The Urinary System

Introduction

Everything we eat or drink must be either used or eliminated. Waste elimination occurs as a natural process of our daily lives. Most of us take it for granted, but without proper excretion, waste products would build up to toxic levels within our bodies.

 

Function

Recall the role that the intestines in the GI system and the lungs in the respiratory system play to eliminate waste from your body. The urinary system plays an important role in maintaining the appropriate balance of water and electrolytes, and also eliminates wastes in the form of urine. Your very survival depends on a stable internal environment of body fluids, temperature, blood pressure, and countless other factors. The term body fluid  refers not only to the water that makes up 60 percent of the human body's weight, but also the substances dissolved in the water—most importantly, the electrolytes.

Fluid both fills and surrounds each cell. About two-thirds of all body fluid is within cells in the intracellular compartment; the remaining must keep a correct electrolyte balance in these fluids for the proper function of muscle and nerve cells.

The processes of the urinary system can be broken down into three parts.

1. Filtration—removing toxic wastes and excess substances from the blood
2. Formation of urine—converting wastes and excess waste substances into urine
3. Micturition—excreting urine from the body, also called urination or voiding

 

The urinary system functions as the body's main waste disposal, flushing away the "garbage" that results from the metabolic processes in every cell of the body. It also helps to maintain the proper pH (acid-base) balance of the blood. Failure of the urinary system to do its job is fatal to the body. If the nitrogenous (nitrogen-containing) wastes of protein metabolism build up in the bloodstream, a toxic condition called uremia will result.

The kidneys, the most important organs of the urinary system, also have an endocrine function because they secrete substances that influence the function of other body parts. For example, they secrete the enzyme renin, which plays an important role in regulating blood pressure; the hormone erythropoietin, which stimulates the maturation of red blood cells in the bone marrow; and vitamin D, which makes it possible for the intestines and the bones to absorb calcium.

 

Organs of the Urinary System

The organs of the urinary system include two kidneys, two ureters, the urinary bladder, and the urethra. Because the male urinary system shares some common anatomy with the male reproductive or genital system, these systems are sometimes studied together as the urogenital or genitourinary (GU) system. The branch of medicine concerned with the male genital tract and the urinary tracts of both genders is urology.

 

Your kidneys look like a pair of reddish-brown kidney beans, each about the size of a computer mouse. The kidneys lie in the posterior aspect of the upper abdomen.

The right kidney is just below the liver, and the left kidney is just below the spleen. The left kidney is a little higher than the right one. They are located beneath a layer of muscle and a cushion of fat just under your ribs, not in the peritoneum (membrane lining the abdominal cavity).

 

The kidneys are complex organs made up of several microscopic parts. The word renal is used in terms relating to the kidneys because the combining form ren/o means "kidney." The renal cortex is the outer layer, and the renal medulla is the inner portion.

The heart pumps over 20 percent of its blood flow straight to the kidneys because a high rate of blood flow through the kidneys is essential for the formation of urine. This high flow rate exerts high pressure on the narrow renal capillaries, forcing the blood to filter through nephrons.

 

Clean blood leaves the kidney via the renal vein. Meanwhile, the waste material that's now urine is funneled through the renal pelvis into the ureter. The ureter is a 10- to 12-inch duct that uses peristalsis (muscle contractions) to move urine along to the urinary bladder.

 

The urinary bladder  is a reservoir made of muscular walls lined with a loose mucous membrane that wrinkles into folds called rugae when it's empty. These folds enable the bladder to expand when full, similar to stomach folds.

The bladder lining has a smooth, close-to-the-muscle section called the vesical trigone. The term trigone means triangular area," and the three corner of the vesical trigone are where the two ureters enter and the urethra exits.

As a storage reservoir, the bladder has its limits—about 1 1/2 pints. When the bladder expands enough to stimulate the nerves that initiate the emptying reflex, the internal urethral sphincter is automatically released. Urine then passes into the urethra. 

 

The urethra is the tube leading from the bladder to the surface that carries urine out from your body via urination.

The adult male's urethra is approximately seven inches long; it passes through the penis and also conveys semen.

The female's urethra is much shorter—1 1/2 inches in length—and opens between the clitoris and the vagina.

The external urethral sphincter, at the end of the urethra, is the last valve holding the urine back. This sphincter is under voluntary control after the age of two or three. The opening through which urine leaves the body is the urinary meatus. (A meatus is an opening or a passage.)

 

The prostate is a gland of the male reproductive system that's slightly larger than the size of a walnut. It's located between the urinary bladder and the penis. 

The urethra, from the urinary bladder to penis, runs through the center of the prostate. The prostate gland secretes a milky fluid that's discharged into the urethra at the time of ejaculation that nourishes and protects sperm.

 

 

Control of Urinary Secretion

The selective reabsorption or secretion of materials in the urine is regulated by chemical and nervous system control. The two main systems of chemical control are the antidiuretic hormone (ADH) system and the renin-aldosterone (RA) system. 

ADH is secreted by the hypothalamus in response to high sodium concentrations in the blood, which is called hypernatremia. To lower the salt concentration, ADH works on the tubules to promote reabsorption of water. This excess water enters the bloodstream and dilutes the sodium, thus bringing the sodium concentration back to normal.

The name "antidiuretic hormone" implies that this hormone counteracts the effects of diuretics. Since ADH allows reabsorption of water, it reduces the amount of water in the urinary filtrate; on the other hand, diuretics increase the water volume in the urinary filtrate. Diuretics allow the kidneys to eliminate more water volume from the bloodstream. Common substances such as alcohol and caffeine are diuretics.

The RA system works in a different way. The kidneys can detect when blood pressure is too low. In response, the kidneys produce a hormone called renin, which stimulates the adrenal glands to produce aldosterone. This substance promotes the reabsorption of sodium and water from the tubules into the bloodstream, which raises blood pressure.

The kidneys also produce another hormone called erythropoietin (erythr/o refers to red blood cells). This hormone stimulates the bone marrow to produce red blood cells. Chronically diseased kidneys don't produce enough erythropoietin, which ultimately results in anemia.

 

 

Diseases and Treatments

Common symptoms of urinary system disorders include

• Retention (unable to empty the bladder)
• Incontinence (unable to control the urination)
• Dysuria (pain on urination)
• Oliguria (no urine formation)
• Frequency
• Urgency

Common diseases of the urinary system include

• Benign prostate hyperplasia (BPH) in men
• Urinary incontinence
• Infections
• Renal failure

 

Benign prostate hyperplasia (BPH) is defined as the enlargement of the prostate gland and is the most common urinary system disorder in men. Even though enlargement of the prostate occurs throughout a man's life, it usually doesn't become problematic until old age.

Enlargement or inflammation of the prostate causes the urethra to constrict, resulting in difficult urination, inability to empty the bladder, and frequent urination due to weakness of the bladder muscles. Along with surgical removal of the prostate, there are many drugs that can be used to treat BPH.

Some of the common drugs used for BPH treatment include

• Tamsulosin (Flomax)
• Dutasteride (Avodart)
• Finasteride (Proscar)
• Terazosin (Hytrin)
• Doxazosin (Cardura)

 

The bladder stores and expels urine with the coordinated response from the nervous system and urinary system muscle contractions. Incontinence results from either uncontrolled contraction or overactivity of the bladder muscles.

The frequent urge to void can lead to leakage during the night. Other conditions such as urinary tract infection, prostate gland inflammation, and obstruction of the urinary system can also cause incontinence. Certain medications can lead to weaker bladder muscle control as well.

Some of the medications that are commonly used to treat uncontrolled or overactive bladder include

• Tolterodine (Detrol)
• Oxybutynin (Ditropan)
• Solifenacin (Vesicare)
• Trospium (Santura)

 

Infections may occur in any of the urinary system organs. Urinary tract infections (UTIs) are more prevalent in women than men due to a smaller urethra. UTIs are also more common in sexually active women. Intercourse can cause the GI tract bacteria to move around the surface of the vagina. From there, bacteria can move up to the urethra, uterus, bladder, and even kidneys causing urethritis, ureteritis, cystitis, and pyelonephritis, respectively. UTIs also occur very frequently in elderly people who are bed-bound and/or have catheters in place to remove urine from the bladder.

 

There are many antibiotics available to treat frequent UTIs. Foods that create an acidic environment in the bladder can also help alleviate minor UTIs. Cranberry juice or tablets are commonly used in young women to prevent reoccurring, mild UTIs. Severe UTIs can lead to hospitalization and intravenous antibiotics.

Some of the commonly used antibiotics to treat UTI include

• Sulfamethoxazole and trimethoprim (Bactrim, Septra)
• Ciprofloxacin (Cipro)
• Amoxicillin/Clavulanate (Augmentin)
• Cefaclor (Ceclor)
• Piperacillin/tazobactam (Zosyn)

A medication called phenazopyridine (AZO, Pyridium) is also available OTC to treat pain associated with UTI.

 

Renal failure is defined as decreased or total lack of kidney function. Acute renal failure is the quick onset of decline in kidney function due to dehydration, prolonged hospitalization, severe illness, trauma, or drugs. Acute renal failure is generally reversible with hydration and removal of offending medications or treatment of an offending disease.

On the other hand, chronic renal failure signifies a progressive decline in renal function. Chronic renal failure (CRF) can be the result of age, uncontrolled hypertension, uncontrolled diabetes, and genetic or environmental factors.

 

Renal failure causes your body to build up waste due to the kidneys' inability to remove nitrogenous waste. During renal failure, certain drugs aren't excreted from the body quickly and may require dose or frequency adjustments to avoid accumulation of those drugs in the body.

Patients with mild renal failure may live their life normally but are instructed to avoid certain foods and may have to limit the consumption of fluids. Patients with chronic and severe renal failure may require mechanical means of filtering wastes from the blood. Mechanical filtration of your blood using different machines is called dialysis. 

Some of the mechanical filtrations commonly used in renal failure patients are hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD). During HD, patients are hooked to a machine that acts as a filter as blood is run through it. HD generally takes two to three hours, and patients with complete renal failure may require HD two to three times a week. CAPD is the continuous method of dialyzing blood with a dialysis solution infused in the peritoneal cavity. The peritoneum membrane acts as a filter as wastes are drawn into the dialysis solution.

 

Key Points

• The urinary system plays a very important role in maintaining fluid, electrolytes, and other essential elements in your body and helps eliminate waste.
• Many drugs are excreted via the kidneys, which means a decrease in renal function can affect drug levels in your body. Close monitoring and dose adjustment may be essential in patients with compromised renal function.
• UTIs are the most common urinary system disorder and may require antibiotics, hydration, symptom relief medications, and hospitalization for severe cases.
• End stage renal failure requires mechanical forms of filtration to remove waste and toxic chemicals from your blood. Two common types of mechanical filtration methods are continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis (HD).
• BPH is a common prostate gland disorder in elderly men. Medications including tamsulosin, dutasteride, finasteride, and terazosin (Hytrin) are used to treat BPH. Severe forms of BPH may require surgical removal of the gland.