Urine Concentration Flashcards
Maintaining Normal Cellular Environment Extracellular fluid must have a constant
concentration of electrolytes and other solutes
Maintaining Normal Cellular Environment Solute concentration & osmolarity determined by:
Total amount of solute / Volume of extracellular fluid
Maintaining Normal Cellular Environment Changing extracellular water has significant effect on
solute concentration and osmolarity
Maintaining Normal Cellular Environment body water determined by
Fluid intake (controlled by thirst) Renal excretion of water (controlled by changing GFR and tubular reabsorption
If ECF solute concentration increases, kidneys
hold onto
water so ECF volume increases diluting ECF solutes
If ECF solute concentration decreases kidneys
excrete more water so ECF volume decreases concentrating ECF solutes
Assuming normal solute intake and metabolic production
Solute excretion will remain relatively constant each day
Total amount of solute in ECF relatively constant. Quantity of water excreted each day adjusted to keep solute concentration of ECF constant
Increased ECF [solute] (i.e. increased ECF osmolarity)
Normal amount of solute dissolved in less water
Holding onto water will spread the total amount of solute over larger volume of water thus decreasing solute concentration of ECF
Decreased ECF [solute] (i.e. decreased ECF osmolarity)
Normal amount of solute dissolved in too much water
Getting rid of water will spread the total amount of solute over smaller volume of water thus increasing solute concentration of ECF
Posterior pituitary responds to changes in ECF osmolarity by changing ADH release. what effects ADH release?
Increased ECF osmolarity results in an increased release of ADH
Decreased ECF osmolarity results in a decreased release of ADH
Quantity of water excreted controlled
ADH. Increased [ADH] results in an increase in water reabsorption by the distal tubule & collecting duct
Decreased [ADH] results in a decrease in water reabsorption by the distal tubule & collecting duct
Changes in water reabsorption control
urine volume and urine solute concentration.
Increased water reabsorption means
less water enters collecting duct decreasing overall volume of urine - Normal amount of excreted solutes now dissolved in less volume production of small amount of very concentrated urine
At max concentration: 500 mls/day with osmolarity of 1200 to 1400 mOsm/Liter
Decreased water reabsorption means
more water enters collecting duct increasing overall volume of urine – Normal amount of excreted solutes now dissolved in less volume production of large amount of very dilute urine
At min concentration: 20 Liters/day with osmolarity of 50 mOsm/Liter
Excretion of Dilute Urine
Can excrete 20 liters/day with
minimal concentration of 50 mOsm/Liter.Low Antidiuretic Hormone concentration
Reabsorb normal amounts of solute
Limit water reabsorption in late distal tubule and collecting ducts
Water Diuresis process drink 1 liter of water…
Changes begin to occur within 45
minutes
Slight increase in solute excretion
Slight decrease in plasma osmolarity
Large decrease in urine osmolarity [600 mOsm/L to 100 mOsm/L]
Large increase in urine output [1 ml/min to 6 mls/min]
Production of Dilute Urine
Filtrate osmolarity =
Plasma osmolarity
≈ 300 mOsm/L
o produce dilute urine, solute has to be
reabsorbed at a faster rate than water
Production of Dilute Urine
Proximal Tubule
Solute & water reabsorbed at same rate
No change osmolarity
Production of Dilute Urine Descending Loop
Water reabsorbed following gradient into hypertonic interstitial fluid
Osmolarity increases 2 to 4 times osmolarity of plasma
Production of Dilute Urine
Ascending Loop
Sodium, potassium, chloride reabsorbed
No water reabsorbed regardless of [ADH]
Tubular osmolarity decreases to 100 mOsm/L
1/3 osmolarity of plasma
Production of Dilute Urine
Distal Tubule & Collecting Tubules
Variable amount of water reabsorption based on [ADH]
NoADH–Nowater reabsorption
Solute reabsorption continues further decreasing tubular osmolarity
Max dilution of 50 mOsm/Liter
Excretion of Concentrated Urine Always losing water (breathing, sweat, feces, urine). Must be able to concentrate urine when water intake
is limited
Excretion of Concentrated Urine Can excrete 500 mls/day with maximum
concentration of 1200 to 1400 mOsm/Liter. High ADH concentration Reabsorb normal amounts of solute
Excretion of Concentrated Urine Increased water reabsorption in
late distal tubule and collecting ducts
Obligatory Urine Volume
Some urine has to be produced each day to excrete the waste products of metabolism and ingested ions
Volume dictated by
ability to concentrate the urine
Normal 70 kg person needs to excrete
600 mOsm/day
Sea water has salt content of
3.5%
salt molarity
58.5g/mole
osmolarity of salt water
1200 mOSM/liter
If the only water you have is sea water and you drink 1 Liter of sea water each day you need to remove
1200 mOsm of salt PLUS 600 mOsm of waste each day
if you drink salt water you lose
500 mls of volume each day which means you quickly become dehydrated
What Is Needed To Produce
Concentrated Urine?
High concentration of ADH Increased permeability of distal tubules & collecting ducts
High osmolarity of renal medullary interstitial fluid Water reabsorption is driven by osmotic forces Interstitial osmolarity setup by the countercurrent mechanism
Interstitial fluid surrounding collecting ducts normally hyperosmotic which provides the gradient for water reabsorption
Once water leaves the distal tubule & collecting ducts it is quickly picked up by the vasa recta capillary network
Countercurrent Mechanism
Made possible by
anatomical arrangement of: Loops of Henle
Especially the loops of the juxtamedullary nephrons that go deep into the renal medulla
25% of total nephrons
Collectingducts
Carry urine down
through the renal medulla
Corresponding vasa recta capillaries
Parallel
the loops
Urine osmolarity cannot exceed
osmolarity of interstitial fluid in
renal medulla
To produce concentrated urine of 1200 mOsm/Liter the osmolarity at the bottom of the renal medulla must be at least 1200 mOsm/L
Creating A Hyperosmotic Renal Medulla
Must accumulate
solute in the medulla
Creating A Hyperosmotic Renal Medulla Once solute accumulated,
hyperosmolarity maintained by a balanced
inflow/outflow of water and solutes
Creating A Hyperosmotic Renal Medulla. factors:
Active ion transport & co-transport (Na+, K+, Cl-) out of thick portion of ascending loop into medullary interstitium
Able to create a 200 mOsm concentration gradient Thin descending limb highly permeable to water – As water is reabsorbed, osmolarity of
tubular fluid decreases until it matched osmolarity of interstitial fluid
Active transport of ions from collecting duct into medullary interstititum Facilitated diffusion of urea from inner medullary collecting ducts into
medullary interstitium More solute is reabsorbed into medullary interstitium than wate
Osmolarity of tubular fluid entering distal tubule is
low. NO water permeability in thick ascending segment
Minimal water permeability in late distal tubule
Collecting duct water permeability depends on
ADH conc.
HIGH ADH
IGH ADH Large quantity of water reabsorbed by
cortical collecting duct
Reabsorbed water carried away by peritubular capillaries
Medullary collecting duct highly permeable to water but only small percentage of water is left
Since amount of water relatively small, water permeability is high, and vasa recta able to carry water away, osmolarity inside collecting duct quickly equilibrates with interstitial osmolarity
Affects of Urea on Medullary Osmolarity
Urea accounts for
40 to 50% of total osmolarity of inner renal medulla
urea load normally excreted
50%
excretion rate of urea depends on
Plasmaconcentration GFR
Proximal Tubule urea absorption
50%
Urea concentration increases as
larger percentage of water is reabsorbed
Affects of Urea on Medullary Osmolarity
Thin Loop Segments
Descending – more water is reabsorbed
Descending & ascending – secretion of urea into tubule so urea concentration continues to increase slightly
Facilitated by urea transported UT-A2
Affects of Urea on Medullary OsmolarityThick Ascending Loop, Distal Tubule, Cortical and Outer Medullary Collecting Duct
Urea not permeable
In collecting duct urea concentration rises quickly as large volume of water is reabsorbed
Affects of Urea on Medullary Osmolarity
Inner Medullary Collecting Du
Urea permeability increases so urea will diffuse out of duct into interstitial space
Facilitated by urea transporters UT-A1 and UT-A3
UT-A3 activated by ADH Water is still being reabsorbed so duct
concentration of urea remains high
Some of the urea is secreted back into the thin segments of the loop of Henle
Recirculation of urea (from collecting duct back into the loop of Henle) works to increase concentration of urea in the urine and inner medullary interstitium
Vasa Recta & Urine Concentration
Blood flow to renal medulla needed for
metabolic needs of tissue
Vasa Recta & Urine Concentration How meet metabolic needs without washing out concentrated solute???
Medullary blood flow very
low (5% of total renal flow)
Vasa recta function as countercurrent exchangers
Characteristics of Vasa Recta
Start at cortical-medullary boundary Descend all way through medulla parallel to
medullary loops of Henle
Highly permeable to solute (except protein)
as vasa recta descend through medulla
exposed to ever increasing solute concentration of interstitium
Water follows concentration gradient from blood to interstitium
Solute follows concentration gradient from interstitium to blood
as vasa recta ascend through medulla
now exposed to decreasing interstitial solute concentration
Water now follows gradient into blood Solute follows gradient out of blood
Characteristics of Vasa Recta Carry away the amount of solute and water
absorbed FROM the medullary tubules
increasing the blood flow through the vasa recta will
washout” solute thus reducing the overall solute concentration in the renal medulla
what increases BF through vasa recta
Somevasodilators
Largeincreasesinarterialblood pressure
Flow through renal medulla affected more than flow through other areas of kidney
Affect of Vasa Recta Blood Flow Rate. Decreased medullary osmolarity
means less reabsorption of water more urine output
Proximal tubule
65% of filtered electrolytes are reabsorbed along with proportional amount of water
Filtrate flow goes from
125 mls/minute to 44 mls/minute
Descending Loop tubular flow
25 mls/minute tubular flow. High permeability to water
Low permeability to sodium, chloride, urea
Tubular osmolarity matched interstitial osmolarity
Low levels of ADH
Urea absorption from collecting duct reduced so interstitial osmolarity also reduced
Thin Ascending Loop
No water permeability Some reabsorption of sodium, chloride Some diffusion of urea into tubule Net result – decrease in osmolarity No change in tubular flow (25 mls/minute) Changes in Osmolarity Through Nephron
Thick Ascending Loop
No water permeability
Active reabsorption of sodium, chloride, potassium
Largeamount reabsorbed
Tubular osmolarity continues to decrease
100 to 200 mOsm/L
No change in tubular flow (25 mls/minute)
Changes in Osmolarity Through Nephron
Early Distal Tubule
Diluting segment
No water permeability
Active reabsorption of sodium, chloride, potassium
Largeamount reabsorbed
Tubular osmolarity continues to decrease
50 mOsm/L
No change in tubular flow (25 mls/minute)
Changes in Osmolarity Through Nephron
Late Distal Tubule / Cortical Collecting Tubules
Osmolarity based on level of ADH
Urea permeability low so total urea load at this point does not change until medullary collecting ducts
LOW: Minimal water reabsorption and further decrease in osmolarity (ions still being reabsorbed)
Tubular flow still around 25 mls/minute
HIGH: High water reabsorption so osmolarity increases
Tubular flow drops to 8 mls/minute
Changes in Osmolarity Through Nephron
Medullary Collecting Tubules
Osmolarity depends on [ADH] and interstitial osmolarity
HIGH [ADH]: High water permeability / reabsorption – Solute concentration increases (especially of urea)
Tubular flow drops to 0.2 mls/minute
LOW [ADH]: Low water permeability – Solute concentration drops as urea is reabsorbed
Slight decrease in tubular flow to 20 mls/minute
Increased flow through vasa recta decreases overall solute concentration of interstitial fluid which decreases water reabsorption
Not able to concentrate urine to as high a level or reabsorb as much water
Kidneys can produce concentrated urine that contains little sodium or chloride even though under normal conditions
make up 50 to 60% of interstitial solute at max concentration. Osmolarity of other solutes increase (urea) Dehydration / low sodium intake – stimulate release of angiotensin
II and aldosterone
Kidneys can produce large quantities of dilute urine without changing
sodium excretion
Changing [ADH] which changes water reabsorption in later segments of nephron without changing sodium reabsorption
Obligatory urine volume dictated by
max ability to concentrate the urine
