Introduction

There is a massive production of chemicals synthesized to meet the demands of industry. Most of these chemicals are foreign to the body (xenobiotics). They enter the environment where they are available for uptake by organisms. Most animals possess enzymes (multi-function oxidases) which reduce the toxicity of the xenobiotics and facilitate their excretion. Many xenobiotics, however, are resistant to metabolic degradation, are accumulated in the body and can be extremely toxic.

Most organic chemicals must enter the body before they can exert their toxic effect(s). Uptake can occur through the food chain or by direct uptake from the environment across the respiratory surface and skin. The uptake of xenobiotics from water by fish is determined by numerous factors, the most important of which are the transfer capacity of the gills and the physio-chemical properties of the compound.

Exchange at the gills

In adult fish, the gills comprise the major surface area of the body, and constitute a thin but continuous barrier between the environment and the blood. Water flows over the gills counter to blood flow ensuring a constant partial pressure gradient of O2 and CO2 over the duration of blood transit through the gills. Conditions for diffusion of O2 are further maximized by the presence of haemoglobin which binds molecular oxygen keeping the PO2 in the blood low. The gills in fish are hyperventilated with water relative to blood to ensure adequate rates of oxygen uptake from the environment.