It is important for the animal to regulate the rate of plasma filtration in the kidney. Too much filtration results in loss of important substances and water
The distal tubule can sense changes in sodium concentrations and filtrate volume. Macula densa cells sense sodium and filtrate volume and release paracrine factors that effect JG cell contraction and afferent arterial vasoconstriction
The system allows the body another chance to reabsorb water from the distal tubule and connecting duct before excretion and greatly affects urine volume
2. Descending limb of Loop of Henle is permeable to water, water exits lumen and lumen filtrate becomes concentrated
3. Hairpin and thin ascending limb has low water permeability but is permeable to sodium, sodium starts to leave lumen
4. Thick ascending limb actively transports sodium and chloride out of the lumen and is impermeable to water, lots of sodium pumped out but water remains inside
Osmoconcentration is achieved because of the close proximity and countercurrent flow of tubular filtrate between the descending and ascending limbs of loop
A vertical osmotic gradient is established as the countercurrent flow of tubular filtrate continues to deliver more sodium-chloride into the medulla interstitial fluid
1. Tubular filtrate exiting the loop and entering the distal tubule is hypotonic (100 mOsm) and then empties into the collecting duct
2. The collecting duct continues through the medulla tissue with a high solute concentration, increases potential for water to be reabsorbed from the duct
3. Vasopressin triggers insertion of aquaporins (AQP-2) into the apical membrane of collecting duct cells, water is reabsorbed and urine becomes hypertonic or concentrated with solutes