MTC.AP2.Lec10

Learning Objectives

• Identify & describe structures of the urinary system
• Relate macroscopic & microscopic kidney anatomy to function
• Trace renal blood flow & filtrate pathway
• Explain mechanisms/regulation of
  • Glomerular filtration
  • Tubular reabsorption & secretion (active vs. passive transport)
  • Tubular excretion
• Catalogue solutes filtered/reabsorbed/secreted along nephron segments
• Clarify renal portal (arteriole–capillary–arteriole) arrangement & its hemodynamic significance
• Connect blood/osmotic pressure changes to filtrate production & solute–water recovery
• Discuss hormonal control of urine osmolality & ion balance (ADH, aldosterone, PTH, renin-angiotensin)
• Summarize renal role in systemic acid–base balance vis-à-vis pulmonary regulation

Urinary System – General Overview

• Primary role: body’s “water-treatment plant”; eliminates metabolic wastes while conserving needed solutes & water
• Major waste route: kidneys → ureters → urinary bladder → urethra
• Output ≈ 1\,\text{L} urine/day (can vary widely)

Component Functions

• Kidneys
  • Filter ~25\% cardiac output; convert blood filtrate → urine
  • Regulate ions (Na^+,K^+,Ca^{2+},Cl^- etc.), pH, and blood pressure
  • Endocrine: renin, erythropoietin (EPO), calcitriol activation
• Ureters
  • Muscular tubes conducting urine kidney → bladder via peristalsis
  • Possess one-way vesicoureteral valve preventing reflux
• Urinary Bladder
  • Distensible smooth-muscular sac (detrusor); capacity ≈ 1\,\text{L}
  • Location: anterior to uterus (♀); superior to prostate (♂)
• Urethra
  • Conducts urine to exterior; length & regional histology differ by sex
  • Sphincters
  • Internal urethral – smooth muscle, autonomic
  • External urethral – skeletal muscle, voluntary

Macro-Anatomy Quick Map

• Arterial blood (renal artery) → kidney → filtered blood exits via renal vein → systemic circulation
• Urine drains via ureters → bladder → urethra → exterior

Urethra – Detailed Comparison

• Shared histology: proximal transitional epithelium → distal non-keratinized stratified squamous; ♂ have intermediate pseudostratified columnar
• Female
  • ~4\,\text{cm}; external orifice within vaginal vestibule → higher UTI susceptibility
• Male (≈20\,\text{cm})
  • Pre-prostatic → prostatic (receives ejaculatory ducts) → membranous → spongy/penile

Urinary Bladder Anatomy & Micturition

• Walls: mucosa (transitional ep.), submucosa, detrusor (3 layers smooth muscle), serosa/adventitia
• Micturition reflex
  • Stretch receptors fire ≈150\,\text{mL}; voluntary suppression via external sphincter
  • At 300!–!400\,\text{mL}, reflex overrides → incontinence
  • Pathway: stretch afferents → sacral cord → parasympathetic efferents: detrusor contracts & internal sphincter relaxes; somatic inhibition of external sphincter

Ureters – Histology

• Mucosa: transitional epithelium + goblet cells (mucus = barrier)
• Muscularis: inner longitudinal, outer circular smooth muscle → peristaltic waves q10–15 s
• Adventitia: loose CT anchoring within retro-/intra-peritoneal space

Kidney – External Features

• Retroperitoneal (T12–L3); right lower than left (liver)
• Layers: renal capsule (fibrous), perirenal fat pad (shock absorber), renal fascia, pararenal fat
• Supraadrenal (adrenal) gland perched superior pole

Kidney – Internal Architecture

• Cortex (granular, lighter)
• Medulla (darker): renal pyramids separated by cortical columns
• Papilla (tip of pyramid) drains → minor calyx → major calyx → renal pelvis → ureter
• Hilum: entry/exit for renal artery, vein, nerves, lymph, ureter

Vascular Tree

• Renal → segmental → interlobar → arcuate → cortical radiate (interlobular) → afferent arteriole → glomerulus → efferent arteriole →
  • Peritubular capillaries (cortical nephron)
  • Vasa recta (juxtamedullary nephron)
• Venous return mirrors arterial (cortical radiate → arcuate → interlobar → renal vein)

Nephron – Structural Units (~1!–!1.5\,\text{million/kidney})

• Renal corpuscle = glomerulus + Bowman’s capsule (parietal squamous & visceral podocytes)
• Tubular segments
  • Proximal convoluted tubule (PCT)
  • Loop of Henle (descending thin limb, ascending thick limb)
  • Distal convoluted tubule (DCT)
  • Collecting duct (receives filtrate from multiple nephrons; merges → papillary duct)
• Types
  • Cortical (short loops; \approx85\%)
  • Juxtamedullary (long loops into medulla; produce concentrated urine)

Filtration Membrane

• Fenestrated capillary endothelium
• Basement membrane (negatively charged glycoproteins)
• Podocyte slit diaphragms (~8\,\text{nm} gaps)
• Lets water, ions, glucose, aa, urea across; excludes cells & proteins >5\,\text{nm} or negatively charged

Juxtaglomerular Apparatus (JGA)

• Location: DCT contacts vascular pole
• Cell types & roles
  • Macula densa (DCT) – sense filtrate [NaCl] / osmolarity
  • Granular (juxtaglomerular) cells – modified smooth muscle of afferent arteriole; secrete renin & alter arteriole caliber
  • Extraglomerular mesangial cells – parac