In mammals, the kidney is important in both osmoregulation and excretion of nitrogenous waste in the form of urea. Urea is produced from highly toxic ammonia, the direct product of protein metabolism, by the liver. In fish, however, the kidney only serves an osmoregulatory function; the excretion of nitrogenous waste occurs at the gill where ammonia is excreted as quickly as it is produced (See Ch. VIIA). The functional unit of the kidney is the nephron which is composed of the glomerulus and renal tubule. The glomerulus is a tuft of capillaries through which the glomerular filtrate is passed. In fish, the tubule is divided into a Bowman's capsule that surrounds the glomerulus, followed by a series of segments (proximal I, proximal II, distal, and collecting duct) which perform different functions. The illustration below is highly schematic and is designed to show function not structure (the tubules are not straight, but convoluted and the different segments are not really different diameters).
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In FW teleosts, the kidney is of major importance. It removes water that passively entered at the gill through generation of a dilute urine. In SW teleosts, the kidney is of only minor importance. It helps rid the fish of divalent cations (Ca++ and Mg++), excess hydronium ions and a few other minor waste products, but plays no role in water balance. Since SW fish need to retain water and the piscine kidney cannot produce a urine more concentrated than body fluids, urine flow is kept to a minimum.
The first step in urine formation is the production of glomerular filtrate. This occurs when the permeable capillaries of the glomerulus "leak" water, salts, glucose, amino acids, and small protein molecules. Some marine teleosts lack glomeruli and therefore produce almost no filtrate and no urine. The filtrate is collected in the Bowman's capsule and proceeds down the tubule. The function of the tubule is to selectively add and remove solutes, and therefore water, to turn a general blood filtrate into urine waste.
After passing through the neck, which may be ciliated to help move the filtrate, the filtrate enters proximal segment I (PSI). In PSI, glucose and other desirable larger molecules are scavenged out of the filtrate and, to a lesser extent, anionic organic waste products are pumped in [An Example]. The larger molecules are retrieved by pinocytosis while the ions are moved by ATPases. This occurs in both FW and SW fish. In PSI (and PSII), the tubule is permeable to water and water follows the solutes out of the filtrate and there is no change in the osmotic pressure of the filtrate. PSII is responsible for handling divalent cations. In salt-starved FW teleosts, the divalent cations in the filtrate are recovered and in ion-rich SW teleosts, additional cations are excreted into the waste stream. In at least some FW fish (e.g. eels), Na+ and Cl- are secreted into the filtrate in the PS's so water will follow and increase urine flow (the ions will be recovered later).
The distal segment (DS) and collecting tubule (CT) are where Na+ and Cl- are pumped back out of the filtrate. FW teleosts do this to recover the ions while SW teleosts do not need the ions, but pump the salt to move the valuable water back out of the filtrate. FW and SW teleosts differ in the permeability of the DS and CT. In FW teleosts, the walls are impermeable to water, so ions can be recovered without water following, while in SW teleosts the walls are permeable so water will follow the salts. This impermeable section allows FW teleosts to excrete a urine that is much more dilute than the blood. In SW teleosts, the tubules are permeable throughout the nephron so the urine, while having a different ion make-up than the filtrate, will differ little in osmotic pressure.
In summary, FW and SW teleosts differ in their kidney function in that: 1) glomerular filtration rate (GFR) is higher in FW teleosts, 2) in PSII, FW teleosts pump divalent cations out of the filtrate while SW teleosts pump them in, and 3) the DS and CT are impermeable in FW teleosts and permeable in SW teleosts. It should be noted, that it is necessary for euryhaline teleosts to change kidney function in these three ways as they move from freshwater to saltwater and back. GFR and the divalent ion pump can be changed fairly rapidly, requiring only a day or so to adapt to the new environment. Changing the permeability of the DS and CT involves cell mitosis and takes several days.
Hagfish are iso-osmotic to seawater. The body fluids of the hagfish are about 1000 mOsmol, so they don't need to osmoregulate, however, they do ion regulate, i.e. the concentrations of different ions in the body fluids differ from seawater, but the total osmotic pressure is the same. Lampreys osmoregulate in a fashion similar to teleosts. Chondrichthyans have an inorganic salt content of about one third seawater like teleosts, yet they osmoregulate in an entirely different way. They retain organic salts, primarily non-toxic nitrogenous compounds (urea and trimethylamine oxide) to bring their osmotic pressure up to slightly greater than sea water. They gain water over the gill, so they pass a copious urine like FW teleosts. They passively gain inorganic salts, as well, and excrete these with a specialized rectal gland that fills the same osmoregulatory function as the gill in marine teleosts.
There are interesting contrasts in kidney function between fish and mammals. First, GFR is much higher in mammals (about 650 ml/kg/hr); the average human produces 50 gallons of filtrate a day [ha!]. GFR in mammals is higher because their glomerulus is fed by high pressure arterial flow, while fish (with lower blood pressure to begin with) have the glomerulus supplied with venous blood from the renal portal system (See Ch. IVB). Another difference is in urine flow. Urine flow in mammals is quite variable, of course, and depends on the amount of liquid the animal has imbibed. Mammals are capable of varying the osmoality of their urine as their state of hydration requires. FW fish, on the other hand, have a steady state of hydration because the water flow in through the gills remains fairly constant which results in less fluctuation in urine flow. Finally, fish are unable to produce a urine with a higher osmotic pressure than their blood, while dehydrated humans can concentrate urine to1200 mOsmol and desert rodents can produce urine with over 2000 mOsmol, five to six times saltier than their blood. Mammals can do this because of the special structure of their kidneys. Mammalian kidney tubules form a loop in the medullary region of the kidney termed the loop of Henle. In the ascending part of the loop, salt is pumped out, but water cannot follow because the tubule is impermeable to water. This maintains a high osmotic gradient in the medullary region. When the collecting tubule, which is permeable to water, passes back through this salty region, the water is drawn out of the urine. Mammals can control the osmoality of the urine by regulating the permeability of the CT with hormones.
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