Compromises and Trade-Offs in the Design of Gas Exchangers

Abstract

Unlike the human-made devices that are normally contrived to perform a specific task, biological structures are generally dynamic, composite, multifunctional entities. In their development, genetic programming engages various constraining factors including the available space in which they form. In mammals, for example, the maximal volume of the lung is determined by that of the thoracic cavity. In turn, the volume of the lung sets the dimensions of the components that constitute it. In some amphibian groups, the lungs play various roles that include hydrostatic adjustment, sexual display and sensory perception. The lung is an important source of pharmacologically active agents (e.g. Bakhle 1975), constitutes an important organ for defence and is involved in clearance of mucus and electrolyte transport. Regarding the endocrine and metabolic roles of the lung, Bakhle (1975) declared that ‘instead of referring to pharmacokinetics as one of the functions of the lung, we ought to refer to gas exchange as one of the nonpharmacokinetic functions of the lung’. The exceptional diversity of cells in the lung (e.g. Weibel 1984) provides the necessary receptor sites for perception and execution of its various functions. During the development of the mammalian lung, as many as 20 different cell types are in place as early as the 14th week of gestation (e.g. Kauffman 1980). This affords protection as well as control of endogenous and exogenous biologically active chemical factors. In the dog and the rat, pulmonary capillary endothelial cells constitute as much as 51% of the total population of cells (Crapo et al. 1983). The endothelial cells of the pulmonary blood vessels have abundant micropinocytotic vesicles (Figs. 89,90,95,97,98,134) that are involved in the degradation, transformation, interaction and biosynthesis of the macromolecules that the lung has affinity for as well as transendothelial transfer of some across the blood-gas (tissue) barrier. As in the lung, a multiplicity of functions occurs in the gills. These include osmoregulation, acid-base balance, modification of plasma hormones and elimination of products of nitrogen metabolism. To optimize these functions, refinements particularly regarding density and location of the structural components are evident in fish gills. Gas exchange occurs across the thin, flat cells that overlie the secondary lamellae (Fig.55), whereas the metabolic functions occur in the more elaborate primary epithelium (Maina 1990a, 1991) (Figs. 51, 52, 54). Owing to their multifunctionality, the designs of gas exchangers must not be looked at only from the narrow perspective of their respiratory function but rather from a holistic perspective, i.e. from the viewpoint of their diverse roles.

The quality of a system depends on the quality of the components which form it, as well as the excellence of its organization. On the other hand, systems can improve the performance of components such as mechan isms and structures. French (1988)