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Mechanism of Concentration and Dilution - Coggle Diagram
Mechanism of Concentration and Dilution
The fundamental processes involved in the excretion of a concentrated
or diluted urine include:
i. Variable permeability of the nephron to the passive reabsorption of water along an osmotic gradient and passive diffusion of urea along its concentration gradient.
ii. Passive reabsorption of NaCl by the thin segment of ALH.
iii. Active reabsorption of Na+ by the thick segment of ALH via the Na+ - 2Cl- - K+ symporter and the Na+ - K+ - ATPase pump.
The concentration of the tubular fluid occurs mainly in the juxta
medullary nephron alone, therefore counter current system is the
feature of juxta medullary nephron.
The concentrating mechanism depends upon the existance of a
gradient of increasing osmolality along the medullary pyramids. This
gradient is:
i. Produces by operation of the loop of Henle and collecting ducts as
Counter Current Multipliers, and
ii. Maintained by Vasa Recta as
Counter Current Exchangers.
(i) and (ii) together called as
Counter Current Multiplier Exchange System
or Counter Current System.
Counter Current Multipliers
In the PCT as about 70% of filtered solutes and water get reabsorbed, therefore, osmolar concentration of PCT fluid is equal to osmolar concentration of plasma i.e. 300 mosm/kg water.
The Loop of Henle
A. DLH - it is the ‘concentrating’ segment of the nephron. DLH is:
highly permea0ble to water due to presence of Aquaporin-1, and
relatively impermeable to solute.
Therefore,
i. solute-free water moves into the interstitium,
ii. solute concentration in DLH increases; predominantly NaCl.
B. ALH – The thick and thin segments of ALH constitute the ‘Diluting’ segment of the nephron.
i. The thin segment is
impermeable to water, and
permeable to NaCl and urea.
ii. The thick segment is
impermeable to water and solute, but
actively transports NaCl out of the lumen into interstitium by
Na+ ─ 2Cl- ─ K+ symporter.
DCT and collecting duct
A. Cortical and outer medullary portion of DCT and collecting duct.
The early part of DCT is relatively impermeable to water. However,
the collecting duct segment is impermeable to urea and NaCl, but
permeable to water. Therefore, as the fluid passes through this
segment
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B. Inner medullary portion of collecting duct.
In this portion ADH increases both water and urea permeabilities, therefore,
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Concentration of the fluid in this segment is further increased by the forceful active transport of Na+ from the adjacent ALH.
Therefore, Counter Current System operates:
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Counter Current Exchangers
The osmotic gradient in the medullary pyramids would not last long if
the NaCl and urea in the interstitium were removed by circulation. The
concentrations of Na+ and urea the medullary interstitium are kept high by the slow blood flow in the Vasa recta.
These solutes remain in the pyramids to maintain hyperosmolality of the medullary interstitium
primarily because the vasa recta operates as counter current exchanger.
The arrangement of the descending and ascending limbs of the vasa
recta in dose proximity to each other functions to maintain the
hyperosmolality of the medullary interstitium.