3/10/2016concentration&dilution of urine1. Renal mechanisms of diluting and concentrating urine …

05/14/23 concentration&dilution of urine 1

Renal mechanisms of diluting and concentrating urine

The kidneys excrete excess water by forming dilute urine and conserves water by excreting

concentrated urine. Can excrete urine

with an osmolarity as low as 50mOsm/L Conversely, can excrete urine

with concentration of 1200 –1400mOsm/L.


Renal mechanisms of diluting and concentrating urine

ADH alters renal excretion of water independent of rate of solute excretion.

ADH secretion when the solute in the body fluids become too concentrated.

It increases the permeability of late distal tubules and collecting ducts to

water.


Excretion of dilute urine When there is a large excess of water in

the bodythe kidneys can excrete as much as 20L/day

of dilute urine.Continue to reabsorb solutes while failing to reabsorb large amount of

water in distal parts of the nephron. Glomerular filtrate has osmolarity that

of plasma.


Excretion of dilute urine In proximal

tubules, solutes and water

are reabsorbed in equal

proportions, so that little change in

osmolarity occur.


Excretion of dilute urine In the descending

loop of Henle water is

reabsorbed by osmosis

and the tubular fluid reaches equilibrium

with surrounding interstitial fluid.


Excretion of dilute urine Ascending limb of

loop of Henle is impermeable to

water even in presence

of large amounts of ADH.


Excretion of dilute urine Active reabsorption

of solutes, causing the tubular

fluid to become more dilute

as it flows into the early distal tubule,

osmolarity decreasing progressively to

100mOsm/L.


Excretion of dilute urine In the distal tubules,

cortical collecting duct and medullary duct there is additional

reabsorption of NaCl. In the absence of

ADH these segments are

also impermeable to water.


Excretion of dilute urine Tubular fluid

become even more dilute decreasing its

osmolarity to as low as 50 mOsm/L


Excretion of concentrated urine Essential for survival of mammals

that live on land. Fluid intake is required

to match with fluid loss. The ability of the kidney

to form a small volume of concentrated urine minimizes the intake required to maintain

homeostasis.


Excretion of concentration urine

When there is water deficit in the body the kidney forms small volume concentrated

urine by continuing to excrete solutes while increasing water reabsorption.

Obligatory urine volume 0.5l/day. Excretion of concentrated urine requires.

High ADH level.Hyperosmotic renal medullary interstitium.



Excretion of concentrated urine

NaCl H2O Urea

Urea

H2O

H2O

NaCl

NaCl

NaCl

NaCl

H2O

300300

600 600 600

1200 1200 1200

100300

600

1200

cortex

medullar

Stimuli for ADH secretion ECF osmolarity Blood volume/Arterial pressure Stimuli to CNS Drugs,

Alcohol inhibits ADH secretion


Hyperosmotic renal medullary interstitium Provide osmotic gradient

necessary for water reabsorption to occur○ in the presence of high levels of ADH

Water moves through the tubular membrane by osmosis

○ into the renal interstitiumthen by vasa recta back into the blood.


Causes of hyperosmotic renal medullary interstitium

Special anatomical arrangements of the loops of Henle

and vasa recta.


Causes of hyperosmotic renal medullary interstitium Major factors that contribute to hyperosmolarity

of renal medullary interstitium are Active transport of Na+, and cotransport of K+, Cl-

and others out of the thick portion of loop of Henle. Capable of establishing concentration gradient

○ of 200 mOsm/L Active transport of ions

from the collecting duct into the medullary interstitium. Passive diffusion of urea

from inner medullary collecting ducts into the interstitiusm


Causes of hyperosmotic renal medullary interstitium Diffusion of only small amount of water

from the medullary collecting ducts into the medullary interstitium. large amount being reabsorbed

○ in late distal tubules and cortical collecting ducts. Some passive reabsorption of NaCl

in the descending loop of Henle.


Mechanisms Active pump

of thick ascending limb of loop of Henle

establishes 200mOsm/L concentration gradient

between the tubular fluid and interstitial fluid.


Mechanisms Tubular fluid in the

descending limb of loop of Henle and

the interstitial fluid quickly reach

osmotic equilibrium. Hyperosmotic fluid

formed in the descending limb

flows into ascending limb.


Mechanisms Once this fluid

is in the thick segment

additional ions are pumped into the

interstitium with water remaining

behind until osmotic

gradient of 200mOm/L○ is established.


Mechanisms Tubular fluid in the

descending loop and hyperosmotic

medullary interstitial fluid

reaches equilibrium.


Mechanisms

These are repeated over and over

with the net effect of adding more and

more solutes to the medulla in excess of water.


Mechanisms With sufficient time

this process multiplies the concentration

gradient established by active

transport of ions in the thick segment of

loop of Henle raising the osmolarity to 1200 – 1400mOm/L

This process Counter current

multiplier.


Mechanisms Passive reabsorption of urea

from inner medullary collecting ducts contributes 40% of the renal medullary

interstitium osmolarity when the kidney is forming maximally concentrated

urine.


Mechanisms Very little urea

is reabsorbed in the loop of Henle.

Distal tubules and cortical collecting ducts are impermeable to

urea even in the

presence of high concentration of ADH.


Mechanisms In the presence of

high [ADH], Urea] increases

rapidly in cortical collecting duct.


Mechanisms As fluid flows

into inner medullary collecting ducts

further reabsorption

of water takes place,

[Urea] increase.


Mechanisms Increased [Urea]

causes large amount of urea

to diffuse into the interstitium

because this segment

is highly permeable to urea

ADH increases this permeability

even more.


Urea cycle Urea is recycled

several times before it is excreted.

Some diffusion of urea into the thin loop of

Henle from the medullary interstitium.


Maintenance of hyperosmolarity of the renal medulla

Medullary blood flow is low The vasa recta serves as counter

current exchangers,Minimizing wash out of solutes from the

medullary interstitium. Blood enters and leaves the medulla by

the way of vasa recta at the boundary of cortex and medulla.



As blood descends into the medulla toward the papillae it becomes

progressively more concentrated

party by solute ○ entry from the

interstitium



and party by water diffusing ○ into the

interstitium By the time

blood reaches the tips of vasa recta

it has conc.of 1200mOm/L.


Maintenance of hyperosmolarity of renal medulla

As blood ascends back toward the cortex it become

progressively less concentrated

water moving into vasa recta○ solutes diffusing

out.


Maintenance of hyperosmolarity of renal medulla

Thus although there is large amount of fluid

and solute exchange ○ across the vasa

recta,there is little

○ net dilution of the concentration

○ of the interstitial fluid at each level of the medulla.



Vasa recta mOsm/L

Renal medullaryinterstitium

Renal medullaryinterstitium

H2O

H2O

H2O

600

800

10001200

600

300

solutes

solutes

solutes

solutes

solutes

solutes

600600

800 900

1200

800

300300

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