Urinary System: Electrolyte Regulation

Urinary System: Regulation of Electrolytes

• The urinary system regulates K+, Ca²⁺, PO₄³⁻, and Mg²⁺, crucial for maintaining homeostasis.

• Potassium excretion is determined by filtration rate (GFR multiplied by plasma K+ concentration), reabsorption by tubules, and secretion by tubules.

  • Approximately 65% of filtered potassium is reabsorbed in the proximal tubule, with an additional 25-30% being reabsorbed in the loop of Henle, particularly the thick ascending limb (active co-transport with Na+ and Cl-).
  • Reabsorption in the late distal and collecting tubules involves H+-K+ ATPase, exchanging K+ for H+ ions, which then diffuse into the bloodstream.
  • Under normal conditions, principal cells play a smaller role in potassium excretion and rely on the Na+-K+ ATPase pump and the electrochemical gradient.
  • Factors affecting potassium secretion include extracellular fluid K+ concentration, aldosterone, and tubular flow rate. Increased extracellular fluid K+ and aldosterone increase potassium secretion; increased tubular flow rate offsets this.
  • Acidosis inhibits potassium secretion; alkalosis reverses this effect.

• Calcium excretion: About 99% of filtered calcium is reabsorbed by the tubules (65% in proximal, 25-30% in loop of Henle, 4-9% in distal).

  • The bone acts as a large calcium reservoir and source.
  • Parathyroid hormone (PTH) regulates calcium reabsorption:
    • Stimulates bone resorption.
    • Stimulates vitamin D3 activation to increase intestinal calcium absorption.
    • Directly increases renal tubular calcium reabsorption.

• Phosphate excretion: Proximal tubules reabsorb 75-80% of filtered phosphate.

  • Distal tubules reabsorb 10%, with smaller amounts in other segments.
  • Approximately 10% of filtered phosphate is excreted in urine.
  • Increased plasma PTH decreases tubular phosphate reabsorption, resulting in more phosphate excretion.

• Magnesium excretion: Proximal tubule reabsorbs approximately 25% of filtered magnesium, with 65% reabsorbed in the loop of Henle.

  • A small amount (typically <5%) is reabsorbed in the distal and collecting tubules.
  • Magnesium excretion is increased by higher extracellular magnesium concentrations, extracellular volume expansion, and higher extracellular calcium levels.

Urinary System: Fluid Balance & Transport

• Renal mechanisms regulate extracellular fluid (ECF) homeostasis through various hormonal and other adjustments.

• When intra-renal compensation fails, systemic mechanisms regulate salt and water balance (including changes in blood pressure, circulating hormones like renin-angiotensin-aldosterone system, ADH, ANP, and changes in sympathetic nervous system activity).

• Urine transport from kidney to bladder:

  • Urine follows the same composition as fluid from collecting ducts.
  • Peristaltic contractions in the ureters propel the urine into the renal pelvis and then to the bladder.
  • The detrusor muscle (smooth muscle in bladder walls) helps force urine towards the urethra.
  • Both sympathetic and parasympathetic nerves modulate ureter contractions.
  • Bladder distention triggers a spinal reflex, leading to bladder contraction.

• Micturition reflex: Bladder distension stimulates stretch receptors in the bladder walls, triggering smooth muscle contractions of the bladder and relaxation of the internal and external sphincters. Inhibitory impulses can come from the cerebral cortex and prevent micturition until bladder volume is significant

  • Various factors affect the rate of urine excretion: voluntary control and higher center inhibition, bladder pressure, muscle tone, and spinal reflex.

Pathological Terms

• UTI: Urinary tract infection
• Nephritis: Kidney inflammation
• Hydronephrosis: Renal pelvis dilation
• Anuria: No urine output
• Dysuria: Painful urination
• Enuresis: Lack of bladder control
• Oliguria: Scanty urine output
• Polyuria: Excessive urine output
• Incontinence: Involuntary urine discharge
• Diuretic: Increases urine output
• Antidiuretic: Decreases urine output

Explore how the urinary system maintains homeostasis by regulating essential electrolytes such as potassium, calcium, phosphate, and magnesium. Learn about the mechanisms of reabsorption and secretion in the kidneys, including the roles of different tubules and hormonal influences like aldosterone.