Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 51

The urinary system manages osmotic and ionic regulation. Marine elasmobranchs maintain a high plasma osmolality due to high levels of sodium, chloride, urea, and methylamine oxides. They are slightly hyperosmotic to their environment, which results in an uptake of water that balances fluid lost in the urine. In contrast to marine teleosts, drinking is negligible. The gills, kidney, and rectal gland are important for maintaining homeostasis, and the kidney is the main site for urea regulation (Lacy and Reale 1999). In freshwater elasmobranchs, urea, electrolytes, and osmolality are much lower; the kidneys are microscopically simpler; and the rectal gland is significantly reduced in size and function.

The kidneys are paired and firmly attached to the dorsal wall of the caudal coelom under a thick capsule. In sharks, they lie midway in the coelomic cavity and widen as they reach caudally. In skates and rays, they are more caudal in the coelomic cavity and more lobulated. In males, the ureters leave the kidneys medially and travel along the midline to meet the seminal vesicles and sperm sacs caudally and exit through a single (or sometimes double) urogenital pore or papilla. A small urine sample may be obtained by passing a catheter past the reproductive tract; large volumes are not possible since urine production is low. A semen sample can be collected more distally and where the sacs or ampullae are.

The kidneys of marine elasmobranch are unable to produce a concentrated urine; ionic concentrations cannot be higher than those of the plasma. Glomerular filtration rates (GFRs) vary from 0.2 to 4.0 mL/kg/h, higher than seawater teleosts but similar to freshwater teleosts. Exposure to dilute environments increases the urine flow rate and decreases reabsorption of urea. Freshwater stingrays have a GFR of 8–10 mL/kg/h. Details of renal function in elasmobranchs can be found elsewhere (Evans et al. 2004; Shuttleworth 2012).