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Gill blood flow in teleosts Farrell, Anthony Peter


Unlike the respiratory organs of airbreathing vertebrates where gas exchange is perfusion limited, gas transfer across fish gills is diffusion limited. Fish can therefore enhance gas exchange by increasing the gill diffusing capacity. Previous suggestions, indicate that fish may achieve this by altering the pattern of gill blood flow to increase the area of gill perfused and to reduce the blood-water diffusion barrier. To verify these suggestions an investigation of the patterns of gill blood flow, their regulation and their significance in gas exchange in the ling cod, Ophiodori elongatus, was undertaken. The circulatory arrangement in the gill filament of the ling cod consists of an arterio-arterial respiratory network and an arterip-venous veriolymphatic system. All cardiac output passes through the respiratory exchange sites, the lamellae. Blood flow through lamellae is described by sheet blood flow equations, where flow is proportional to the vascular sheet thickness (h). The lamellar vascular sheet is very compliant .and h increases with transmural pressure (ΔP[sub lam]). It is predicted if ΔP[sub lam] and flow are raised, then intralamellar shunting of blood flow and a reduction of the blood-water barrier will result, thereby increasing the gill diffusing capacity; Not all lamellae appear to be equally perfused under certain in vivo conditions. Furthermore, if resting perfusion conditions are simulated, only 67% of the more proximal lamellae are perfused. Thus the total gill area is not utilised at rest. To account for this situation it is proposed that the critical closing pressures associated with distal lamellar units are greater than those for the proximal lamellae. The afferent arterioles were determined to be the major resistance site in the gills and they therefore control flow to lamellae. Elevations in flow and lamellar input pressure will reduce the likelihood of' critical closing and more lamellae will be perfused. Lamellar recruitment increases the gill diffusing capacity. , The demonstrated changes in flow patterns to and within lamellae are effected by elevated flow and input pressures (or ΔP[sub Lam]). Changes in cardiac performance and in the pressure profile of the gills alter flow and pressure. Cardiac performance in ling cod is influenced by intrinsic, cholinergic and adrenergic controls which alter stroke volume and heart rate. The pressure profile of the gills can be altered by cholihergically or adrenergically mediated changes in vessel dimensions. The gill outflow arteries vasoconstrict in localised regions with cholinergic .stimulation and thereby increasing gill resistance (Rg) and lamellar' input pressures. Afferent vessels apparently dilate with-g-adrenergic stimulation and thereby lower Rg. Cardiovascular changes are associated with conditions of reduced oxygen availability (hypoxia) and of increased oxygen demand (struggling) in ling cod. The cardiovascular changes are such that they alter the pattern of gill blood flow and increase the gill diffusing capacity. Increased oxygen uptake, cardiac output and gill ventilation are associated with hypoxia and struggling. The quantitative increases in cardiac output per se associated with these conditions does not fully account for the observed increase in oxygen uptake. It is concluded that change in gill diffusing capacity through alterations in gill blood flow patterns are important in enhancing oxygen uptake across the gills.

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