What is the urinary system?

Structure and Functions

The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The kidneys function to remove metabolic waste from the blood, maintain proper water balance for the body, and maintain the proper acid-base balance in the blood. The ureters, urinary bladder, and urethra are involved in the moving of the urine formed in the kidneys to the external environment. The kidneys play the major role in the function of the urinary system.

Most people have two kidneys, located at the lower end of the rib cage and lying against the back of the body wall. Typically, the right kidney is positioned a little lower than the left kidney because the right kidney is pushed down by the liver. An adult kidney is about 12.5 centimeters long, 7.5 centimeters wide, and 2.5 centimeters thick and is shaped like a kidney bean. Each kidney is surrounded by a thick layer of fat, which is important for holding the kidneys in their normal body position.

Inside each kidney is a lighter outer region called the renal cortex. Deep in the cortex is a darker layer called the renal medulla. Within the cortex and medulla are found tiny structures called nephrons. Each kidney contains approximately one million nephrons, most of which are in the renal cortex. Nephrons are the functional units of the kidney, carrying out the processes involved in urine formation.

Each nephron consists of two main parts, the glomerulus and the renal tubule. The glomerulus is composed of a knot of capillaries that fit inside the Bowman’s capsule, the cup-shaped head of the renal tubule. The rest of the
renal tubule is about 2.5 centimeters long. The neck of the renal tubule undergoes a high degree of coiling and twisting just before it makes a hairpin loop. This part of the renal tubule is called the proximal convoluted tubule. The hairpin loop of the renal tubule is termed the loop of Henle. After coming out of this loop, the renal tubule again undergoes a high degree of coiling and twisting and is called the distal convoluted tubule. The distal convoluted tubule then enters another tube, the collecting duct. Surrounding and encasing the renal tubule is the peritubular capillary bed.

Urine formation occurs in the nephron and is the result of three processes: glomerular filtration, tubular reabsorption, and tubular secretion. The glomerulus acts as a filter. This process of glomerular filtration occurs as a result of the capillaries in the glomerulus being somewhat leaky as compared to other capillaries in the body. This process of filtration is a passive process that does not require any metabolic energy. High pressure in the glomerular capillaries causes the formation of a filtrate that consists primarily of blood, except that it lacks the red blood cells and blood proteins. (Both red blood cells and blood proteins are too large to pass through the leaky glomerular capillaries.) The filtrate contains the metabolic waste as well as the many useful substances found in the blood, including glucose, amino acids, vitamins, and water. This filtrate will be continually formed as long as the systemic
blood
pressure is normal.

The filtrate that is formed is caught in the Bowman’s capsule of the renal tubule. From here, the filtrate will pass into the proximal convoluted tubule. Rather than losing the useful substances in the urine, the nephron works to put them back into the blood through the process of tubular reabsorption. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule. Cells within the tubule take up needed substances from the filtrate and pass them out to the space between the proximal convoluted tubule and the surrounding peritubular capillaries. Once these useful substances are brought into this space, termed the extracellular space, they can be absorbed back into the blood contained within the peritubular capillaries. Some of this reabsorption is passive, not requiring any metabolic energy; water is an example of a substance that is reabsorbed passively. Most substances, however, depend on membrane transporters to carry them out to the extracellular space. These membrane transporters require metabolic energy in the form of adenosine triphosphate (ATP). There are a large number of membrane transporters for those substances that need to be reabsorbed, and few if any transporters for those substances that do not need to be
transported. This imbalance helps to explain why substances such as glucose and amino acids are almost completely reabsorbed back into the blood while metabolic waste products such as urea and uric acid are not.

The process of tubular secretion occurs in the loop of Henle and is essentially opposite to that of tubular reabsorption, with substances taken from the blood and put back into the filtrate. Some substances that are secreted from the blood and into the filtrate include hydrogen and potassium ions, ammonium ions, and certain drugs (for example, penicillin). It is the process of tubular secretion that allows the kidneys to remove toxins and drugs from the body, as well as to maintain the acid-base balance of the blood.

The regulation of the volume of urine secreted is controlled by the distal convoluted tubules and the collecting ducts to which they attach. After the filtrate has gone through the proximal convoluted tubules and the loop of Henle, it is fairly concentrated and therefore does not contain a large amount of water. The distal convoluted tubule and collecting duct are impermeable to water when a substance called vasopressin, or antidiuretic hormone (ADH), is present, in which case the filtrate will contain little water and the final urine volume will be small. If ADH is not present, the distal convoluted tubule and collecting ducts become permeable to water and, because the concentration of solutes is higher in the distal convoluted tubule and collecting duct, water enters into these two structures from the blood. The result is a dilution of the filtrate, with an increased water content and a large volume of urine. The role of ADH in determining urine volume can be seen with the ingestion of alcohol or coffee, both of which inhibit ADH release from the pituitary gland: The distal convoluted tubule and collecting duct become permeable to water, and the
urine volume and the frequency of urination increase. It is by this mechanism that the kidneys regulate the body’s water balance.

Once the urine is formed in the kidney, it will flow into a tube, the ureter. The ureters, one for each kidney, are passageways that carry urine from the kidney to the urinary bladder. Because the ureters run downward from the kidney, it might seem that the movement of urine to the urinary bladder is created by gravity. In reality, the ureters, which are stretchy and muscular tubes, contract at a rate of one to five times per minute to force the urine toward the bladder, a process termed peristalsis (the same type of contractions that move food through the digestive system). Where the ureters enter the urinary bladder, small, valvelike folds prevent the backflow of urine from the urinary bladder toward the kidneys.

The urinary bladder is a muscular, collapsible sac located in the pelvic cavity. When the bladder is empty, it is only 5.0 to 7.5 centimeters long and its walls are thrown into folds. As urine enters the bladder, it causes the organ to expand. A moderately full bladder is about 12.5 centimeters long and will contain approximately one-half of a liter of urine. A completely full bladder is capable of holding approximately 1 liter of fluid. The kidneys are continually forming urine. Thus, the bladder acts as a temporary storage unit for urine, allowing the individual to empty the bladder when it is convenient.

The urethra is a thin-walled tube that carries urine from the urinary bladder to the exterior of the body. Near where the urethra exits the urinary bladder is a band of smooth muscle that makes up the internal urethral sphincter. This sphincter, which is not under conscious control, acts to keep the urethra shut when urine is not being voided. A second sphincter, the external urethral sphincter, is found farther down the length of the urethra and is composed of skeletal muscle. This sphincter is under voluntary control: when it is not convenient to void the urine, this sphincter is used to prevent urination.

The urge to urinate is brought about by the stretching of the bladder. Ordinarily, the urge to urinate occurs when the bladder contains about 200 milliliters (almost 7 ounces) of urine. This amount of urine causes a stretching of the bladder that sends impulses to the spinal cord initiating the contraction of the urinary bladder. The contractions of the urinary bladder force urine past the internal urethral sphincter. At this time, a person will feel the need to void the urine, a process termed urination or micturition.

Disorders and Diseases

Renal and urinary disorders can be categorized based on their mechanism of action and the portion of the urinary system that they affect. These disorders include obstructive disorders that interfere with normal urine flow anywhere within the urinary tract, urinary tract infections, and glomerular disorders, which affect the glomeruli in the kidneys.

Obstructive disorders of the urinary system can be caused by many different factors. Obstruction of the passage of the urine will usually cause a backing up of the urine into the kidney or kidneys. The result is a swelling of the kidney termed hydronephrosis.

Perhaps the most common obstruction is caused by kidney stones, also referred to as renal calculi. Kidney stones consist of crystallized minerals such as calcium, magnesium, or uric acid salts that form hard stones in the distal end of the collecting ducts. If the stones are small, they will pass through the remainder of the urinary tract. Larger stones, however, get caught in the ureters, thus blocking the passage of urine from the kidneys to the urinary bladder. This blockage usually results in intense pain as the ureters rhythmically contract in an effort to dislodge the stone; this condition is sometimes referred to as renal colic. If the stone does not move from its position of blockage, a buildup of urine in the kidney may occur; if this continues, damage may be done to the kidneys.

Damage to the nerves that innervate the bladder, termed neurogenic bladder, can also result in an obstructive disorder. Damage to these nerves results in the loss of normal control over the voiding of urine from the bladder. Consequently, there is a retention of urine in the bladder since there is no signal telling the bladder to contract.

Tumors of the urinary system may also cause obstruction of urine flow. Another cause of obstruction is the loss of the fat surrounding the kidney. When this occurs, one or both kidneys may drop from their normal position, a condition referred to as renal ptosis. When the kidneys drop, there is a chance that the ureters exiting the kidney may become kinked and prevent the normal flow of urine from the kidneys to the urinary bladder.

Urinary tract infections are usually caused by bacteria and can involve the urethra, ureters, urinary bladder, kidneys, or all the above. Urinary infections in the urethra are termed urethritis and result in the inflammation of the urethra. The two most common bacterial infections involved in urethritis are gonorrhea and chlamydia. Males are more likely than females to have urethritis.

Cystitis refers to any inflammation of the urinary bladder. This condition usually results from bacterial infections, but it may also be caused by tumors or by the presence of stones in the bladder. Cystitis occurs more frequently in women than in men and is characterized by pelvic pain, a frequent urge to urinate, and possibly blood in the urine.

Nephritis is a general term used to describe inflammatory kidney diseases. The inflammation of the nephrons within the kidney is referred to as pyelonephritis. Pyelonephritis is often attributable to bacterial infection but may also be caused by viral infections, tumors, kidney stones, or pregnancy.

Glomerulonephritis is a term that refers to any type of glomerular disorder. It can be further subdivided into two categories: acute glomerulonephritis and chronic glomerulonephritis. Acute glomerulonephritis is the most common form and may be caused by bacterial infection. Chronic glomerulonephritis refers to noninfectious kidney disorders. It commonly occurs when the immune system reacts to and destroys the body’s own glomeruli. This type of glomerulonephritis eventually leads to kidney failure. Acute glomerulonephritis, if it is left untreated or it does not respond to treatment, can become chronic glomerulonephritis.

Renal (kidney) failure is simply the inability of the kidneys to form urine. Renal failure can be classified as either acute or chronic. Acute renal failure is the abrupt loss of kidney function, which may result from excessive loss of blood, severe burns, pyelonephritis, glomerulonephritis, or infection or obstruction of the urinary tract. Chronic renal failure is the slow destruction of the nephrons in the kidney. This form of renal failure may result from infections, glomerulonephritis, tumors, obstructive disorders, or autoimmune diseases. Unless the progression of nephron loss is stopped, chronic renal failure will eventually lead to death.

Diabetes insipidus is a disease that does not directly attack the urinary system, but it has a profound effect on the urinary system through its influence on the pituitary gland and the hypothalamus. With diabetes insipidus, the pituitary gland fails to release antidiuretic hormone, as a result of an injury or tumor of the posterior portion of the pituitary gland or hypothalamus. Because of the decreased amount of antidiuretic hormone, large amounts of urine, and thus water, are flushed from the body daily. If left untreated, diabetes insipidus can lead to dehydration and electrolyte imbalances. To offset the loss of water in the urine, individuals with diabetes insipidus must drink large amounts of water.

Perspective and Prospects

The complexity of the human kidney can be seen in science’s inability to build an artificial kidney that is continuously functional and can be inserted into the body in place of the normal kidney. Until the development of tubing that contained miniature holes (dialysis tubing), kidney failure nearly always resulted in death. Dialysis tubing allowed the development of renal dialysis, which cleanses the blood of toxic substances and helps to regulate electrolyte balance. The process of renal dialysis is carried out using a thin membrane that is permeable to only a few select substances. The tubing is immersed in a bathing solution that is very similar to normal blood plasma. As blood circulates through the tubing, toxic substances and some
electrolytes move out of the blood and into the bathing solution. This dialysis tubing and the bathing solution are often referred to as an artificial kidney. Dialysis is usually done three times a week, with each session requiring about four to eight hours. Although effective, dialysis is a far cry from the functioning of the human kidney and is no cure for chronic renal failure. When the kidneys are no longer functioning, the only hope is a kidney transplant.

Because the kidneys are so effective at filtering unneeded substances out of the blood, the urine formed by the kidney is the principal
fluid used for
drug testing and drug screening. Furthermore, the kidney also secretes some white blood cells into the urine. As techniques continue to develop, it will be possible to perform genetic tests on these white blood cells to determine genetic traits such as sex and the color of hair and eyes, as well as the possibility of the presence of genetic diseases or personality traits. Such technology could have considerable impact on the future of individual privacy, as many companies and employers require a mandatory analysis of urine, primarily for the presence of drugs in the urine, prior to the possibility of employment. Thus, with a simple urine sample, the company could know not only the possible drug use of prospective employees but also their genetic makeup.

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"Your Urinary System and How It Works." National Kidney and Urologic Diseases Information Clearinghouse, June 29, 2012.