The kidneys are a pair of fist-sized organs located outside the peritoneal cavity on each side of the spine. 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. The importance of urine formation and excretion as a life-sustaining function is highlighted in situations in which kidney function is suddenly lost. Without at least one functioning kidney, death can occur within a few days.

Urine is a very complex fluid comprised of 95% water and 5% solids. The approximately one to one and one-half liters of urine excreted each day are 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. The afferent arteriole carries blood from the renal artery into the glomerulus, where it divides to form a circulatory network. At the distal end of the glomerulus, the capillaries rejoin to form the efferent arteriole through which blood leaves the glomerulus.

A large amount of circulating blood flows through the kidneys. Approximately 25% of the cardiac output or 1200 ml of blood per minute is received by 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 large afferent arteriole and the pressure in Bowman's capsule.

The renal tubule is responsible for reabsorption and secretion. Reabsorption is the process of moving solutes from the tubules and reabsorbing or returing them to the bloodstream. Some substances such as glucose and sodium are reabsorbed until the plasma level reaches a specific concentration known as the renal threshold. Secretion is the process of transporting solutes into the renal tubule so that they can be excreted in the urine. Secretion allows substances such as hydrogen ions to be eliminated at a rate that exceeds glomerular filtration. Both reaborption and secretion are controlled by the selective permeability of different areas of the renal tubule to water, sodium, and urea (a by-product of protein metabolism) and the response of the distal collecting tubules in the kidney to hormones such as aldosterone, antidiuretic hormone, and parathyroid hormone.

The kidney has a remarkable ability to dilute or concentrate urine, according to an individual's changing physiological needs, and to regulate electrolyte excretion. Impaired renal function has adverse effects on blood chemistry, blood pressure, fluid balance, nutrient intake, and the person's general state of health. When kidney function becomes compromised by disease, the processes of glomerular filtration and renal tubular reabsorption and secretion become affected. Blood and urine biochemical tests reflect the extent of this dysfunction. Renal function tests are used to screen for kidney disease, to help determine the cause of kidney disease, the to determine the extent of renal dysfunction.

Renal failure is a loss of renal function characterized by uremia, the retention of nitrogenous wastes in the blood. Acute renal failure is rapid in onset and can often be reversed. Acute renal failure is classified as prerenal, renal, and postrenal failure. Prerenal failure is caused by decreased blood flow to the kidneys, as occurs with congestive heart failure. Renal failure results from injury to the kidney’s glomeruli and tubules. The most common causes of renal failure are glomerulonephritis, pyelonephritis, and tubular damage caused by drugs, heavy metals, and viral infection. Post renal failure is caused by obstruction in the urinary tract below the kidneys. This can occur from urinary tract stones, tumors, and anatomic obstruction due to benign prostatic hypertrophy. Chronic renal failure has a slow onset, may follow episodes of acute renal failure, and is not reversible. Chronic renal failure often results from acute glomerulonephritis or pyelonephritis. Other reasons for chronic renal failure include diabetes mellitus, atherosclerosis of the renal blood vessels, hypertension, polycystic kidney disease, and kidney stones.