The Urinary System Overview

Urinary System Overview

• The urinary system is essential for filtering blood, regulating ionic composition, pH, osmolarity, and volume, and excreting waste.

Lecture Objectives

• Renal Structure
  • Intra/retro/sub-peritoneal positioning of organs
  • Internal and external features of the kidney
  • Features of the nephron and renal blood supply
  • Features of the bladder
• Renal Function
  • Features of glomerular filtration in the nephron and regulation
  • Resorption and secretion processes in nephron tubules
  • Renin-angiotensin-aldosterone system and hormonal regulation
  • Histology of nephron related to function

Anatomy of the Kidneys

• Located retroperitoneally
• Surrounded by peritoneum:
  • Intraperitoneal: organs surrounded by serosa (e.g. ileum, jejunum)
  • Retroperitoneal: organs behind the peritoneum (e.g. kidneys)
  • Subperitoneal: organs below the peritoneum (e.g. urinary bladder)
• Kidney Structure:
  • Outer cortex and inner medulla organized in pyramids
  • Renal papilla at pyramid apex
  • Minor calyx -> major calyx -> renal pelvis -> ureter for urine drainage

Kidney Function

• Monitoring blood composition:
  • Regulation of ionic composition (e.g. Na+, K+, Ca2+, Cl-, phosphate)
  • Regulation of blood pH, osmolarity, glucose, volume, and blood pressure
  • Secretion of hormones (e.g. renin, erythropoietin, calcitriol)

Blood Supply to the Kidneys

• Renal blood flow is crucial; features include:
  • Renal artery, interlobar arteries, and glomerular capillaries
  • Nephron: responsible for blood filtration and urine formation
  • Afferent arterioles -> glomerulus for filtration
  • Efferent arterioles -> peritubular capillaries for reabsorption

Nephron Functions

• Three Processes:
  1. Glomerular Filtration: Plasma filtered into the nephron
  2. Tubular Reabsorption: Na+, Cl-, and water reabsorbed into the blood
  3. Tubular Secretion: Additional waste products secreted from blood into nephron
• Filtration Fraction: 20% of plasma is filtered, 99% of fluid is reabsorbed

Glomerular Filtration

• Enhancing Factors:
  • Thin filtration membrane
  • Large surface area of glomerular capillaries
  • High blood pressure within glomerular capillaries
• Filtration Membrane Composition:
  • Prevents passage of formed elements and large proteins
  • Allows water and small solutes through

Regulation of Glomerular Filtration Rate (GFR)

• Important Pressures:
  • Promoted by glomerular hydrostatic pressure (Pgc)
  • Opposed by tubular hydrostatic pressure (Pt) and blood colloid osmotic pressure (πgc)
  • Net filtration pressure (NFP) ~10 mmHg
• GFR Calculation:
  GFR = Kf imes P{uf}
  where Kf = filtration constant

Regulation Methods for GFR

• 1. Renal Autoregulation:
  • Myogenic mechanism: smooth muscle contraction in response to stretch
  • Tubuloglomerular feedback: by macula densa cells
• 2. Neural Regulation:
  • Sympathetic nervous system activation causes vasoconstriction
• 3. Hormonal Regulation:
  • Atrial natriuretic peptide (ANP): vasodilation and increased GFR
  • Renin-angiotensin-aldosterone system: decreases GFR

Tubular Reabsorption and Secretion

• Countercurrent Multiplier Mechanism: Long loop of Henle creates osmotic gradient in medulla
• Hormonal Regulation:
  • ADH (Vasopressin): Increases water reabsorption in collecting ducts
  • Aldosterone: Enhances Na+ reabsorption
  • ANP: Decreased reabsorption and increased urine output

Histology of Renal Tubule

• Proximal Convoluted Tubule: simple cuboidal with brush border
• Loop of Henle: thin segment (simple squamous), thick segment (cuboidal)
• Distal Convoluted Tubule and Collecting Duct: receptors for ADH and aldosterone

Clinical Application - Renal Filtration

• Substances filtered or not filtered into Bowman’s Capsule:
  • Erythrocytes: Not filtered
  • Water, glucose, Na+, urea, amino acids: Filtered

Summary of Hormonal Effects on Urine Concentration

• ADH: More concentrated urine when water is retained
• ANP: Less concentrated urine when more fluid is excreted

Reading Materials

• Tortora, Chapter 26: The Urinary System