Urinary System Notes

Urinary System

Function and Structures

  • The urinary system serves as a large blood filter.
  • It retains essential substances in the blood while excreting waste products and excess water to maintain overall health.
  • It functions as the body’s waste treatment center.
  • Blood enters the kidney via the renal artery.
  • The glomerulus, a network of tiny blood vessels, filters blood from the renal artery.
  • The nephron is the functional unit of the kidney, responsible for filtration, absorption, and secretion to form urine.
  • Urine produced in the kidney is transported to the urinary bladder.

Components of the Urinary System

The urinary system comprises:

  • 2 kidneys
  • 2 ureters
  • A urinary bladder
  • A urethra
  • Prostate (in males)

Sterility and Defense Mechanisms

  • The genitourinary system, except for the distal urethra, is sterile.
  • Defense mechanisms include:
    • Continuous flushing action of urine.
    • Urine composition (pH 4.6 - 8, high urea and organic acids) inhibits aerobes.
  • Organism virulence factors such as fimbriae or pili.
  • Urine passes through the resident flora of the distal urethra, requiring differentiation between true infection and contamination.

Urinary Tract Infections (UTIs)

Lower UTIs

  • Often occur via an ascending route of infection, where organisms travel up the urethra.
    • Cystitis: Bladder infection resulting from organisms ascending the urethra and infecting the bladder. Common in women due to their shorter urethra and in catheterized individuals.
    • Urethritis: Infection of the urethra.
  • The female perineum contains resident flora that can lead to UTIs.
  • Female anatomy predisposes them to UTIs due to a short urethra and the close proximity of vaginal and rectal areas.

Upper UTIs

  • Pyelonephritis: Infection of the kidney (renal pelvis).
    • Presence of white blood cell casts indicates pyelonephritis.
  • Ureteritis: Infection of the ureters.
  • Prostatitis: Infection of the prostate in males.

Acute Urethral Syndrome

  • Presents with UTI symptoms but without significant numbers of organisms in urine.
  • Individuals with low organism counts (100 colonies/mL), presence of white blood cells, and symptoms should be treated.
  • Infections can reach the kidneys secondarily from the blood.
  • Acute pyelonephritis complications:
    • Bacteremia develops in 10-65% of patients with acute pyelonephritis.
    • Infection with Gram-Negative Rods (GNR) can lead to septic shock and disseminated intravascular coagulation (DIC).

Bacteriuria

  • Presence of bacteria in the urine.
    • Asymptomatic bacteriuria in women: Two consecutive clean-catch voided urine specimens with isolation of the same bacterial strain in count \geq 100,000 CFU/mL.
    • Asymptomatic bacteriuria in men: A single clean-catch voided urine specimen with isolation of one bacterial species isolated in count \geq 100,000 CFU/mL.
    • Asymptomatic bacteriuria for any patient: A single catheterized urine specimen with one bacterial species isolated in counts \geq 1000 CFU/mL.
  • Vague symptoms especially in the elderly:
    • Poor appetite, fatigue, urinary incontinence.
    • Mental changes and/or confusion are possible.

Symptomatic Bacteriuria

  • Pyelonephritis:
    • Flank pain, tenderness, fever.
    • Vomiting, nausea, fatigue.
    • Dysuria, frequency, nocturia.
    • Urine changes.
  • Cystitis:
    • Dysuria.
    • Suprapubic tenderness.
    • Increased frequency/urgency of urination.
    • Cloudy urine with a strong odor.
    • Hematuria.

Specimen Collection

  • Acceptable Methods of Collection:
    • Mid-Stream Clean Catch: Most common specimen submitted.
    • Catheterization:
      • Straight catheter (in and out) specimen.
      • Indwelling catheter: After 4 days, it becomes colonized with biofilm; ideal to collect specimen from collection port 12-24 hours after catheter change.
    • Cytoscopy specimen: Collected with an instrument that passes into the bladder through the urethra.

Setup and Incubation of Cultures

  • Quantitation of bacteria is necessary to determine if a UTI is present.
  • Routine media:
    • BAP (Blood Agar Plate): Grows both gram-positive and gram-negative bacteria.
    • MacConkey agar (MAC): Selects and differentiates Enterobacteriaceae.
    • Add chocolate agar if Haemophilus, Gardnerella, or N. gonorrhoeae are suspected by the physician; must be incubated in carbon dioxide.
  • Procedure:
    • Dip loop vertically into well-mixed urine.
    • Draw an inoculation line down the center of the agar and then streak the entire agar surface perpendicular to the line.
    • Incubate media at 37°C in ambient air or carbon dioxide for 24-48 hours.

Interpretation of Cultures

  • Determine the numbers and types of colonies present.
  • Classic definition of UTI: > 100,000 CFU/mL of a single organism (no longer the only significant indicator).
  • > 1000 CFU/mL of a member of the Enterobacteriaceae is considered significant.
  • White blood cells present:
    • > 100 CFU/mL can also be significant.
  • Considerations:
    • Urinary frequency present.
    • Drinking high volumes of fluid.
    • Partially treated UTI.
    • Dysuria or urethral syndrome.

Microbes Associated with UTIs

Common Isolates

  • Gram-Negative Bacteria: Enterobacteriaceae
    • Escherichia coli.
    • In indwelling catheters: Antibiotic-resistant members of Enterobacteriaceae and non-fermenters (Pseudomonas, Proteus, Klebsiella, Enterobacter spp.).
    • Promotes the formation of kidney stones:
      • Proteus
      • Providencia
      • Morganella
  • Gram-Positive Bacteria: Enterococcus, Streptococcus, and Staphylococcus
    • Enterococcus: Primarily older men with urinary tract manipulation or prostatic hypertrophy
    • Streptococcus agalactiae: Both pregnant and non-pregnant females
    • Staphylococcus (including S. saprophyticus, S. epidermidis, and S. aureus)

Unusual Isolates

  • Corynebacterium urealyticum: Kidney infections and kidney stone production in renal transplant recipients and all immunosuppressed, urological, or renal transplant patients at risk.
  • Candida spp.: Pyelonephritis via hematogenous route, indwelling catheter
  • Gardnerella vaginalis: Emerging urinary tract pathogen, often symptomatic
  • Lactobacillus spp.
  • Haemophilus spp.: Children with urinary tract abnormalities

Treatment

  • Often, treatment is initiated before culture results are available.
    • Directed toward gram-negative rods and gram-positive cocci.
  • Trimethoprim-sulfamethoxazole (Septra or Bactrim) is commonly used for cystitis.
  • Quinolones or ampicillin if organism is antibiotic-resistant.
  • Pyelonephritis requires intravenous therapy with broad-spectrum antibiotics.