1AA3/2YY3 - 25: Urinary - Urine Concentration, Bladder & Ureters

Overview of the Urinary System

• Focus: Concentrated and dilute urine production, anatomy of urethra, ureters, and bladder, as well as the micturition reflex.

Urine Production

• Fluid Intake: Daily fluid intake affects fluid retention and urination.

• Hormonal Control:

  • Antidiuretic Hormone (ADH):

    • Low ADH = dilute urine production.

    • High ADH = concentrated urine production (e.g., due to dehydration).

Formation of Dilute Urine

• Filtrate Concentration:

  • Requires large amounts of water and few solutes in collecting ducts.

  • Water movement via osmosis, influenced by osmolarity.

• Osmolarity Changes:

  • Glomerular filtrate starts at about 300 milliosmoles.

  • In kidney tubules, changes occur due to differential movement of water and ions.

• Kidney Tubule Function:

  • Proximal Convoluted Tubule:

    • Equal osmolarity inside and outside.

  • Descending Limb:

    • Permeable to water; osmolarity increases in the tubule as water leaves.

  • Ascending Limb:

    • Impermeable to water; solutes exit but water does not.

  • Distal Convoluted Tubule (DCT) and Collecting Duct:

    • Further solute reabsorption occurs; low osmolarity leads to dilute urine production without ADH.

Characteristics of Concentrated Urine

• ADH Presence: Facilitates water reabsorption through aquaporin-2 in collecting ducts, causing concentrated urine.

• Role of Juxtamedullary Nephrons: Create osmotic gradients to draw water out, enhancing urine concentration.

Normal Urine Characteristics

• Volume: 1 to 2 liters per day.

• Color: Yellow/amber, influenced by urobilin and diet.

• Turbidity: Typically clear; cloudiness indicates abnormalities.

• Odor: Influenced by diet and metabolic products (e.g., asparagus).

• pH: Varies with diet and hydration.

Ureters and Bladder

• Ureters:

  • 25-30 cm long, transport urine from kidneys to bladder.

  • Muscular layers facilitate peristalsis.

• Bladder:

  • Capacity: 700-800 ml; distensible and hollow.

  • Detrusor Muscle: Contracts to expel urine.

  • Trigone: Smooth region between urethral openings and urethra.

Micturition Reflex

• Urination Process: Triggered by stretch receptors signaling fullness (200-400 ml) to spinal cord (S2-S3).

• Motor Response: Parasympathetic response causes detrusor contraction and internal sphincter relaxation.

• External Sphincter Control: Voluntary control allows for conscious urination.

Urethra Differences

• Male Urethra:

  • 20 cm long, passes through prostatic and spongy regions.

• Female Urethra:

  • 4 cm long, simple pathway from bladder to exterior.

Summary

• Reviewed urine production, the anatomy and histology of the urinary organs, and the micturition reflex. Discussion included the differences between male and female urinary systems.