<P> The last step in the processing of the ultrafiltrate is excretion: the ultrafiltrate passes out of the nephron and travels through a tube called the collecting duct, which is part of the collecting duct system, and then to the ureters where it is renamed urine . In addition to transporting the ultrafiltrate, the collecting duct also takes part in reabsorption . </P> <P> The kidney participates in whole - body homeostasis, regulating acid - base balance, electrolyte concentrations, extracellular fluid volume, and blood pressure . The kidney accomplishes these homeostatic functions both independently and in concert with other organs, particularly those of the endocrine system . Various endocrine hormones coordinate these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others . </P> <P> The kidneys excrete a variety of waste products produced by metabolism into the urine . These include the nitrogenous wastes urea, from protein catabolism, and uric acid, from nucleic acid metabolism . The ability of mammals and some birds to concentrate wastes into a volume of urine much smaller than the volume of blood from which the wastes were extracted is dependent on an elaborate countercurrent multiplication mechanism . This requires several independent nephron characteristics to operate: a tight hairpin configuration of the tubules, water and ion permeability in the descending limb of the loop, water impermeability in the ascending loop, and active ion transport out of most of the ascending limb . In addition, passive countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function . </P> <P> Two organ systems, the kidneys and lungs, maintain acid - base homeostasis, which is the maintenance of pH around a relatively stable value . The lungs contribute to acid - base homeostasis by regulating carbon dioxide (CO) concentration . The kidneys have two very important roles in maintaining the acid - base balance: to reabsorb and regenerate bicarbonate from urine, and to excrete hydrogen ions and fixed acids (anions of acids) into urine . </P>

What is the role of the kidney in excretion
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