Excretory system, xenobiotic

The processes of selective reabsorption of nutrients and xenobiotics goes on within the complex tubule system. 98-99% of filtered materials (salts, water, sugars, amino acids) are eventually reabsorbed by passive or active transport. Biomolecules such as glucose and amino acids are entirely reabsorbed if their concentrations are within the normal range in the blood. However, should the concentrations be higher than normal, those molecules might not be completely reabsorbed because they have exceeded the ability of the nephron transport systems to accommodate them. This is referred to as exceeding the renal threshold. Urine is therefore a convenient body fluid to assay for the initial assessment of metabolic or excretory system malfunctions. 

Two more factors contributing to the persistence of xenobiotics in polychaetes, molluscs, crustaceans and echinoderms, and presumably other marine invertebrates, are the formation of macromolecular adducts and the slow release of free metabolites. The incorporation of PAH into more stable compartments , possibly as a result of cytochrome P-450-mediated adduct formation, is particularly evident in molluscs (see Fig. 3). The metabolites of many xenobiotics are lost more slowly than the parent compound, resulting in a build-up in the tissues during both exposure and subsequent depuration periods. The primary function of metabolism, therefore, appears to be detoxication, by preventing the formation of specific reactive metabolites and, probably more importantly, the non-specific toxic action of lipophilic xenobiotics caused by their penetration of membrane systems. Loss of metabolites down a concentration gradient occurs, reducing body burden of the xenobiotic, but is obviously limited by the absence or ineffectiveness of specific transport and excretory systems for the polar metabolites. 

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