Some recent advances on the study and understanding of the functional design of the avian lung: morphological and morphometric perspectives

Abstract

The small highly aerobic avian species have morphometrically superior lungs while the large flightless ones have less well-refined lungs. Two parabronchial systems, i.e. the paleopulmo and neopulmo, occur in the lungs of relatively advanced birds. Although their evolution and development are not clear, understanding their presence is physiologically important particularly since the air- and blood flow patterns in them are different. Geometrically, the bulk air flow in the parabronchial lumen, i.e. in the longitudinal direction, and the flow of deoxygenated blood from the periphery, i.e. in a centripetal direction, are perpendicularly arranged to produce a cross-current relationship. Functionally, the blood capillaries in the avian lung constitute a multicapillary serial arterialization system. The amount of oxygen and carbon dioxide exchanged arises from many modest transactions that occur where air- and blood capillaries interface along the parabronchial lengths, an additive process that greatly enhances the respiratory efficiency. In some species of birds, an epithelial tumescence occurs at the terminal part of the extrapulmonary primary bronchi (EPPB). The swelling narrows the EPPB, conceivably allowing the shunting of inspired air across the openings of the medioventral secondary bronchi, i.e. inspiratory aerodynamic valving. The defence stratagems in the avian lung differ from those of mammals: fewer surface (free) macrophages (SMs) occur, the epithelial cells that line the atria and infundibula are phagocytic, a large population of subepithelial macrophages is present and pulmonary intravascular macrophages exist. This complex defence inventory may explain the paucity of SMs in the avian lung.

Aspergillusinfections in birds: a review

“…The air sacs are usually the primary infection sites, since inhaled air reaches the posterior thoracic and abdominal air sacs prior to contacting epithelial surfaces in the lungs (Nardoni et al, 2006). In the lung parenchyma, spores get embedded in the atria and parts of the infundibula in the parabronchus and are engulfed by (surface) phagocytic epithelial cells (Maina, 2002). When there are too many spores or the bird has an impaired immune response, the innate defence mechanisms do not succeed in eliminating infection at the site of the air capillaries.…”

“…Instead, the susceptibility of birds to Aspergillus spp. may be attributed to differences in anatomical, physiological and respiratory immune system characteristics compared with mammals (Toth, 2000;Maina, 2002;Tell, 2005).…”

Local immunity of the respiratory mucosal system in chickens and turkeys

“…Macrophages have been shown to be present on the epithelial lining in atria and in infundibula of tertiary bronchi (Maina andCowley 1998, Maina 2002). They are also present in the connective tissue, which is situated under the epithelium near the atria (Maina and Cowley 1998), and in the interatrial barrier (Reese et al 2006), which indicates that phagocytic cells are situated at a strategic site of gas exchange.…”

Induction of respiratory immune responses in the chicken; implications for development of mucosal avian influenza virus vaccines

“…The chicken RT contains few free-residing MPh ('avian respiratory phagocytes'; ARP) compared to mammals and MPh are absent from the surface of the air capillaries (Maina 2002). In avian lung, MPh are present in the lining of the atria and infundibulae (Maina 2002) and are also abundantly present in the connective tissue (Klika et al 1996).…”

“…In avian lung, MPh are present in the lining of the atria and infundibulae (Maina 2002) and are also abundantly present in the connective tissue (Klika et al 1996). Phagocytic cells are strategically localized at the start of the gasexchange area to clear the air of inhaled particles, before it reaches the thin and vulnerable air capillaries .…”