Final_ Reabsorption, Secretion, Excretion
Page 1: Intrinsic Control of Glomerular Filtration
Control Mechanism: Glomerular filtration is intrinsically regulated by changes in blood flow and vessel stretch.
Renal Changes: These intrinsic changes help sense variations in blood flow.
Page 2: Glomerular Filtration Rate (GFR) and Regulation
GFR Autoregulation: Normally, GFR is autoregulated.
MAP Below 80 mm Hg:
Decrease in GFR.
Decrease in water filtered and excreted.
MAP Above 160 mm Hg:
Increase in GFR.
Increase in water filtered and excreted.
Occurs under pathological conditions.
Zone of Autoregulation: This area shows little change in filtration rates despite fluctuations in GFR.
Page 3: Lecture Overview
Learning Goals:
Understand and explain renal exchange processes, including secretion and reabsorption.
Assess their impact on urine composition and excretion rates.
Page 4: Tubular Reabsorption and Secretion
Definitions:
Tubular Reabsorption: Retaining substances (from filtrate to blood).
Tubular Secretion: Removing substances (from blood to filtrate).
Fluid Terminology:
Primary Urine (Filtrate): Fluid inside the nephron.
Blood: Medium inside peritubular capillaries.
Interstitial Fluid: Fluid between the nephron and blood.
Page 5: Reabsorption Mechanisms
Active Reabsorption: Involves active transport (requiring energy) across one membrane and passive transport across another.
Water Reabsorption: Driven by osmolarity differences.
As solutes are reabsorbed, plasma osmolarity increases, prompting water reabsorption.
Passive Reabsorption: Occurs for ions and solutes; water movement depends on solute transport.
Page 6: Secretion Process
Secretion Defined: Solutes exit peritubular capillaries into renal tubules; reverse of reabsorption.
Actively Secreted Substances:
Potassium and hydrogen ions.
Waste and foreign products (solutes only, no water).
Page 7: Excretion Overview
Excretion Function: Eliminates solutes and water as definitive urine.
Excretion Rate: Directly relates to plasma volume and composition.
Rule for Excretion: Any material entering renal tubule lumen is excreted unless reabsorbed (exception: glucose and amino acids).
Page 8: Regional Specialization of Renal Tubules
Variation in Tubule Properties: Osmotic pressure and transport proteins vary across regions, leading to different transport mechanisms and item transport.
Page 9: Osmolarity and Water Movement
Osmosis Defined: Water movement from low to high solute concentration, following solute reabsorption in renal processes.
Page 10: Importance of Osmolarity Regulation
Normal Osmolarity: Body fluids maintained at 300 mOsm.
Fluid Compartments: No osmotic force for water movement under normal osmolarity, but changes can trigger kidney regulation of water reabsorption.
Page 11: Water Reabsorption Breakdown
Proximal Convoluted Tubules: 65-70% reabsorption, not regulated.
Loop of Henle: 20% reabsorption, not regulated.
Distal Tubules and Collecting Ducts: 10-15% reabsorption, regulated by ADH (vasopressin) and aldosterone.
Page 12: Proximal Tubule Water Reabsorption
Mechanism: Water follows solute reabsorption (mainly sodium).
Sodium Transport: Sodium actively transported across the basolateral membrane, creating an osmotic gradient for water.
Page 13: Reabsorption Steps in the Tubule
Active Reabsorption: Na+, X, and Y are reabsorbed, increasing osmolarity of peritubular fluid and plasma.
Water Reabsorption: Followed by osmosis.
Urea Reabsorption: Passive due to permeating solute.
Page 14: Regulation in Distal Tubule and Collecting Duct
Na+ Regulation: Controlled by aldosterone in distal tubule influences water reabsorption in the collecting duct.
Osmotic Gradient Necessity: Gradient from Loop of Henle is essential for effective reabsorption.
Page 15: Renal Medulla Osmotic Gradient
Osmolarity Variation: Interstitial fluid osmolarity varies from cortex to renal pelvis.
Gradient Importance: Critical for effective water reabsorption.
Page 16: Countercurrent Multiplier Effect
Loop of Henle Structure:
Ascending Limb: Impermeable to water; actively transports Na+, Cl–, K+.
Descending Limb: Permeable to water; no solute transport.
Page 17: Osmotic Changes During Transport
Osmotic Gradient: Established by active transport of ions increasing interstitial osmolarity.
Page 18: Changes in Osmotic Gradient During Water Movement
Fluid Changes: As fluid moves, osmotic pressures affect the interstitial environment; influences solute and water transport across membranes.
Page 19: Iso-osmotic States in the Descending Limb
Fluid Dynamics: As ions are reabsorbed, osmotic differences influence fluid movement and absorption efficiency.
Page 20: Fluid Transport through Active Mechanisms
Active Transport Role: Facilitates movement of ions and water retention in the renal tubules, enhancing the osmotic gradient.
Page 21: Continuous Fluid Dynamics in the Loop of Henle
Isosmotic State: Maintains balance as substances move between descending and ascending limbs, optimizing reabsorption.
Page 22: Urea's Contribution to Osmotic Gradient
Urea Role: Maintains osmotic gradient; produced by the liver for nitrogen elimination.
Urea Transporters: UT-A, UT-B, and UT-C help urea pass through tubular barriers, contributing to the overall osmolarity gradient.
Page 23: Vasa Recta's Role
Capillary Arrangement: Prevents diffusion issues, preserving the medullary osmotic gradient.
Osmolarity Changes in Vasa Recta: Water and solutes shift as blood moves through descending and ascending limbs, influencing osmotic balance in kidneys.