Urinary system in the male. Urine flows from thekidneys via theureters into thebladder where it is stored until it exits the body through theurethra (longer in males, shorter in females) during urination
The humanurinary system, also known as theurinary tract orrenal system, consists of thekidneys,ureters,bladder, and theurethra. The purpose of the urinary system is to eliminate waste from the body, regulate blood volume and blood pressure, control levels ofelectrolytes andmetabolites, and regulateblood pH. The urinary tract is the body's drainage system for the eventual removal ofurine.[1] The kidneys have an extensive blood supply via therenal arteries which leave the kidneys via therenal vein. Each kidney consists of functional units callednephrons. Followingfiltration of blood and further processing, waste (in the form ofurine) exits the kidney via the ureters, tubes made ofsmooth muscle fibres that propel urine towards the urinary bladder, where it is stored and subsequently expelled through theurethra duringurination. The female and male urinary system are very similar, differing only in the length of the urethra.[2]
800–2,000 milliliters (mL) of urine are normally produced every day in a healthy human. This amount varies according to fluid intake and kidney function.
The urinary system refers to the structures that produce and transport urine to the point of excretion. In the human urinary system there are two kidneys that are located between the dorsal body wall and parietal peritoneum on both the left and right sides.
The formation of urine begins within the functional unit of the kidney, the nephrons. Urine then flows through the nephrons, through a system of converging tubules called collecting ducts. These collecting ducts then join to form theminor calyces, followed by the major calyces that ultimately join the renal pelvis. From here, urine continues its flow from the renal pelvis into the ureter, transporting urine into the urinary bladder. The anatomy of the human urinary system differs between males and females at the level of the urinary bladder. In males, the urethra begins at the internal urethral orifice in the trigone of the bladder, continues through the external urethral orifice, and then becomes the prostatic, membranous, bulbar, and penile urethra. Urine exits the male urethra through theurinary meatus in theglans penis. The female urethra is much shorter, beginning at the bladder neck and terminating in thevulval vestibule.
Under microscopy, the urinary system is covered in a unique lining calledurothelium, a type oftransitional epithelium. Unlike theepithelial lining of most organs, transitional epithelium can flatten and distend. Urothelium covers most of the urinary system, including the renal pelvis, ureters, and bladder.
Average urine production in adult humans is about 1–2litres (L) per day, depending on state of hydration, activity level, environmental factors, weight, and the individual's health. Producing too much or too little urine requires medical attention.Polyuria is a condition of excessive urine production (> 2.5 L/day). Conditions involving low output of urine areoliguria (< 400 mL/day) andanuria (< 100 mL/day).
The first step in urine formation is the filtration of blood in the kidneys. In a healthy human, the kidney receives between 12 and 30% ofcardiac output, but it averages about 20% or about 1.25 L/min.
The basic structural and functional unit of the kidney is thenephron. Its chief function is to regulate theconcentration ofwater and soluble substances likesodium by filtering theblood, reabsorbing what is needed and excreting the rest asurine.
In the first part of the nephron,Bowman's capsule filters blood from thecirculatory system into the tubules. Hydrostatic and osmotic pressure gradients facilitate filtration across a semipermeable membrane. The filtrate includes water, small molecules, and ions that easily pass through the filtration membrane. However, larger molecules such asproteins andblood cells are prevented from passing through the filtration membrane. The amount of filtrate produced every minute is called theglomerular filtration rate or GFR and amounts to 180 litres per day. About 99% of this filtrate is reabsorbed as it passes through the nephron and the remaining 1% becomes urine.
Aldosterone plays a central role in regulating blood pressure through its effects on the kidney. It acts on the distal tubules and collecting ducts of the nephron and increases reabsorption of sodium from the glomerular filtrate. Reabsorption of sodium results in retention of water, which increases blood pressure and blood volume. Antidiuretic hormone (ADH), is aneurohypophysial hormone found in mostmammals. Its two primary functions are to retain water in the body andvasoconstriction. Vasopressin regulates the body'sretention of water by increasing water reabsorption in the collecting ducts of the kidney nephron.[4] Vasopressin increases water permeability of the kidney's collecting duct and distal convoluted tubule by inducing translocation of aquaporin-CD water channels in the kidney nephron collecting duct plasma membrane.[5]
Diseases of the kidney tissue are normally treated bynephrologists, while diseases of the urinary tract are treated byurologists.Gynecologists may also treat female urinary incontinence.
Diseases of other bodily systems also have a direct effect on urogenital function. For instance, it has been shown thatprotein released by the kidneys indiabetes mellitus sensitizes the kidney to the damaging effects ofhypertension.[6]
Urinary incontinence can result from a weakening of thepelvic floor muscles caused by factors such aspregnancy,childbirth,aging, and beingoverweight. Findings recent systematic reviews demonstrate that behavioral therapy generally results in improved urinary incontinence outcomes, especially forstress andurge UI, than medications alone.[8][9] Pelvic floor exercises known asKegel exercises can help in this condition by strengthening the pelvic floor. There can also be underlying medical reasons for urinary incontinence which are often treatable. In children, the condition is calledenuresis.
Kidney stones have been identified and recorded about as long as written historical records exist.[10] The urinary tract including the ureters, as well as their function to drain urine from the kidneys, has been described byGalen in the second century AD.[11]
The first to examine the ureter through an internal approach, called ureteroscopy, rather than surgery wasHampton Young in 1929.[10] This was improved on byVF Marshall who is the first published use of a flexibleendoscope based onfiber optics, which occurred in 1964.[10] The insertion of a drainage tube into therenal pelvis, bypassing the ureters and urinary tract, callednephrostomy, was first described in 1941. Such an approach differed greatly from theopen surgical approaches within the urinary system employed during the preceding two millennia.[10]
^C. Dugdale, David (16 September 2011)."Female urinary tract".MedLine Plus Medical Encyclopedia.
^Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993).Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall.ISBN0-13-981176-1.
^Nahon, I; Waddington, G; Dorey, G; Adams, R (2011). "The history of urologic surgery: from reeds to robotics".Urologic Nursing.31 (3):173–80.doi:10.7257/1053-816X.2011.31.3.173.PMID21805756.
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