Urinary System: Notes on Urine Formation and Micturition

Regulation of Urine Production

  • Antidiuretic Hormone (ADH)

    • Known as vasopressin.
    • Function: Promotes water reabsorption at the late distal convoluted tubule (DCT) and collecting duct (CD).
    • Effects of Water Deficit:
    • Increased ADH secretion.
    • Increased permeability of late DCT and CD due to aquaporin insertion.
    • Results in concentrated urine (increased urine concentration) and decreased urine volume.

  • Normal & Abnormal Urine Volume:

    • Normal Volume: 1-2 L/day
    • Polyuria: > 2 L/day
    • Oliguria: < 500 mL/day
    • Anuria: 0 - 100 mL/day
    • Note: ADH cannot completely halt urine production; a minimum volume is necessary to eliminate waste solutes.

Transportation of Urine

  • Pathway:

    • Urine travels from the renal pelvis of kidneys to the urinary bladder via ureters.

  • Forces Facilitating Movement:

    • Peristaltic waves.
    • Hydrostatic pressure.
    • Gravity.

  • Prevention of Backflow:

    • Ureterovesical valves prevent backflow; bladder compression prevents backflow into ureters.

The Micturition Reflex

  • Overview:

    • Combination of involuntary and voluntary muscle contractions.
    • A spinal cord reflex activated when bladder wall is stretched (urine volume > 200-400 mL).

  • Reflex Mechanism:

    • Stretch receptors in bladder wall send impulses to the micturition center located in the S2 and S3 segments of the sacral spinal cord.
    • This stimulates:
    • Parasympathetic response:
      • Contraction of the detrusor muscle (smooth) in the bladder wall.
      • Opening of the internal urethral sphincter.
    • Pons and cerebrum to create the sensation of urgency.
      • Inhibition of skeletal muscle (of the external urethral sphincter) leads to its opening.

Voluntary Regulation of Micturition

  • Control Mechanism:

    • Micturition reflex can be consciously inhibited.
    • Holding urination:
    • Brain sends signals to spinal cord.
    • Somatic neurons keep the external urethral sphincter contracted.
    • Urination possible, but limited; excessive fullness will override voluntary control.

  • Initiating Urination:

    • Urination can occur even without the urge by inhibiting the external sphincter and pelvic diaphragm; this allows bladder contraction and urination.

Clinical Correlation: Urinary Incontinence

  • Definition:

    • Lack of voluntary control over micturition.

  • Normal in Infants:

    • Infants < 2-3 years lack developed neurons for the external urethral sphincter; voiding occurs when the bladder is full.

  • Loss of Control Causes:

    • Injury or lack of toilet training (e.g., infants).

  • Types of Urinary Incontinence:

    1. Stress Incontinence:
    • Common in young/middle-aged females.
    • Caused by weakness in pelvic floor muscles, resulting in leakage during physical stress (coughing, sneezing, exercise).
    1. Urge Incontinence:
    • Common in older adults.
    • Abrupt, intense urge followed by involuntary urine loss; can be caused by bladder irritation (infection, stones, etc.).
    1. Overflow Incontinence:
    • Involuntary leakage from an overly full bladder; often due to obstruction (e.g., enlarged prostate).
    1. Functional Incontinence:
    • Resulting from inability to reach a toilet in time, caused by conditions like arthritis or Alzheimer’s disease.

Conclusion

  • Students should be able to:
    1. Discuss regulation of urine volume and osmotic concentration.
    2. Discuss the micturition regulatory mechanisms (both reflexive and voluntary).
    3. Understand clinical correlates related to urinary control issues.