cortex to form a collecting duct. The collecting ductbegins to merge within the renal medulla. Thecollecting ducts become increasingly larger as they arejoined by other collecting ducts. The resulting tube iscalled the papillary duct. The papillary duct emptiesinto the minor calyx through an opening in the renalpapilla.FunctionThe kidneys are effective blood purifiers and fluidbalance regulators. In addition to maintaining a normalpH of the blood (acid-base balance), the kidneys keepthe blood slightly alkaline by removing excesssubstances from the blood. The end product of thesefunctions is the formation of urine, which is excretedfrom the body.Urine is formed through a series of processes in thenephron. These processes are filtration, reabsorption,and secretion.FILTRATION.—Urine formation begins whenwater and various dissolved substances are filtered outof blood plasma from a glomerular capillary into theglomerular capsule. The filtered substance (glomerularfiltrate) leaves the glomerular capsule and enters therenal tubule.REABSORPTION.—As glomerular filtratepasses through the renal tubule, some of the filtrate isreabsorbed into the blood of the peritubular capillary(fig. 1-57). The filtrate entering the peritubularcapillary will repeat the filtration cycle. This processof reabsorption changes the composition of urine. Forinstance, the filtrate entering the renal tubule is high insugar content, but because of the reabsorption process,urine secreted from the body does not contain sugar.SECRETION.—Secretion is the process bywhich the peritubular capillary transports certainsubstances directly into the fluid of the renal tubule(fig. 1-58). These substances are transported by similarmechanisms as used in the reabsorption process, butdone in reverse. For example, certain organiccompounds, such as penicillin and histamine, aresecreted directly from the proximal convoluted tubuleto the renal tubule. Also, large quantities of hydrogenions are secreted in this same manner. The secretion ofhydrogen ions plays an important role in regulating pHof body fluids.The glomerulus filters gallons of blood each day. Itis estimated that 2,500 gallons of blood pass throughthe kidneys in 24 hours, and about 80 gallons ofglomerular filtrate. All the water from this filtrate isreabsorbed in the renal tubules except that containingthe concentrated waste products.1-55HM3F0158EFFERENTARTERIOLEAFFERENTARTERIOLEGLOMERULARCAPSULEGLOMERULUSGLOMERULARFILTRATERENALTUBULEPERITUBULARCAPILLARYSECRETIONFigure 1-58.—The secretion process.HM3F0157EFFERENTARTERIOLEBLOODFLOWAFFERENTARTERIOLEGLOMERULARCAPSULEGLOMERULUSGLOMERULARFILTRATEREABSORPTIONRENALTUBULEPERITUBULARCAPILLARYBLOODFLOWFigure 1-57.—The reabsorption process.
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