Water Balance

  1. The Kidney Maintains Water Balance

Kidney maintain water content of body & tonicity of plasma.

Kidney can produce hypotonic urine in response to a water overload.

  1. The Proximal Tubule Reabsorbs More Than 60% of Filtered Water

Takes up solutes from tubule fluid by both active & passive means. Sodium-potassium-adenosine triphosphatase(Na+,K+-ATPase) pump in basolateral plasma membrane actively transports Na+ & drives carrier-mediated secondary active transport & passive transport uptake of solutes.

Small chemical gradient results in rapid movement of water from tubule fluid to interstitial fluid.

High oncotic pressure & low hydrostatic pressure in peritubular capillaries favor movement of reabsorbed water & solute from interstitial fluid to blood.

Water is reabsorbed nearly isotonically with salt, osmolality of tubule fluid remains similar from Bowman's space to beginning of thin descending limb of Henle's loop.

  1. The Kidney Can Produce Either Concentrated or Diluted Urine

Have 3 components

Generation of a hypertonic medullary interstitium allowing excretion of concentrated urine

Dilution of tubule fluid by thick ascending limb & distal convoluted tubule allowing excretion of dilute urine

Variability in water permeability of collecting duct in response to antidiuretic hormone (ADH,vasopressin) determining final urine concentration.

  1. A Hypertonic Medullary Interstitium Is Needed to Form Concentrated Urine

Formation of concentrated urine

Generation of a hypertonic medullary interstitium

Enhanced water permeability in collecting duct in presence of ADH

Hypertonicity of medullary interstitium produced & maintained by

The reabsorption of osmotically active substances by tubules in medulla

The removal of water from medullary interstitium by vasa recta

  1. Shoot-Loop & Long-Loop Nephrons Have Different Roles in Urine Concentration

Superficial mephrons: Have short loops of Henle that extend only into inner stripe of outer medulla. Short-loop nephrons: Have a descending thin limb that parallels thick ascending limb, but do not have an ascending thin limb.

Juxtamedullary nephrons: Have long loops of Henle extend deep into inner medulla. Long-loop nephrons: Have several segments of descending & ascending thin limbs with specific urea & water transporter expression.

  1. Sodium Chloride Reabsorption by the Medullary Thick Ascending Limb Generates Medullary Hypertonicity

Hypertonic interstitium allows water to be abstracted from water-permeable descending thin limbs & returned to circulation.

  1. Urea Reabsorption by Inner Medullary Collecting Duct & Urea Recycling Enhance Medullary Hypertonicity

Mammals: Urea recycling enhances efficiency of urine-concentrating mechanisms.

Birds: Urea nearly absent in medullary interstitium, urates do not contribute appreciably to osmotic pressure because they have low water solubility. Medullary hypertonicity: depend on single-solute (NaCl) recycling.

  1. The Countercurrent Mechanism Increases Medullary Interstitial Osmolality with Minimal Energy Expenditure

Because of:

The anatomical arrangement of thin limbs of Henle's loop

The differential water and salt permeabilities of descending & ascending thin limbs

Passive - thin limbs have reabsorbed both water & salt. Water reabsorbed from descending thin limb & salt reabsorbed from ascending thin limb.

Variable water & salt permeabilities helped to maintain medullary hypertonicity.

  1. Countercurrent Exchange in the Vasa Recta Removes Water from the Medullary Interstitium Without Reducing Medullary Interstitial Hypertonicity

Ascending vasa recta leave medulla, have net movement of fluid into capillary

Plasma oncotic pressure has fallen

Blood flow in ascending vasa recta is double that in descending vasa recta.

High initial plasma oncotic pressure, countercurrent arrangement of vasa recta, passive equilibration of plasma with changing interstitial osmolalities in different regions of medulla allowing removal of water & solute from medullary interstitium without dissipating medullary hypertonicity.

Vasa recta permeable to water, salts & urea.

  1. Active Sodium Chloride Reabsorption in the Thick Ascending Limb & Distal Convoluted Tubule Dilutes the Tubule Fluid

Tubule fluid delivered to collecting duct is hypotonic even in a dehydrated animal.

  1. Antidiuretic Hormone Regulates Collecting Duct Water Permeability to Determine the Final Urine Osmolality

Water overload: ADH present, collecting duct impermeable to water. Tubule fluid delivered by distal convoluted tubule remains hypotonic because water is retained in collecting duct lumen. Absence of ADH - dilute urine, excess water excreted.

Dehydration/hypotension/volume depletion: ADH released from pituitary triggered by a rise in plasma osmolality resulting from dehydration/sakt overload & by decreased blood pressure due to heart failure, etc.

ADH present - water flows from dilute tubule fluid into cell into interstitium down the concentration gradient, producing structural alteration.

Birds: Salt & water reabsorption occurs distal to collecting ducts. Lacks of urinary bladder, urine travels from kidney via ureters to cloaca where both salt and water reabsorbed.

  1. Cells in the Inner Medulla Adapt to Interstitial Hyperosmolality by Accumulation of Organic Osmolytes

Intracellular concentrations of osmolytes vary with diuretic state of animal, increasing during periods of urine concentration, when medullary interstitial osmolality is maximized, and decreasing during diuresis when medullary interstitial osmolality decreases.