries. Of the air sacs (ASs), the abdominal ones appeared earliest (day 5) followed by the cervical ones on day 6. In quick succession, the other ASs were well formed by day 10. After hatching, no further consequential structures formed: only shifts in topographical locations and an increase in size and number occurred. Morphogenetically, the avian respiratory system differs from the mammalian one in certain key aspects: besides the ASs that are unique to it, the lung is exceptionally complex in structure and is essentially mature at the end of the embryonic life.

The bronchial system (BS), the pulmonary artery (PA) and the pulmonary vein (PV) of the lung of the domestic pig, Sus scrofa were simultaneously cast with silicone rubber and studied. Asymmetrical dichotomous bifurcation preponderated in the tree-like arrangement of the three conducting systems. Lengths and diameters of the various generations were measured. At the extremities of the BS and the PA, alveoli and blood capillaries related very closely. In the cranial and middle lobes of the right and left lungs, topographically, the PA and the PV closely followed the BS, but in the accessory and the caudal (diaphragmatic) lobes, only the PA accompanied the BS: the PV run intersegmentally. Certain similarities and differences were observed between the diameters and lengths of the various generations of the three conducting systems. The strong correlations between some of the structural parameters indicated a high level of structural optimization. While morphometric variations suggest that the air and the blood flow dynamics may somewhat differ between the three conducting systems, they may also register structural features unique to the lung of the domestic pig, an animal that has been highly genetically exploited for fast growth and now leads an indolent lifestyle in captivity.

The form, geometry and epithelial morphology of the extrapulmonary primary bronchi (EPPB) of the domestic fowl (Gallus gallus var. domesticus) and the rock dove (Columba livia) were studied microscopically and by three-dimensional computer reconstruction to determine the structural features that may be involved in the rectification of the inspired air past the openings of the medioventral secondary bronchi (MVSB), i.e. the inspiratory aerodynamic valving (IAV). In both species, the EPPB were intercalated between the clavicular and the cranial thoracic air-sacs. A notable difference between the morphology of the EPPB in G. g. domesticus and C. livia was that, in the former, the EPPB were constricted at the origin of the MVSB, while a dilatation occurred at the same site in the latter. In both species, a highly vascularized, dorsally located hemispherical epithelial swelling was observed cranial to the origin of the MVSB. The MVSB were narrow at their origin and variably angled relative to the longitudinal axis of the EPPB. Conspicuous epithelial tracts and folds were observed on the luminal aspect of the EPPB in both C. livia and G. g. domesticus. From their marked development and their orientation relative to the angled MVSB, these properties may influence the flow of the air in the EPPB. It was concluded that features such as syringeal constriction, an intimate topographic relationship between the EPPB and the cranial air-sacs, prominent epithelial tracts and folds, an epithelial swelling ahead of the origin of the first MVSB (corresponding to the 'segmentun accelerans'), and narrowing and angulation of the MVSB at their origin, may together contribute to IAV to a variable extent. In as much as the mechanism of pulmonary ventilation and mode of airflow in the parabronchial lung are basically similar in all birds, the morphological differences observed between G. g. domesticus and C. livia suggest that either the mechanism of production of IAV or its functional efficiency may be different, at least in these two species of birds.