urinary system

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The bodily system consisting of the organs that produce, collect, and eliminate urine and including the kidneys, ureters, urinary bladder, and urethra.

The urinary system consists of the kidneys, urinary ducts, and bladder. Similarities are not particularly evident among the many and varied types of excretory organs found among vertebrates. The variations that are encountered are undoubtedly related to problems with which vertebrates have had to cope in adapting to different environmental conditions.

Kidneys

In reptiles, birds, and mammals three types of kidneys are usually recognized: the pronephros, mesonephros, and metanephros. These appear in succession during embryonic development, but only the metanephros persists in the adult.

The metanephric kidneys of reptiles lie in the posterior part of the abdominal cavity, usually in the pelvic region. They are small, compact, and often markedly lobulated. The posterior portion on each side is somewhat narrower. In some lizards the hind parts may even fuse. The degree of symmetry varies.

The kidneys of birds are situated in the pelvic region of the body cavity; their posterior ends are usually joined. They are lobulated structures with short ureters which open independently into the cloaca.

A rather typical mammalian metanephric kidney (Fig. 1) is a compact, bean-shaped organ attached to the dorsal body wall outside the peritoneum. The ureter leaves the medial side at a depression, the hilum. At this point a renal vein also leaves the kidney and a renal artery and nerves enter it. The kidneys of mammals are markedly lobulated in the embryo, and in many forms this condition is retained throughout life. See also Kidney.

Sagittal section of a human metanephric kidney (semidiagrammatic). (After C. K. Weichert and W. Presch, Elements of Chordate Anatomy, 4th ed., McGraw-Hill, 1975)

Urinary bladder

At or near the posterior ends of the nephric ducts there frequently is a reservoir for urine. This is the urinary bladder. Actually there are two basic varieties of bladders in vertebrates. One is found in fishes in which the reservoir is no more than an enlargement of the posterior end of each urinary duct. Frequently the urinary ducts are conjoined and a small bladder is formed by expansion of the common duct. The far more common type of bladder is that exhibited by tetrapods. This is a sac which originates embryonically as an outgrowth from the ventral side of the cloaca. Present in all embryonic life, it is exhibited differentially in adults. All amphibians retain the bladder, but it is lacking in snakes, crocodilians, and most lizards; birds, also, with the exception of the ostrich, lack a bladder. It is present in all mammals. See also Urinary bladder.

Physiology

Urine is produced by individual renal nephron units which are fundamentally similar from fish to mammals (Fig. 2); however, the basic structural and functional pattern of these nephrons varies among representatives of the vertebrate classes in accordance with changing environmental demands. Kidneys serve the general function of maintaining the chemical and physical constancy of blood and other body fluids. The most striking modifications are associated particularly with the relative amounts of water made available to the animal. Alterations in degrees of glomerular development, in the structural complexity of renal tubules, and in the architectural disposition of the various nephrons in relation to one another within the kidneys may all represent adaptations made either to conserve or eliminate water.

A mammalian metanephric tubule, showing the renal corpuscle and secretory and collecting portions. (After C. K. Weichert and W. Presch, Elements of Chordate Anatomy, 4th ed., McGraw-Hill, 1975)

Regulation of volume

Except for the primitive marine cyclostome Myxine, all modern vertebrates, whether marine, fresh-water, or terrestrial, have concentrations of salt in their blood only one-third or one-half that of seawater. The early development of the glomerulus can be viewed as a device responding to the need for regulating the volume of body fluids. Hence, in a hypotonic fresh-water environment the osmotic influx of water through gills and other permeable body surfaces would be kept in balance by a simple autoregulatory system whereby a rising volume of blood results in increased hydrostatic pressure which in turn elevates the rate of glomerular filtration. Similar devices are found in fresh-water invertebrates where water may be pumped out either as the result of work done by the heart, contractile vacuoles, or cilia found in such specialized “kidneys” as flame bulbs, solenocytes, or nephridia that extract excess water from the body cavity rather than from the circulatory system. Hence, these structures which maintain a constant water content for the invertebrate animal by balancing osmotic influx with hydrostatic output have the same basic parameters as those in vertebrates that regulate the formation of lymph across the endothelial walls of capillaries. See also Osmoregulatory mechanisms.

Electrolyte balance

A system that regulates volume by producing an ultrafiltrate of blood plasma must conserve inorganic ions and other essential plasma constituents. The salt-conserving operation appears to be a primary function of the renal tubules which encapsulate the glomerulus. As the filtrate passes along their length toward the exterior, inorganic electrolytes are extracted from them through highly specific active cellular resorptive processes which restore plasma constituents to the circulatory system.

Movement of water

Concentration gradients of water are attained across cells of renal tubules by water following the active movement of salt or other solute. Where water is free to follow the active resorption of sodium and covering anions, as in the proximal tubule, an isosmotic condition prevails. Where water is not free to follow salt, as in the distal segment in the absence of antidiuretic hormone, a hypotonic tubular fluid results.

Nitrogenous end products

Of the major categories of organic foodstuffs, end products of carbohydrate and lipid metabolism are easily eliminated mainly in the form of carbon dioxide and water. Proteins, however, are more difficult to eliminate because the primary derivative of their metabolism, ammonia, is a relatively toxic compound. For animals living in an aquatic environment ammonia can be eliminated rapidly by simple diffusion through the gills. However, when ammonia is not free to diffuse into an effectively limitless aquatic environment, its toxicity presents a problem, particularly to embryos of terrestrial forms that develop wholly within tightly encapsulated eggshells or cases. For these forms the detoxication of ammonia is an indispensable requirement for survival. During evolution of the vertebrates two energy-dependent biosynthetic pathways arose which incorporated potentially toxic ammonia into urea and uric acid molecules, respectively. Both of these compounds are relatively harmless, even in high concentrations, but the former needs a relatively large amount of water to ensure its elimination, and uric acid requires a specific energy-demanding tubular secretory process to ensure its efficient excretion. See also Urea; Uric acid.

Urine concentration

The unique functional feature of the mammalian kidney is its ability to concentrate urine. Human urine can have four times the osmotic concentration of plasma, and some desert rats that survive on a diet of seeds without drinking any water have urine/plasma concentration ratios as high as 17. More aquatic forms such as the beaver have correspondingly poor concentrating ability.

The concentration operation depends on the existence of a decreasing gradient of solute concentration that extends from the tips of the papillae in the inner medulla of the kidney outward toward the cortex. The high concentration of medullary solute is achieved by a double hairpin countercurrent multiplier system which is powered by the active removal of salt from urine while it traverses the ascending limb of Henle's loop (Fig. 2). The salt is redelivered to the tip of the medulla after it has diffused back into the descending limb of Henle's loop. In this way a hypertonic condition is established in fluid surrounding the terminations of the collecting ducts. Urine is concentrated by an entirely passive process as water leaves the lumen of collecting ducts to come into equilibrium with the hypertonic fluid surrounding its terminations. See also Urine; Urogenital system.

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A body system consisting of a pair of kidneys and their associated blood supply and tubes. The urinary system helps to maintain fluid and electrolyte balance, provides long-term regulation of blood pressure, and eliminates nitrogenous waste products (e.g. urea) from the blood while helping to retain useful substances (e.g. glucose).

It has been suggested that this article or section be merged into Excretory system. (Discuss)

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.

Physiology

Kidneys viewed from behind with spine removed

Main article: Kidney

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.

Bladder

Main article: Bladder

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.

Urethra

Main article: Urethra

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:

• Urinary tract infections (UTIs), interstitial cystitis
• incontinence (involuntary loss of urine), benign prostatic hyperplasia (where the prostate overgrows), prostatitis (inflammation of the prostate).
• Transitional cell carcinoma (bladder cancer), renal cell carcinoma (kidney cancer), and prostate cancer are examples of neoplasms affecting the urinary system.

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

Testing

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.

Radiology:

• KUB is plain radiography of the urinary system, e.g. to identify kidney stones.
• An intravenous pyelogram studies the shape of the urinary system.
• CAT scans and MRI can also be useful in localising urinary tract pathology.

Development

Main article: Development of the urinary and reproductive organs

See also

• Urothelium
• Major systems of the human body

References

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

Anatomy: urinary system
Kidneys • Ureters • Urinary bladder (Uvula)  • Urethral sphincters • Urethra

Human organ systems
Cardiovascular system • Digestive system • Endocrine system • Immune system • Integumentary system • Lymphatic system • Muscular system • Nervous system • Reproductive system • Respiratory system • Skeletal system • Urinary system

Urinary system, physiology: renal physiology and acid base physiology
Filtration	Ultrafiltration - Countercurrent exchange
Hormones affecting filtration	Antidiuretic hormone (ADH) - Aldosterone - Atrial natriuretic peptide
Endocrine	Renin - Erythropoietin (EPO) - Calcitriol (Active vitamin D) - Prostaglandins
Assessing Renal function / Measures of dialysis	Glomerular filtration rate - Creatinine clearance - Renal clearance ratio - Urea reduction ratio - Kt/V - Standardized Kt/V - Hemodialysis product
Acid base physiology	Fluid balance - Darrow Yannet diagram - Body water - Interstitial fluid - Extracellular fluid - Intracellular fluid/Cytosol - Plasma - Transcellular fluid - Base excess - Davenport diagram - Anion gap
Buffering/compensation	Bicarbonate buffering system - Respiratory compensation - Renal compensation

Urinary system - Pathology - Nephrology (N00-N39, 580-599)
Diseases of the glomerulus	Glomerulonephritis - Focal segmental glomerulosclerosis - Membranoproliferative glomerulonephritis - Membranous glomerulonephritis - Nephritic syndrome - Post-streptococcal glomerulonephritis - Nephrotic syndrome (Minimal change disease) - IgA nephropathy - Lupus nephritis - Diabetic nephropathy - Rapidly progressive glomerulonephritis
Tubulointerstitial diseases of the kidney	Interstitial nephritis - Pyelonephritis - Hydronephrosis - Pyonephrosis - Balkan nephropathy - Reflux nephropathy
Renal failure	Acute renal failure (Acute tubular necrosis) - Chronic renal failure
Diseases of the renal tubule and other disorders of kidney and ureter	Renal osteodystrophy - Nephrogenic diabetes insipidus - Renal tubular acidosis - Nephroptosis - Ureterocele
Other diseases and disorders of urinary system	Cystitis (Interstitial cystitis, Trigonitis) - Neurogenic bladder - Vesicointestinal fistula - Urethritis - Urethral stricture - Urinary tract infection - Kidney stone
Tumors of the kidney	Renal cell carcinoma - Wilms' tumor (children)
See also congenital conditions (Q60-Q64, 753)

Prenatal development/mammalian embryogenesis - Development of the urinary and reproductive organs
General Urinary/Reproductive system	Cloacal membrane - Cloaca - Urogenital folds - Urethral groove - Urogenital sinus - Urachus
Kidney development	Pronephros - Mesonephros - Ureteric bud - Metanephric blastema
Fetal genital development	Pronephric duct - Wolffian duct - Müllerian duct - Gubernaculum - Genital tubercle - Labioscrotal folds - Gonadal ridge - Sex cord - Cortical cords

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