Urinary System

Structure and Function

The urinary system consists of two kidneys, two ureters, a bladder and a urethra. The kidneys are located on each side of the spinal column just above the waist. Each kidney contains more than one million nephrons. Nephrons filter waste products from the blood. The kidneys regulate fluid and electrolyte balance, blood pressure, and pH, and remove waste products such as ammonia and creatinine. Creatinine is measured as an indicator of kidney function. Kidneys work as an overflow mechanism to maintain optimal fluid balance. The kidneys control blood pressure by adjusting the amount of fluid excreted into the urine. Electrolytes are often salt compounds. Water tends to follow salt. When electrolytes are excreted into the urine, water tends to follow. The kidneys function to clean the blood and filter waste products into the urine. Kidneys also release a hormone involved in red blood cell production.

Ureters lead from each kidney to the bladder and transport urine from the kidneys into the bladder. The bladder acts as a storage bag for urine. When the bladder is full of urine it sends a message to the spinal cord that it needs to be emptied. The bladder has a capacity of 250-500 ml or more before signaling a need to urinate. The control for emptying the bladder is in the brain, which tells the bladder sphincters to relax, allowing the bladder to empty. Urine leaves the body through the urethra. The male urethra is longer than the female urethra. In women the urethra is approximately 3 cm long, and in men it is about 20 cm long. In the male, the urethra passes through the donut-shaped prostate gland.

Urine formation occurs through three processes: filtration, reabsorption and secretion. Filtration occurs when there is movement of a substance from a high concentration area to a low concentration area, aided by a force. Blood passes through the nephrons via filtration with blood pressure as the force. Most of the liquid portion of the blood is filtered through the kidneys. The liquid in the kidneys is called filtrate. Reabsorption occurs as the filtrate passes through the nephron. The body reabsorbs 97-99 percent of water and dissolved substances back into the blood that were filtered through the kidneys. Secretion allows the nephrons to add to the filtrate substances the body does not need, which now looks like urine. Secretion is similar to reabsorption; however, the direction is reversed.

 The kidneys actively reabsorb substances (glucose, water, electrolytes, etc.) needed by the body and refuse to reabsorb harmful substances such as waste products of metabolism.

Age-Related Changes

The kidneys reduce in size and weight. The number of working nephrons in the kidneys decreases with age, which results in a less efficient filtering system. A decrease in blood flow also decreases filtering efficiency. Loss of muscle tone in the bladder and pelvic muscles makes it more difficult to hold large amounts of urine in the bladder. In men, the prostate may enlarge, making urination difficult and even painful. A partially emptied bladder is a perfect home for bacteria that causes bladder infections. Bladder capacity decreases, and an increase in involuntary bladder contractions can lead to urgency and frequency. A person may be unable to hold as much urine, which leads to more frequent trips to the bathroom.

Urinary Tract Infection

Urinary tract infection (uti) is an infection of one or more structures in the urinary system. A UTI is usually an infection of the bladder. Women are more susceptible to bladder infections than men because the female urethra is shorter, and close to the vagina and rectum. Once an individual has a bladder infection, he or she is more prone to recurring infections. If left untreated, the infection can travel up to the kidney, causing a kidney infection. The infection can then travel into the blood, causing a blood infection (sepsis), which can be life-threatening.

Factors Increasing Risk of UTI

  • Female
  • Urinary stasis: stagnation of normal urine flow
  • Institutionalization: living in a facility
  • Sexual activity
  • Catheterization

Symptoms of UTI

  • Frequency
  • Burning, pain with urination
  • If severe, blood and pus in urine
  • Chills, fever and back pain
  • Cloudy urine
  • Odor
  • Incontinence
  • Confusion or agitation
  • Some people have no symptoms

Treatment for UTI

  • Antibiotics  
  • Extra fluids.

Drug Classifications that Affect the Urinary System

  1. Beta-blockers relax muscles of the bladder and reduce the number of contractions.
  2. Antibiotics treat infection; nausea, vomiting, and diarrhea are common side effects.
  3. Minerals such as sodium and potassium are essential in maintaining proper fluid balance in the body.
  4. Diuretics increase urine excretion; they work on different chemicals and parts of the nephron.
  5. Antigout drugs work on the kidney by promoting excretion of the waste product uric acid. (Gout is a buildup of uric acid in a joint that causes severe pain, warmth, redness and swelling.)
  6. Antispasmodics/anticholinergics reduce the strength of bladder contractions and increase bladder capacity.
  7. Cholinergics cause contractions of the bladder to relieve urinary retention. analgesics relieve pain or burning associated with an infection of the bladder.
  8. Alpha-adrenergic blockers block receptors in the smooth muscle of the bladder and prostate, causing them to relax.
  9. Antiandrogens reduce the size of the prostate; can cause fetal damage if handled by pregnant women.
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