Digestive/Excretory System

                                Digestive & Excretion Systems - Osmoregulation


“Because of pressure differentials, fluid is filtered from the blood plasma through capillary walls in the glomerulus and into the kidney tubule. By a combination of pressure and ciliary action, the fluid is forced down the tubule. Cells lining the lumen resorb water and solutes; these pass back into the blood flowing through capillaries surrounding the tubule. Simultaneously, urea and other solutes are respected from the blood into the tubular fluid. Water resorption occurs primarily in the proximal convoluted tubule. Coelomic fluid can enter the renal tubules via ciliated nephrostomes in salamanders and thus may add to the glomerular filtrate. In anurans the nephrostomes connect to the renal veins, and they may play a role in water resorption from the bladder. Peritubular circulation is derived from renal portal veins. The kidneys are not capable of producing hypertonic urine.

Energetically, ammonia is the most economical vehicle for nitrogen excretion, but its toxicity precludes this product when water turnover is low. On the basis of equivocal ecidence, Jungreis (1976) suggested that the main advantages of excretion of ammonia by aquatic amphibians are in cation conservation and pH regulation. Most amphibians are ureotelic, but they excrete appreciable amounts of ammonia when they are in water and become completely ureotelic when water influx is low. Reduction of cessation of urine production leads to an accumulation of urea, which places the animal in a more favorable situation for obtaining water from soil or saline solutions. Accumulated urea is eliminated rapidly by the kidneys when the animals are rehydrated. There is active, tubular secretion of urea, but when plasma levels of urea are high, urinary urea concentrations usually approximate those in the plasma.”


                                        Urinary Bladder of Most Amphibians

Amphibian kidneys function much like those of freshwater fishes. When in fresh water, the skin of the frog accumulates certain salts from the water by active transport, and the kidneys excrete dilute urine. On land, where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.”


Nitrogenous Excretion


“Most amphibians lack the means for excreting nitrogen or salts economically with respect to water because they cannoy produce heperosmotic urine, do not posses salt glands, and excrete nitrogen in a soluble form. Some aquatic amphibians normally excrete most of their nitrogenous wastes in the form of ammonia; because of its high toxicity, this excretory product is possible only in aquatic situations where ammonia is diluted immediately and carried away from the animal. A few other aquatic and most terrestrial and arboreal amphibians produce urea, and a few tree frogs produce uric acid. In the sequence of ammonotelism-ureotelism-uricotelism, less water is needed as uricotelism is approached.

The production and storage of urea is not limited to nonaquatic amphibians. Experimental studies revealed that frogs deprived of water and then placed in water excreted urea that had been stored in body fluids; subsequently, the frogs shifted to excreting ammonia again.

Little ammonia is present in the blood, and the ammonia in the urine is formed and secreted in the kidneys, presumably by a means of conserving blood sodium. Urea normally represents a passive component of blood and reduces evaporative water loss. Under conditions of dehydration, elevated levels of serum and body urea result initially from reductions in urine formation and subsequently in response to increased amino acid metabolism.”



(Duellman, William Edward)

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Project authors: Andrea Popyordanova, Desislava Karakoleva and Yordan Penev; American College of Sofia; Date published: April 23rd, 2009; Date last revised: April 23rd, 2009