Urinary System Overview Lecture Notes

Introduction

• Function: Eliminate metabolic wastes from the body; different from defecation (digestive system).
• Main Purpose: Excretion of metabolic wastes, especially nitrogenous wastes.

Functions of the Urinary System

• Excretion:
  • Removes metabolic wastes like urea (amino acid breakdown), creatinine (muscle metabolism), uric acid (nucleotide metabolism).
• pH Maintenance:
  • Keeps body fluids at a pH of 7.4 via hydrogen ion secretion or bicarbonate reabsorption.
• Water-Salt Balance:
  • Regulates sodium chloride, blood volume, and blood pressure.
  • Maintains ion concentration: potassium, bicarbonate, calcium.
• Hormone Secretion:
  • Renin: Triggers aldosterone release, increasing sodium reabsorption.
  • Erythropoietin: Increases RBC production if oxygen levels are low.
  • Activates vitamin D for calcium absorption.

Anatomy of the Urinary System

• Kidneys: Main excretory organs, maintaining homeostasis by regulating water and ions.
• Ureters: Transport urine from kidneys to bladder via peristalsis.
• Urinary Bladder: Stores urine, expandable with sphincters controlling release.
• Urethra: Tube from bladder to external opening; also transports semen in males.

Structure of the Kidneys

• Regions:
  • Renal Cortex: Outer red region due to numerous capillaries.
  • Renal Medulla: Inner, striped area due to tubules and vessels.
  • Renal Pelvis: Innermost, collects urine before exiting the kidney.
• Nephrons:
  • Functional units (~1 million per kidney), composed of tubules.

Nephron Structure

• Bowman's Capsule: Surrounds glomerulus; initial filtration site.
• Proximal Convoluted Tubule (PCT):
  • Cuboidal cells with microvilli for absorption.
• Loop of Henle: U-shaped tubule, dips into the renal medulla.
• Distal Convoluted Tubule (DCT): Tubular excretion, lacks microvilli.
• Collecting Ducts: Transport urine to renal pelvis, permeability affected by ADH.

Blood Flow in the Nephron

• Renal Artery: High in oxygen, glucose, amino acids, and wastes.
• Afferent Arteriole: Leads into glomerulus.
• Efferent Arteriole: Takes blood to peritubular capillary network (PCN).
• Renal Venule/Vein: Blood returns to circulation.

Urine Formation

• Glomerular Filtration: Blood to nephron; pressure filtration forming filtrate.
• Tubular Reabsorption:
  • Occurs in PCT and PCN, active transport reabsorbs glucose, amino acids, salts, water.
  • Loop of Henle reabsorbs water (descending) and salts (ascending).
• Tubular Secretion:
  • DCT active transport, excess substances enter filtrate.
  • Collecting duct adjusts water reabsorption, influenced by ADH.

Urine Regulation

• ADH (Anti-Diuretic Hormone):
  • Reduces urine volume, increases blood pressure.
  • Released by posterior pituitary, regulated by hypothalamus.
  • Inhibited by alcohol, causing dehydration.
• Aldosterone:
  • Produced by adrenal cortex, increases sodium and water reabsorption.
  • Increases blood volume and pressure.
• Blood Pressure Sensors:
  • Juxtaglomerular apparatus detects low pressure, releases renin, triggers aldosterone.
• ANH (Atrial Network Hormone):
  • Inhibits renin, increases sodium excretion.

pH Balance

• Buffer Systems: Hydrogen ion and bicarbonate buffer system.
• Regulation: Kidney adjusts hydrogen ion levels to maintain pH.

Conclusion

• The urinary system involves complex processes and anatomical structures, essential for waste removal and homeostasis. Review handouts for detailed understanding.
• Next unit: The Reproductive System.