Respiratory Media Versus Design of Gas Exchangers

Abstract

Of the three states of matter, namely, solids, liquids and gases, only fluids (liquids and gases) are atomically and molecularly suitably configured to contain molecular O₂ and to submit it to convective transport for delivery to the respiratory site. Water, a liquid over the biological range of temperature and pressure, and air, a gas under similar conditions, are the only two naturally occurring respirable fluids. The biophysical properties of water and air have in general greatly influenced the life patterns and the body forms of animals that subsist in the two media. Regarding gas exchangers, the differences have so fundamentally affected the respiratory processes that the basic mechanisms for obtaining O₂ and for eliminating CO₂ that are efficient in water often fail in air and vice versa. Compared with air, water is a more exacting respiratory medium. In saturated water, at 20°C, 1 ml of O₂ is contained in 200 g of water, whereas 1 ml of O₂ occurs in 5 ml (7 g) of air. The rate of diffusion of O₂ in water (3.3 × 10⁻⁵ cm² s⁻¹) is lower by a factor of 105 compared with that in air (1.98 × 10⁻¹ cm² s⁻¹) — Although the capacitance coefficient of O₂ (i.e. the increase of concentration per unit rise in the partial pressure) in water is only 1.82 nmol min⁻¹ 0.133 kPa⁻¹, that in air is much higher (54.74 nmol min⁻¹ 0.133 kPa⁻¹) (Dejours 1988). Furthermore, with respect to density and viscosity, water is about 800 times heavier and 50 times more viscous than air. Owing to these differences, to extract the same amount of O₂ from water and air, all other factors being equal, water breathers have to expend more energy than air breathers. An octopus, for example, ventilates 17 1 of water for each millimole of O₂ consumed (e.g. Dejours et al. 1970). Water breathers have evolved within the constraints of an O₂-deficient and more viscous medium. The cross-current presentation (Sect. 9) and double capillary exposition (Sect. 12) of blood and water in the gills enhance O₂ uptake.

An important goal in biology is to uncover the fundamental design principles that provide the common underlying structure and funct ion in all cells and microorganisms. Joeng et al. (2000)