Together with the skin and the respiratory system, the kidneys are the body's primary excretory organs. The kidney is a highly specialized organ that maintains the internal environment of the body by selectively excreting or retaining various substances according to specific body needs.

Urine formation takes place in the kidneys - two small fist-sized organs located outside the peritoneal cavity on each side of the spine. Urine is a very complex fluid comprised of 95% water and 5% solids. Approximately one and one-half liters of urine is excreted each day. This urine is the end product of metabolism carried out by billions of cells in the renal and urinary systems. The nephron is the kidney's primary functional unit. Each kidney has about 1 million nephrons. Each nephron contains a filtering system known as a glomerulus, and a tubule, through which the filtered liquid passes. Each glomerulus consists of a capillary network surrounded by a membrane called Bowman's capsule.

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A very large amount of circulating blood flows through the kidneys. One liter of urine is the end product of more than 1000 liters of circulating blood processed through the kidneys. Urine formation begins in the glomerular capillaries, with dissolved substances passing into the proximal tubule as a result of the force of blood pressure in the kidney's arterioles and the pressure in Bowman's capsule. As the filtrate passes down the tubule, more solutes are added by excretion from the blood and secretions from the tubular epithelial cells. Some solutes and water pass back into the blood through the mechanics of tubular reabsorption. The kidney has a remarkable ability to dilute or concentrate urine, according to an individual's changing physiological needs, and to regulate sodium excretion.

A urinalysis (UA) is an inexpensive, noninvasive test that provides a wealth of diagnostic information. It is also the oldest clinical test of body fluids, and has been used throughout history to diagnose illness. A urinalysis is an essential procedure for patients who are acutely or chronically ill and for healthy people as a means of detecting early signs of disease. Urinalysis can reveal diseases that have gone unnoticed because they may not produce dramatic signs or symptoms, such as diabetes mellitus, glomerulonephritis, and urinary tract infections. The basic UA is a cost-effective screening test that may indicate the need for a more thorough assessment. Because a routine urinalysis is done so frequently, the nurse needs to understand the meaning of each component of the urinalysis.

The analysis of a patient's urine has 2 important purposes:

Some of the kidney diseases that a UA helps to diagnose are nephrosis (degeneration of the kidneys without inflammation), nephritis (inflammation of the kidney, including pyelonephritis or glomerulonephritis), and cystitis (inflammation of the bladder).

A UA involves three steps:

  1. The physical characteristics of the urine are analyzed and recorded.
  2. A series of chemical tests are run. A chemically impregnated dipstick may be used for many aspects of the UA and standardized results are obtained by processing the urine-touched dipstick through special automated instruments.
  3. The urine sediment is examined under the microscope. The most cost-effective method of screening urine is a paper or plastic dipstick. The color change that occurs on the dipstick is compared to a color chart to obtain results, usually within in a minute. Microscopic analysis may be done with a relatively inexpensive light microscope.

Urine contains thousand of dissolved substances, although the three principal constituents are water, urea, and sodium chloride. More solids are excreted from the body through urine than by any other method. Urine composition depends greatly on the quality and the quantity of the excreted waste material. Almost all substances found in the urine are also found in the blood, although in different concentrations. Urea, for example, is present in the blood, but at a much lower concentration than in the urine. Some substances found in the blood, such as glucose, must exceed a "renal threshold" before they appear in the urine.

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Urinalysis is used to detect disease involving the urinary tract as well as other parts of the body.