🔁 Reabsorption

Reabsorption

Process
 Glomerular filtrate enters proximal convoluted tubule (PCT) → becomes tubular filtrate
 Some substances in tubular filtrate removed and reabsorbed into bloodstream

Key Concept
 Reabsorption recovers water, ions, nutrients from filtrate
 Wastes remain in tubular fluid for excretion

Proximal Convoluted Tubule – Reabsorption

Extent
 About 65% of tubular reabsorption occurs in PCT

Substances Reabsorbed
 Water, sodium (Na⁺), chloride (Cl⁻), bicarbonate (HCO₃⁻)
 Glucose – 100% reabsorbed
 Amino acids – 100% reabsorbed

Key Concept
 PCT recovers most water and solutes from filtrate, keeping blood composition stable

Loop of Henle – Function

Main Function
 Creates and maintains osmolarity gradient in renal medulla (difference in solute concentration between cortex and medulla)

Effect
 Enables collecting duct to concentrate urine later

Key Concept
 Loop of Henle controls water and solute reabsorption by generating medullary concentration gradient

Loop of Henle – Descending Limb

Permeability
 Descending limb is relatively permeable to water
 Is relatively impermeable to particles (solutes)

Water Movement
 Water is removed from tubular lumen (space inside tubule where filtrate flows)

Cause
 Movement occurs because of osmotic gradient (difference in solute concentration that pulls water)
 Between tubular lumen and interstitial fluid of renal medulla (surrounding kidney tissue)

Key Concept
 Water leaves filtrate in descending limb, helping concentrate tubular fluid in renal medulla

Loop of Henle – Ascending Limb

Permeability
 Ascending limb is relatively impermeable to water

Structure
 Has thick portion (thick ascending limb)

Reabsorption
 Major site of sodium (Na⁺) and chloride (Cl⁻) reabsorption
 These ions move from tubule → into capillaries (blood)

Key Concept
 Solutes leave filtrate but water cannot follow, helping maintain osmolarity gradient in renal medulla

Loop of Henle – Countercurrent Effect

Combined Action
 Ascending limb transports Na⁺ and Cl⁻ out of tubular fluid
 Descending limb allows water to leave filtrate

Result
 Combined transport produces osmotic gradient in interstitial fluid of renal medulla (surrounding kidney tissue)

Importance
 Medullary osmotic gradient is essential for concentrating urine later in collecting ducts

Distal Convoluted Tubule – Regulation

Processes
 Reabsorption and secretion occur in distal convoluted tubule (DCT)

Control
 Processes are regulated by hormones

Function
 Allows kidneys to respond to body’s needs
 Adjusts urine composition (what substances remain in urine)

Collecting Duct – Urine Concentration

Function
 Collecting duct acts to concentrate urine by conserving water

Mechanism
 Possible because of osmolarity gradient generated by Loop of Henle

Process
 Renal medulla becomes increasingly salty (higher solute concentration)
 As filtrate flows through collecting duct, water leaves filtrate

Key Concept
 Medullary osmotic gradient allows water reabsorption, producing more concentrated urine

Collecting Duct – ADH Regulation

Target Cells
 Principal cells in collecting duct are target cells for ADH (antidiuretic hormone)
 ADH is hormone that controls water reabsorption in kidneys

Without ADH
 Water permeability of collecting duct is relatively low (water cannot easily pass through)

Result
 Produces large volume of dilute urine
 Because collecting duct remains relatively impermeable to water

Collecting Duct – ADH Action

Target Cells
 Principal cells in collecting duct are target cells for ADH (antidiuretic hormone)

With ADH Present
 Water permeability of collecting duct increases

Water Movement
 Water is reabsorbed from tubular fluid
 Because osmolality inside duct is lower than osmolality in surrounding medulla
 (osmolality = concentration of dissolved solutes)

Key Concept
 ADH increases water reabsorption, producing smaller volume of more concentrated urine

Reabsorption

Substances Reabsorbed
 Na⁺, K⁺, Ca²⁺, Mg²⁺ (important electrolytes)
 Glucose and amino acids (nutrients)
 Cl⁻, bicarbonate (HCO₃⁻)
 Water

Transport Mechanisms
 Osmosis – water movement across membrane due to solute concentration gradient
 Diffusion – movement of solutes from high → low concentration
 Active transport – movement of substances against gradient using cellular energy (ATP)

Pathway Back to Bloodstream
 Substances move from tubular lumen → tubular epithelial cells → interstitial fluid → peritubular capillaries (bloodstream)