Urinary system

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [2]


The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder, and the urethra. The analogous organ in invertebrates is the nephridium (in animals).


Kidneys viewed from behind with spine removed

Typically, every human has two kidneys. The kidneys are bean-shaped organs about the size of a bar of soap. The kidneys lie in the abdomen, posterior or retroperitoneal to the organs of digestion, around or just below the ribcage and close to the lumbar spine. The kidneys are surrounded by what is called peri-nephric fat, and situated on the superior pole of each kidney is an adrenal gland. The kidneys receive their blood supply of 1.25 L/min (25% of the cardiac output) from the renal arteries which are fed by the Abdominal aorta. This is important because the kidneys' main role is to filter water soluble waste products from the blood. The other attachment of the kidneys are at their functional endpoints the ureters, which lies more medial and runs down to the trigone of the bladder.

Functionally the kidney performs a number of tasks. In its role in the urinary system it concentrates urine, plays a crucial role in regulating electrolytes, and maintains acid-base homeostasis. The kidney excretes and re-absorbs electrolytes (e.g. sodium, potassium and calcium) under the influence of local and systemic hormones. pH balance is regulated by the excretion of bound acids and ammonium ions. In addition, they remove urea, a nitrogenous waste product from the metabolism of proteins from amino acids. The end point is a hyperosmolar solution carrying waste for storage in the bladder prior to urination.

Humans produce about 1.5 liters of urine over 24 hours, although this amount may vary according to circumstances. Because the rate of filtration at the kidney is proportional to the glomerular filtration rate, which is in turn related to the blood flow through the kidney, changes in body fluid status can affect kidney function. Hormones exogenous and endogenous to the kidney alter the amount of blood flowing through the glomerulus. Some medications interfere directly or indirectly with urine production. Diuretics achieve this by altering the amount of absorbed or excreted electrolytes or osmalites, which causes a diuresis.


The urinary bladder is a hollow muscular organ shaped like a balloon. It is located in the anterior pelvis. The bladder stores urine; it swells into a round shape when it is full and gets smaller when empty. In the absence of bladder disease, it can hold up to 500 mL (17 fl. oz.) of urine comfortably for two to five hours. The epithelial tissue associated with the bladder is called transitional epithelium. It allows the bladder to stretch to accommodate urine without rupturing the tissue.

Normally the bladder is sterile.

Sphincters (circular muscles) regulate the flow of urine from the bladder. The bladder itself has a muscular layer (detrusor muscle) that, when contracted, increases pressure on the bladder and creates urinary flow.

Urination is a conscious process, generally initiated by stretch receptors in the bladder wall which signal to the brain that the bladder is full. This is felt as an urge to urinate. When urination is initiated, the sphincter relaxes and the detrusor muscle contracts, producing urinary flow.


The endpoint of the urinary system is the urethra. Typically the urethra in humans is colonised by commensal bacteria below the external urethral sphincter. The urethra emerges from the end of the penis in males and between the clitoris and vagina in females.

Role in disease

Kidney diseases are normally investigated and treated by nephrologists, while the specialty of urology deals with problems in the other organs. Gynecologists may deal with problems of incontinence in women.

Diseases of other bodily systems also have a direct effect on urogenital function. For instance it has been shown that protein released by the kidneys in diabetes mellitus sensitises the kidney to the damaging effects of hypertension[1].

Diabetes also can have a direct effect in micturition due to peripheral neuropathies which occur in some individuals with poorly controlled diabetics.

Kidney disease

Renal failure is defined by functional impairment of the kidney. Renal failure can be acute or chronic, and can be further broken down into categories of pre-renal, intrinsic renal and post-renal.

Pre-renal failure refers to impairment of supply of blood to the functional nephrons including renal artery stenosis. Intrinsic renal diseases are the classic diseases of the kidney including drug toxicity and nephritis. Post-renal failure is outlet obstruction after the kidney, such as a renal stone or prostatic bladder outlet obstruction.

Renal failure may require medication, dietary and lifestyle modification and dialysis.

Primary renal cell carcinomas as well as metastatic cancers can affect the kidney.

Non-renal urinary tract disease

The causes of diseases of the body are common to the urinary tract. Structural and or traumatic change can lead to hemorrhage, functional blockage or inflammation. Colonisation by bacteria, protozoa or fungi can cause infection. Uncontrolled cell growth can cause neoplasia.

For example:

The term "uropathy" refers to a disease of the urinary tract, while "nephropathy" refers to a disease of the kidney.


Biochemical blood tests determine the amount of typical markers of renal function in the blood serum, for instance serum urea and serum creatinine. Biochemistry can also be used to determine serum electrolytes. Special biochemical tests (arterial blood gas) can determine the amount of dissolved gases in the blood, indicating if pH imbalances are acute or chronic.

Urinalysis is a test that studies the content of urine for abnormal substances such as protein or signs of infection.

  • A Full Ward Test, also known as dipstick urinalysis, involves the dipping of a biochemically active test strip into the urine specimen to determine levels of tell-tale chemicals in the urine.
  • Urinalysis can also involve MC&S microscopy , culture and sensitivity

Urodynamic tests evaluate the storage of urine in the bladder and the flow of urine from the bladder through the urethra. It may be performed in cases of incontinence or neurological problems affecting the urinary tract.

Ultrasound is commonly performed to investigate problems of the kidney and/or urinary tract.



See also


  1. Baba T, Murabayashi S, Tomiyama T, Takebe K; Uncontrolled hypertension is associated with a rapid progression of nephropathy in type 2 diabetic patients with proteinuria and preserved renal function. Tohoku J Exp Med 1990 Aug;161(4):311-8 PMID 2256104

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