The number of nephrons is constant from birth. If nephrons are injured or become diseased, new ones do not form. Signs of kidney dysfunction usually do not become apparent until function declines < 25% of normal because the remaining nephrons adapt to handle a larger-than-normal load. Surgical removal of one kidney stimulate hypertrophy of the remaining kidney, which is able to filter blood @ 80% of the rate of two normal kidneys.
Overview of Renal Physiology
To produce urine , nephrons and collecting ducts perform 3 basic processes :
1) Glomerular Filtration : water and most solutes in blood plasma move across the wall of the glomerular capillaries into the glomerular capsule and then into the renal tubule. 2) Tubular Reabsorption : Filtered fluid flows along the renal tubule and through the collecting duct where tubule cells reabsorb ~ 99% of the filtered water and many useful solutes. The water and the solutes return to the blood. Note: Reabsorption refers to the return of substances to the blood stream . Absorption by contrast refers to the entry of NEW substances into the body (GIT). 3) Tubular Secretion : as fluid flows along the renal tubule and through the collecting duct, the tubule and duct cells secrete other materials, such as wastes, drugs, and excess ions into the fluid. Note: Tubular Secretion removes a substance from the blood . In contrast, in other parts of the body such as the secretion of hormones cells release substances into interstitial fluid and blood. By filtering, reabsorbing, and secreting, nephrons help maintain homeostasis of the blood’s volume and composition.
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