33.3 Introduction to the Urinary System

Your urinary system is like a sophisticated plumbing network in your body. It filters waste, balances fluids, and keeps you healthy. From the bean-shaped kidneys to the bladder and urethra, each part plays a crucial role in processing and eliminating urine.

Understanding how your urinary system works helps you spot potential issues. Common problems like UTIs, incontinence, and prostate enlargement can disrupt this delicate system. Knowing the signs and risk factors empowers you to take better care of your urinary health.

Anatomy and Physiology of the Urinary System

Urinary system components

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• Kidneys
  • Bean-shaped organs located in the retroperitoneal space behind the peritoneum
  • Primary functions include filtering blood to remove waste products and excess fluids, regulating electrolytes (sodium, potassium) and acid-base balance, and producing urine
• Ureters
  • Thin, muscular tubes that connect the kidneys to the bladder, typically 10-12 inches long
  • Transport urine from the renal pelvis to the bladder via peristaltic contractions, which are rhythmic muscle contractions that propel urine forward
• Bladder
  • Hollow, muscular organ located in the pelvic cavity that stores urine until it is ready to be expelled from the body
  • Composed of the detrusor muscle, which contracts during micturition (urination) to expel urine
  • Trigone is a triangular area where the ureters and urethra connect to the bladder, forming a smooth triangle
• Urethra
  • Thin tube that carries urine from the bladder to the external urethral orifice for elimination from the body
  • Longer in males (8-10 inches) than in females (1.5 inches) due to the passage through the penis

Micturition process

• Micturition (urination) is the process of emptying the bladder of urine
• Involves both voluntary control from the brain and involuntary control through a reflex arc
• Reflex arc
  • Stretching of the bladder wall stimulates stretch receptors, which send sensory signals via neurons to the sacral spinal cord
  • Motor neurons then stimulate the detrusor muscle to contract and the internal urethral sphincter to relax, allowing urine to flow out
• Voluntary control
  • Pontine micturition center in the brainstem coordinates voluntary control of urination
  • External urethral sphincter is under voluntary control and can be consciously contracted to delay urination when it is not socially appropriate
• Stages of micturition
  • Filling stage occurs as the bladder fills with urine from the ureters; the detrusor muscle relaxes to accommodate the increasing volume of urine
  • Storage stage maintains urine in the bladder as the external urethral sphincter remains contracted to prevent leakage
  • Emptying stage occurs when the bladder reaches its threshold volume (about 300-400 mL), triggering the micturition reflex, which causes the detrusor muscle to contract and the internal urethral sphincter to relax, allowing urine to flow out through the urethra

Common Urinary System Disorders

Urinary disorders

• Urinary incontinence
  • Stress incontinence involves leakage of urine during physical activity or increased abdominal pressure (coughing, sneezing, lifting heavy objects)
  • Urge incontinence is a sudden, intense urge to urinate followed by involuntary loss of urine, often associated with overactive bladder
  • Overflow incontinence occurs when the bladder becomes overfilled, leading to leakage of urine, often caused by bladder outlet obstruction or weak detrusor muscle
  • Mixed incontinence is a combination of stress and urge incontinence symptoms
• Urinary tract infections (UTIs)
  • Cystitis is an infection of the bladder, commonly caused by bacteria like Escherichia coli (E. coli) ascending from the urethra
    • Symptoms include dysuria (painful urination), frequency, urgency, suprapubic pain, and hematuria (blood in urine)
  • Pyelonephritis is an infection of the kidney that can result from untreated cystitis
    • Symptoms include fever, chills, flank pain, nausea, and vomiting
  • Risk factors for UTIs include female gender, sexual activity, poor hygiene, and urinary stasis (incomplete emptying of the bladder)
• Benign prostatic hyperplasia (BPH)
  • Noncancerous enlargement of the prostate gland that commonly occurs in older men
  • Can cause urinary symptoms such as frequency, urgency, weak stream, and incomplete emptying of the bladder