Function and ultrastructure of the nephridium inHirudo medicinalis L.

Abstract

1. The mechanism of urine formation in the nephridial excretory system ofHirudo medicinalis has been studied by measuring sodium and potassium concentrations in the primary urine (Na=85 meq/l; K=46 meq/l) and chloride and osmotic concentrations along the central canal. 2. A comparison of the Na/K ratio in blood (≈22), primary urine (≈2), and final urine (≈5) suggests that active potassium transport is one of the mechanisms of primary urine formation. 3. A decrease of osmotic and chloride concentrations along the central canal was found by micropuncture studies (Fig. 2). 94% or more of the solutes secreted in the primary urine are reabsorbed while water is excluded due to relatively low water permeability of the central canal cells. 4. If the nephridium is considered to be a system of 5 compartments (1=blood+connective tissue, 2=canaliculus cells, 3=canaliculus lumen, 4=central canal lumen, 5=central canal cells), two pathways of solute and water circulation can be envisaged (Fig. 3). It is suggested that low water permeability of the apical central canal cells is one factor controlling the flow rates of circulating solutes and water within the nephridial system. Furthermore, the functional consequences of the intimate contact between the canaliculus cells and central canal cells to the mechanisms of urine formation are discussed. 5. It is postulated that increased secretion of primary urine is a prerequisite for the increased diuresis after food uptake.