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Bio Urinary system 2 Lecture

Overview of the Urinary System

  • Focus on renal physiology, specifically nephron function.

Nephron Structure and Function

Filtration Process

  • Daily fluid filtered: 150-180 liters, average 165 liters.

  • Key components:

    • Reabsorption: Movement from nephron to blood; important for retaining essential substances.

    • Secretion: Movement from blood to nephron; vital for eliminating waste and balancing pH.

Substances Involved in Reabsorption and Secretion

  • Water

    • Total blood plasma: 3 liters.

    • Daily filtration: 180 liters, mostly reabsorbed.

    • Only 1-2 liters excreted in urine.

  • Proteins

    • Total plasma: 200 grams; only about 2 grams filtered.

    • Most reabsorbed, 0.1 grams in urine.

  • Glucose

    • Total plasma: 3 grams; 162 grams filtered, ideally fully reabsorbed.

  • Urea

    • Blood plasma: 1 gram; 54 grams filtered, about half is reabsorbed.

  • Creatinine

    • Plasma: 1.6 grams; none reabsorbed, all excreted; good indicator of kidney function.

Mechanisms of Reabsorption

Passive and Active Transport

  • Paracellular Reabsorption: Passive movement between cells (e.g., sodium).

  • Transcellular Reabsorption: Direct movement through tubular cells; involves:

    • Primary active transport (Na+/K+ pump); requires ATP.

    • Secondary active transport, including symporters and antiporters.

      • Symporters: e.g., glucose and sodium moving in the same direction.

      • Antiporters: e.g., sodium and hydrogen ions moving in opposite directions.

Hormonal Regulation of Reabsorption

  • Antidiuretic hormone (ADH): Increases water reabsorption by increasing aquaporins' expression in collecting ducts.

  • Aldosterone: Promotes reabsorption of sodium and chloride, influencing water retention.

  • Parathyroid hormone: Affects calcium reabsorption in distal convoluted tubule.

Regions of the Nephron

Proximal Convoluted Tubule (PCT)

  • Reabsorption: 65% of water, sodium, potassium, glucose, amino acids, and some urea.

  • Secretion: Hydrogen ions, ammonium, urea.

Loop of Henle

  • Descending limb: 15% water reabsorbed.

  • Ascending limb: Solutes reabsorbed, impermeable to water.

Distal Convoluted Tubule (DCT)

  • Reabsorption: Sodium and chloride via symporters; influenced by parathyroid hormone for calcium.

  • Late DCT: Principal cells reabsorb sodium; aquaporin-2 channels for water regulation via ADH.

Collecting Duct

  • Final site of water reabsorption regulated by ADH; can produce concentrated urine.

Urine Concentration Mechanism

Countercurrent Multiplier

  • Uses osmotic gradients to concentrate urine, with vasa recta capillaries aiding in maintaining the gradient by reabsorbing water and solutes in close proximity to nephron segments.

Urine Production Variables

  • Involves intake of fluids; ADH and fluid consumption dictate urine concentration.

  • Dilute urine: Low solute concentration when hydrated; Concentrated urine: High solute concentration and less water.

Urinalysis and Kidney Function

  • Normal urine contains little to no glucose, proteins, red blood cells, or specific waste substances.

  • Abnormal findings may indicate issues such as renal failure or metabolic problems.

Kidney Hormone Production

  • Calcitriol: Active vitamin D form, aids in calcium absorption.

  • Erythropoietin: Stimulates red blood cell production in response to low blood oxygen levels.

Bladder and Urination

  • Bladder capacity approximately 700-800 milliliters.

  • Ureters transport urine from kidneys to bladder via peristalsis.

  • Micturition: Reflex involving involuntary and voluntary muscle control to expel urine; coordinated by spinal reflex and brain signals.

Differences Between Male and Female Urinary Systems

  • Male urethra: Approximately 5 times longer than female urethra; dual function (urination and reproduction).

  • Female urethra: Shorter, direct passage from bladder.