Urinary

• Urinary system


  • Cleanses the blood
  • Rids the body of wastes
  • Regulates pH
  • Regulates blood pressure
  • Regulates concentration of solutes in the blood
  • Regulates red blood cell production
  • Performs final synthesis step of vitamin D production
• If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis
• The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body
• Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence
• Glomeruli


  Tufts of specialized capillaries that filter the blood

  
• Glomerular filtrate


  Filtrate composition very similar to plasma, excluding large elements like blood cells, platelets, antibodies, and albumen
• The glomeruli create about 200 liters of filtrate every day, yet less than two liters of urine is excreted
• Normal urine characteristics


  • Color: Pale yellow to deep amber
  • Odor: Odorless
  • Volume: 750-2000 mL/24 hour
  • pH: 4.5-8.0
  • Specific gravity: 1.003-1.032
  • Osmolarity: 40-1350 mOsmol/kg
  • Urobilinogen: 0.2-1.0 mg/100 mL
  • White blood cells: 0-2 HPF
  • Leukocyte esterase: None
  • Protein: None or trace
  • Bilirubin: <0.3 mg/100 mL
  • Ketones: None
  • Nitrites: None
  • Blood: None
  • Glucose: None
• Urinalysis often provides clues to renal disease
• Urine volume varies considerably, with a normal range of 1-2 liters per day
• Urine volume conditions


  • Polyuria: >2.5 L/day
  • Oliguria: 300-500 mL/day
  • Anuria: <50 mL/day
• Urine pH


  Can vary more than 1000-fold, from 4.5 to 8.0
• Urine specific gravity


  Measure of the quantity of solutes per unit volume, always greater than 1.0 due to presence of solutes
• Urine osmolarity


  More accurate indicator of urinary solutes than specific gravity, ranges from 50-100 mOsmol/L to 1200 mOsmol/L
• Cells are not normally found in urine, presence of leukocytes may indicate a urinary tract infection
• Protein should only be found in trace amounts in urine, excessive protein usually indicates glomerular damage
• Ketones in urine suggest the body is using fat as an energy source, as seen in diabetes mellitus
• Nitrites in urine indicate a urinary tract infection as bacteria metabolize nitrate into nitrite
• Blood should not be found in urine, except possibly from menstrual contamination
• Urethra


  Transports urine from the bladder to the outside of the body

  
• Urethra regions in males

  • Preprostatic urethra
  • Prostatic urethra
  • Membranous urethra
  • Spongy or penile urethra

  
• Female urethra


  Shorter length (4 cm) than male urethra, making it more susceptible to urinary tract infections
• Micturition


  Involuntary and voluntary neural control of urination
• External urethral sphincter


  Function of the pudendal nerve, arises in the sacral region of the spinal cord, traveling via the S2–S4 nerves of the sacral plexus
• Male urethra


  Passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis, length averages 20 cm, divided into 4 regions: preprostatic urethra, prostatic urethra, membranous urethra, and spongy or penile urethra
• Preprostatic urethra


  • Very short and incorporated into the bladder wall

  
• Prostatic urethra


  • Passes through the prostate gland, receives sperm via the ejaculatory ducts and secretions from the seminal vesicles during sexual intercourse
• Cowper's glands (bulbourethral glands)


  • Produce and secrete mucus into the urethra to buffer urethral pH and lubricate the urethra during sexual stimulation
• Membranous urethra


  • Passes through the deep muscles of the perineum, invested by the overlying urethral sphincters

  
• Spongy urethra


  • Exits at the tip (external urethral orifice) of the penis after passing through the corpus spongiosum
• Mucous glands


  • Found along much of the length of the urethra and protect the urethra from extremes of urine pH
• Innervation of the urethra is the same in both males and females

  
• Bladder


  Collects urine from both ureters, lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum in males, partially retroperitoneal with its peritoneal-covered "dome" projecting into the abdomen when distended with urine

  
• Bladder


  • Highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle, interior surface made of transitional cellular epithelium, volumes in adults can range from nearly zero to 500–600 mL

  
• Detrusor muscle


  • Contracts with significant force in the young, strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying
• Micturition


  Urination or voiding, results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters
• Micturition reflex


  Stretch receptors in the bladder wall transmit nerve impulses to the sacral region of the spinal cord, resulting in parasympathetic neural outflow causing contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter, while the spinal cord inhibits somatic motor neurons resulting in relaxation of the skeletal muscle of the external urethral sphincter
• Micturition reflex


  • Active in infants but with maturity, children learn to override the reflex by asserting external sphincter control, may be preserved even in the face of spinal cord injury
• Nerves involved in the control of urination

  Hypogastric, pelvic, and pudendal
• Voluntary micturition


  • Requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center