Mammals have high body temperatures and high metabolic rates, and therefore have the potential for a high rate of water loss and a high level of nitrogenous waste production. Having an excretory system that minimizes water loss made it possible for these highly active species to occupy arid habitats.
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Nephrons are organized in the kidney so that the glomeruli and major portions of the renal tubules (called convoluted tubules) are in the outer region, or cortex, and straight sections of tubules (called loops of Henle) and collecting ducts are in the inner region, or medulla.
Most of the filtrate is reabsorbed isosmotically in the proximal convoluted tubule.
Loops of Henle are countercurrent multipliers that create a concentration gradient in the extracellular fluid.
As the tubular fluid passes down the collecting duct, water can be reabsorbed into the extracellular fluid to produce a more concentrated urine in the tubule.
The mammalian kidney functions in conjunction with the lungs to regulate the pH of the extracellular fluid.
Media Clip 51.1 Inside the Bladder
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