Abstract
This chapter examines what is known about the kidney as it pertains to salt and water balance within the agnathans, elasmobranchs and teleosts. Freshwater teleost fish live in ah environment that is hypoosmotic to their body fluids and are, consequently, plagued with a continuous osmotic influx of water and depletion of salts by diffusion. Freshwater teleost fish have a kidney nephron that includes a glomerulus, proximal tubule I and II, distal tubule and a collecting tubule and duct. Lampreys lack a renal portal system which, in teleosts, enables tubular secretion to continue in the absence of glomerular perfusion. Arterial blood to the kidney in teleosts is supplied by renal arteries arising from the dorsal aorta or by renal branches from segmental arteries. There are few studies outlining the micropuncture analysis of elasmobranch distal tubule fluid, specifically due to its complicated kidney morphology.