ch 25 - urine system

• Tubular Reabsorption


  Selectively returns 99% of substances from filtrate to blood in peritubular capillaries
• Tubular Reabsorption


  • Selective process
  • Nearly all organic nutrients reabsorbed
  • Water and ion reabsorption hormonally regulated and adjusted
  • Includes active and passive tubular reabsorption
• Tubular Reabsorption


  1. Cortical radiate artery

  2. Afferent arteriole

  3. Glomerular capillaries

  4. Efferent arteriole

  5. Glomerular capsule

  6. Renal tubule and collecting duct containing filtrate

  7. Peritubular capillary

  8. To cortical radiate vein
• Active Reabsorption of Sodium

  • Na+ is the most abundant cation (+ charged ion) in filtrate
  • Na+ -K+ Pump in peritubular capillaries
  • Na+ reabsorption provides energy for reabsorbing most other substances
  • Creates electrical gradient for passive reabsorption of anions (- charged ions)
  • Organic nutrients reabsorbed include glucose, amino acids, some ions, vitamins
• Passive Reabsorption of Water

  • Movement of Na+ and other solutes creates osmotic gradient for water
  • Water reabsorbed by osmosis through water-filled pores called aquaporins
  • Aquaporins always open in PCT (obligatory water reabsorption)
  • Aquaporins inserted in collecting ducts only if antidiuretic hormone (ADH) present (facultative water reabsorption)
• Tubule Area and Absorptive Capabilities

  • PCT is most active in reabsorption
  • Nearly all glucose, amino acids, and vitamins reabsorbed, 65% of water and Na+ reabsorbed
  • Descending limb of nephron loop permeable to water, ascending limb impermeable to water but transports ions
  • DCT and collecting duct have Na+ and water permeability regulated by hormones (Aldosterone and Antidiuretic Hormone (ADH))
• Tubular Secretion


  • Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
  • Reabsorption in reverse; almost all in PCT
  • Selected substances K+, H+, NH4+, creatinine, organic acids and bases move from peritubular capillaries through tubule cells into filtrate
• Tubular Secretion


  • Disposes of substances (e.g., drugs) bound to plasma proteins
  • Eliminates undesirable substances passively reabsorbed (e.g., urea and uric acid)
  • Rids body of excess K+
  • Controls blood pH by altering amounts of H+ or HCO3– in urine
• Three major renal processes

  1. Urine

  2. Glomerular filtration

  3. Tubular reabsorption

  4. Tubular secretion
• Urinary System


  A major part of homeostasis is maintaining the composition, pH, and volume of body fluids within normal limits
• Urinary System


  Removes metabolic wastes and substances in excess, including foreign substances like drugs and their metabolites that may be toxic
• Components of the Urinary System


  • Kidneys
  • Ureters
  • Urinary bladder
  • Urethra
• Kidney


  • Retroperitoneal
  • Right kidney crowded by liver, lower than left
  • Convex lateral surface, concave medial surface
• Renal Cortex


  • Granular-appearing superficial region
• Renal Medulla


  • Composed of cone-shaped medullary (renal) pyramids
  • Pyramids separated by renal columns
• Renal Pelvis


  • Funnel-shaped tube continuous with ureter
• Urine Flow


  1. Renal pyramid

  2. Minor calyx

  3. Major calyx

  4. Renal pelvis

  5. Ureter
• Blood and Nerve Supply


  • Kidneys cleanse blood
  • Rich blood supply
  • Renal arteries deliver ~ ¼ (1200 ml) of cardiac output to kidneys each minute
  • Arterial flow into and venous flow out of kidneys follow similar paths
• Peritoneum


  • Renal fascia (anterior and posterior)
  • Perirenal fat capsule
  • Fibrous capsule
• Nephrons


  • Structural and functional units that form urine
  • More than 1 million per kidney
• Parts of a Nephron

  • Renal corpuscle
  • Renal tubule
• Renal Corpuscle


  • Glomerulus (tuft of capillaries with fenestrated endothelium, highly porous, allows filtrate formation)
  • Glomerular capsule (Bowman's capsule, cup-shaped, hollow structure surrounding glomerulus)
• Parts of the Renal Tubule

  • Proximal convoluted tubule (PCT)
  • Nephron loop
  • Distal convoluted tubule (DCT)
• Proximal Convoluted Tubule (PCT)


  • Cuboidal cells with dense microvilli (brush border, large surface area)
  • Large mitochondria
  • Functions in reabsorption and secretion
  • Confined to cortex
• Nephron Loop


  • Descending and ascending limbs
  • Proximal descending limb continuous with proximal tubule
  • Thin distal descending limb
  • Thick ascending limb
• Distal Convoluted Tubule (DCT)

  • Cuboidal cells with very few microvilli
  • Function more in secretion than reabsorption
  • Confined to cortex
• Collecting Ducts


  • Receive filtrate from many nephrons
  • Run through medullary pyramids (striped appearance)
  • Fuse together to deliver urine through papillae into minor calyces
• Classes of Nephrons


  • Cortical nephrons (85% of nephrons, almost entirely in cortex)
  • Juxtamedullary nephrons (long nephron loops deeply invade medulla, important in production of concentrated urine)
• Nephron Capillary Beds


  • Glomerulus
  • Peritubular capillaries
  • Vasa recta
• Glomerulus


  • Specialized for filtration
  • Different from other capillary beds - fed and drained by arterioles
  • Afferent arteriole larger in diameter than efferent arteriole
  • Arterioles are high-resistance vessels
• Peritubular Capillaries


  • Low-pressure, porous capillaries adapted for absorption of water and solutes
  • Arise from efferent arterioles
  • Cling to adjacent renal tubules in cortex
  • Empty into venules
• Vasa Recta


  • Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons
  • Arise from efferent arterioles serving juxtamedullary nephrons
  • Function in formation of concentrated urine
• Aorta -> Renal artery -> Segmental artery -> Interlobar artery -> Arcuate artery -> Cortical radiate artery -> Afferent arteriole -> Glomerulus (capillaries) -> Efferent arteriole -> Inferior vena cava -> Renal vein -> Interlobar vein -> Arcuate vein -> Cortical radiate vein -> Peritubular capillaries or vasa recta
• Glomerular Filtration


  Mechanism of urine formation
• Kidney Physiology: Mechanisms of Urine Formation

  • 180 L fluid processed daily; only 1.5 L urine
  • Filtrate (produced by glomerular filtration) is blood plasma minus proteins
  • Urine contains <1% of original filtrate and contains metabolic wastes and unneeded substances
• Mechanisms of Urine Formation

  1. Glomerular filtration

  2. Tubular reabsorption - Selectively returns 99% of substances from filtrate to blood in peritubular capillaries

  3. Tubular secretion - Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
• Three major renal processes

  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
• Glomerular Filtration


  • Passive process where hydrostatic pressure forces fluids and solutes through filtration membrane
  • Porous membrane allows water, solutes smaller than plasma proteins to pass
• Pressures That Affect Filtration

  • Outward pressures promote filtrate formation: Hydrostatic pressure in glomerular capillaries (55 mm Hg)
  • Inward forces inhibiting filtrate formation: Hydrostatic pressure in capsular space (15 mm Hg), Colloid osmotic pressure in capillaries (30 mm Hg)
• Net Filtration Pressure (NFP)

  Sum of forces - 55 mm Hg forcing out; 45 mm Hg opposing = net outward force of 10 mm Hg