Читать книгу Blackwell's Five-Minute Veterinary Consult: Reptile and Amphibian - Javier G. Nevarez - Страница 27

Reptilian kidneys are metanephric, lack loop of Henle, renal pelvis, and pyramids. The nephron is composed of glomerulus, long and thick proximal convoluted tubule, short and thin intermediate segment, and a short distal tubule. The sexual segment, a terminal segment of the kidney in male snakes and lizards, becomes pale and enlarged during reproduction.

Osmoregulation in reptiles presents unique challenges. A reptile’s body mass is approximately 70% water, similar to mammals but lower than amphibians (75–80%). Total sodium and potassium is similar to mammals but variation exists in larger species. Because of the lack of a loop of Henle, reptiles are unable to concentrate urine beyond their plasma osmolality. Methods of water conservation include the secretion of uric acid, presence of salt glands, fluid reabsorption from the cloaca, the renal portal system, and the ability to tolerate decreases in glomerular filtration rate. All reptiles produce and excrete uric acid, but some may also excrete ammonia and/or urea. Uric acid is the primary or only excretory product of terrestrial species. Uric acid is excreted via the renal tubules, so dehydration does not stop its excretion. It precipitates out of solution in the bladder or cloaca to form urates, which can be composed of potassium salts (herbivores) or sodium salts (carnivores). Uric acid is not a sensitive indicator of renal disease because over 60% of renal function must be lost in order to cause hyperuricemia; thus, dehydration is a more common cause of hyperuricemia. Ammonia and urea excretion occurs primarily in aquatic species. Crocodilians are ammoniotelic while freshwater turtles are ureotelic. Sea turtles excrete variable amounts of urea and ammonia. The cloaca, colon, and urinary bladder are significant sites of osmoregulation and allow active ion transport and passive water absorption through their walls. The bladder actively absorbs sodium while secreting potassium and urates.

The renal portal system is an afferent blood supply from the renal artery to the glomerulus. The renal portal vein (arising near epigastric and iliac veins) bypasses the glomerulus and enters at the tubules; thus, during dehydration the renal portal system perfuses the tubules to prevent necrosis. For this reason, the effects of the renal portal system on drug metabolism is of primary consideration for pharmaceuticals excreted by the tubules and not as critical for those excreted by glomerular filtration. Effects of the renal portal system on drug metabolism and excretion are variable according to the metabolic status of the animal and the pharmacokinetics of the drug.