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The kidneys, the bladder, and the ureters make up the urinary system. Within the kidney, the functional unit is the nephron and each human kidney has approximately 1 million nephrons. The kidneys play an essential role in the regulation of water homeostasis, electrolyte composition (eg, Na, Cl, K, HCO3), regulation of extracellular volume (thus blood pressure), and acid–base homeostasis (Chapter 39). The kidneys filter the plasma of the blood and produce urine which allows the kidneys to excrete metabolic waste products from the body such as urea, ammonium, and foreign chemicals such as drug metabolites. The kidneys are responsible for the reabsorption of glucose and amino acids from the plasma filtrate in addition to regulated calcium and phosphate uptake (high in children). The kidneys play a role in gluconeogenesis and during fasting can synthesize and release glucose into the blood, producing almost 20% of the liver's glucose capacity. The kidneys are also endocrine organs, making kinins (see Chapter 32), 1, 25-dihydroxycholecalciferol (see Chapter 21), erythropoietin (see Chapter 37) and making and secreting renin (see Chapter 38).

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Specifically, in the kidneys, a fluid that resembles plasma is filtered through the glomerular capillaries into the renal tubules (glomerular filtration). As this glomerular filtrate passes down the tubules, its volume is reduced and its composition altered by the processes of tubular reabsorption (removal of water and solutes from the tubular fluid) and tubular secretion (secretion of solutes into the tubular fluid) to form the urine that enters the renal pelvis. From the renal pelvis, the urine passes to the bladder and is expelled to the exterior by the process of urination, or micturition.

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Diseases of the kidney are numerous. Commonly occurring clinical conditions include acute kidney injury, chronic kidney disease, diabetic kidney disease, nephritic and nephrotic syndromes, polycystic kidney disease, urinary tract obstruction, urinary tract infection, renal cancer. When renal function decreases such that the kidneys are no longer functioning to maintain health, patients sometimes undergo dialysis and eventually kidney transplantation.

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The prevalence of kidney disease is increasing across the world as is the cost of treating the diseases which cause kidney damage; thus kidney disease represents a large threat to healthcare resources worldwide. The two diseases that cause the largest prevalence of kidney dysfunction are diabetes and hypertension. Nearly one billion people worldwide have high blood pressure and that number is expected to increase to 1.56 billion by 2025. There are currently over 240 million people with diabetes worldwide and this figure is anticipated to rise to 380 million by 2025. Of those with diabetes, about 40% will develop chronic kidney disease (CKD), which means that their risk of cardiovascular complications also increases. The cumulative global cost for dialysis and transplantation over the next decade is predicted to exceed US$ 1 trillion.

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