Urinary system
Urinary system in the male. Urine flows from thekidneys via theureters into thebladder where it is stored. When urinating, urine flows through theurethra (longer in males, shorter in females) to exit the body
Details
Latin	Systema urinarium
Anatomical terminology

Theurinary system, also known as therenal system orurinary tract, 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 and metabolites, and regulate blood pH. The urinary tract is the body's drainage system for the eventual removal of urine.^[1] The kidneys have an extensive blood supply via the renal arteries which leave the kidneys via the renal vein. Each kidney consists of functional units callednephrons. Following filtration of blood and further processing, wastes (in the form ofurine) exit the kidney via the ureters, tubes made of smooth muscle fibres that propel urine towards the urinary bladder, where it is stored and subsequently expelled from the body byurination (voiding). The female and male urinary system are very similar, differing only in the length of the urethra.^[2]

Urine is formed in the kidneys through a filtration ofblood. The urine is then passed through the ureters to the bladder, where it is stored. During urination, the urine is passed from the bladder through the urethra to the outside of the body.

800–2,000milliliters (mL) of urine are normally produced every day in a healthy human. This amount varies according to fluid intake and kidney function.

Structure

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 together 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 through the external urethral meatus. The female urethra is much shorter, beginning at the bladder neck and terminating in the vaginal vestibule.

Development

Microanatomy

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.

Function

The main functions of the urinary system and its components are to

• Regulateblood volume and composition (e.g.sodium,potassium andcalcium)
• Regulateblood pressure.
• RegulatepHhomeostasis of the blood.
• Contributes to the production of red blood cells by thekidney.
• Helps synthesizecalcitriol the (active form ofVitamin D).
• Stores waste product (mainlyurea anduric acid) before it and other products are removed from the body.

Urine formation

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).Oliguria when < 400 mL (millilitres) are produced, andanuria one of < 100 mL per 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.

The urinary system is regulated by theendocrine system byhormones such asantidiuretic hormone,aldosterone, andparathyroid hormone.^[3]

Regulation of concentration and volume

The urinary system is under influence of thecirculatory system,nervous system, andendocrine system.

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]

Urination

Urination is the ejection ofurine from theurinary bladder through theurethra to the outside of the body. In healthy humans (andmany other animals), the process of urination is under voluntary control. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntaryreflex. Physiologically, micturition involves coordination between thecentral,autonomic, andsomatic nervous systems. Brain centers that regulate urination include thepontine micturition center,periaqueductal gray, and thecerebral cortex. Inplacental mammals the male ejects urine through thepenis, and the female through thevulva.

Clinical significance

Urologic disease can involve congenital or acquired dysfunction of the urinary system. As an example,urinary tract obstruction is a urologic disease that can causeurinary retention.

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]

Diabetes also can have a direct effect inurination due toperipheral neuropathies, which occur in some individuals with poorly controlled blood sugar levels.^[7]

Urinary incontinence can result from a weakening of thepelvic floor muscles caused by factors such aspregnancy,childbirth,aging, and beingoverweight. 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.

Some cancers also target the urinary system, includingbladder cancer,kidney cancer,ureteral cancer, andurethral cancer. Due to the role and location of these organs, treatment is often complicated.^{[citation needed]}

History

Kidney stones have been identified and recorded about as long as written historical records exist.^[8] 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.^[9]

The first to examine the ureter through an internal approach, called ureteroscopy, rather than surgery wasHampton Young in 1929.^[8] This was improved on byVF Marshall who is the first published use of a flexibleendoscope based onfiber optics, which occurred in 1964.^[8] The insertion of a drainage tube into therenal pelvis, bypassing the uterers 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 preceeding two millenia.^[8]

See also

• Excretory system
• Major systems of the human body

References

External links

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