Urine Concentration

Question 1

Maintaining Normal Cellular Environment Extracellular fluid must have a constant

Answer 1

concentration of electrolytes and other solutes

Question 2

Maintaining Normal Cellular Environment Solute concentration & osmolarity determined by:

Answer 2

Total amount of solute / Volume of extracellular fluid

Question 3

Maintaining Normal Cellular Environment Changing extracellular water has significant effect on

Answer 3

solute concentration and osmolarity

Question 4

Maintaining Normal Cellular Environment body water determined by

Answer 4

Fluid intake (controlled by thirst)
 Renal excretion of water (controlled by changing GFR and tubular reabsorption

Question 5

If ECF solute concentration increases, kidneys

Answer 5

hold onto

water so ECF volume increases diluting ECF solutes

Question 6

If ECF solute concentration decreases kidneys

Answer 6

excrete more water so ECF volume decreases concentrating ECF solutes

Question 7

Assuming normal solute intake and metabolic production

Answer 7

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

Question 8

Increased ECF [solute] (i.e. increased ECF osmolarity)

Answer 8

 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

Question 9

Decreased ECF [solute] (i.e. decreased ECF osmolarity)

Answer 9

 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

Question 10

Posterior pituitary responds to changes in ECF osmolarity by changing ADH release. what effects ADH release?

Answer 10

Increased ECF osmolarity results in an increased release of ADH
 Decreased ECF osmolarity results in a decreased release of ADH

Question 11

Quantity of water excreted controlled

Answer 11

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

Question 12

Changes in water reabsorption control

Answer 12

urine volume and urine solute concentration.

Question 13

Increased water reabsorption means

Answer 13

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

Question 14

Decreased water reabsorption means

Answer 14

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

Question 15

Excretion of Dilute Urine

 Can excrete 20 liters/day with

Answer 15

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

Question 16

Water Diuresis process drink 1 liter of water…

Answer 16

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]

Question 17

Production of Dilute Urine

 Filtrate osmolarity =

Answer 17

Plasma osmolarity

 ≈ 300 mOsm/L

Question 18

o produce dilute urine, solute has to be

Answer 18

reabsorbed at a faster rate than water

Question 19

Production of Dilute Urine

Proximal Tubule

Answer 19

Solute & water reabsorbed at same rate

 No change osmolarity

Question 20

Production of Dilute Urine Descending Loop

Answer 20

Water reabsorbed following gradient into hypertonic interstitial fluid
 Osmolarity increases 2 to 4 times osmolarity of plasma

Question 21

Production of Dilute Urine

Ascending Loop

Answer 21

 Sodium, potassium, chloride reabsorbed
 No water reabsorbed regardless of [ADH]
 Tubular osmolarity decreases to 100 mOsm/L
 1/3 osmolarity of plasma

Question 22

Production of Dilute Urine

Distal Tubule & Collecting Tubules

Answer 22

 Variable amount of water reabsorption based on [ADH]
 NoADH–Nowater reabsorption
 Solute reabsorption continues further decreasing tubular osmolarity
 Max dilution of 50 mOsm/Liter

Question 23

Excretion of Concentrated Urine Always losing water (breathing, sweat, feces, urine). Must be able to concentrate urine when water intake

Answer 23

is limited

Question 24

Excretion of Concentrated Urine Can excrete 500 mls/day with maximum

Answer 24

concentration of 1200 to 1400 mOsm/Liter. High ADH concentration Reabsorb normal amounts of solute

Question 25

Excretion of Concentrated Urine Increased water reabsorption in

Answer 25

late distal tubule and collecting ducts

Question 26

Obligatory Urine Volume  Some urine has to be produced each day to excrete the waste products of metabolism and ingested ions  Volume dictated by

Answer 26

ability to concentrate the urine

Question 27

 Normal 70 kg person needs to excrete

Answer 27

600 mOsm/day

Question 28

Sea water has salt content of

Answer 28

3.5%

Question 29

salt molarity

Answer 29

58.5g/mole

Question 30

osmolarity of salt water

Answer 30

1200 mOSM/liter

Question 31

If the only water you have is sea water and you drink 1 Liter of sea water each day you need to remove

Answer 31

1200 mOsm of salt PLUS 600 mOsm of waste each day

Question 32

if you drink salt water you lose

Answer 32

500 mls of volume each day which means you quickly become dehydrated

Question 33

What Is Needed To Produce | Concentrated Urine?

Answer 33

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

Question 34

Countercurrent Mechanism Made possible by

Answer 34

anatomical arrangement of:  Loops of Henle  Especially the loops of the juxtamedullary nephrons that go deep into the renal medulla  25% of total nephrons

Question 35

Collectingducts |  Carry urine down

Answer 35

through the renal medulla

Question 36

Corresponding vasa recta capillaries |  Parallel

Answer 36

the loops

Question 37

Urine osmolarity cannot exceed

Answer 37

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

Question 38

Creating A Hyperosmotic Renal Medulla |  Must accumulate

Answer 38

solute in the medulla

Question 39

Creating A Hyperosmotic Renal Medulla Once solute accumulated,

Answer 39

hyperosmolarity maintained by a balanced | inflow/outflow of water and solutes

Question 40

Creating A Hyperosmotic Renal Medulla. factors:

Answer 40

 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

Question 41

Osmolarity of tubular fluid entering distal tubule is

Answer 41

low. NO water permeability in thick ascending segment |  Minimal water permeability in late distal tubule

Question 42

Collecting duct water permeability depends on

Answer 42

ADH conc.

Question 43

HIGH ADH

Answer 43

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

Question 44

Affects of Urea on Medullary Osmolarity |  Urea accounts for

Answer 44

40 to 50% of total osmolarity of inner renal medulla

Question 45

urea load normally excreted

Answer 45

50%

Question 46

excretion rate of urea depends on

Answer 46

 Plasmaconcentration  GFR

Question 47

Proximal Tubule urea absorption

Answer 47

50%

Question 48

Urea concentration increases as

Answer 48

larger percentage of water is reabsorbed

Question 49

Affects of Urea on Medullary Osmolarity | Thin Loop Segments

Answer 49

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

Question 50

Affects of Urea on Medullary OsmolarityThick Ascending Loop, Distal Tubule, Cortical and Outer Medullary Collecting Duct

Answer 50

 Urea not permeable |  In collecting duct urea concentration rises quickly as large volume of water is reabsorbed

Question 51

Affects of Urea on Medullary Osmolarity | Inner Medullary Collecting Du

Answer 51

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

Question 52

Vasa Recta & Urine Concentration |  Blood flow to renal medulla needed for

Answer 52

metabolic needs of tissue

Question 53

Vasa Recta & Urine Concentration  How meet metabolic needs without washing out concentrated solute???

Answer 53

 Medullary blood flow very low (5% of total renal flow)  Vasa recta function as countercurrent exchangers

Question 54

Characteristics of Vasa Recta

Answer 54

 Start at cortical-medullary boundary  Descend all way through medulla parallel to medullary loops of Henle  Highly permeable to solute (except protein)

Question 55

as vasa recta descend through medulla

Answer 55

exposed to ever increasing solute concentration of interstitium  Water follows concentration gradient from blood to interstitium  Solute follows concentration gradient from interstitium to blood

Question 56

as vasa recta ascend through medulla

Answer 56

now exposed to decreasing interstitial solute concentration |  Water now follows gradient into blood  Solute follows gradient out of blood

Question 57

Characteristics of Vasa Recta Carry away the amount of solute and water

Answer 57

absorbed FROM the medullary tubules

Question 58

increasing the blood flow through the vasa recta will

Answer 58

washout” solute thus reducing the overall solute concentration in the renal medulla

Question 59

what increases BF through vasa recta

Answer 59

 Somevasodilators  Largeincreasesinarterialblood pressure  Flow through renal medulla affected more than flow through other areas of kidney

Question 60

Affect of Vasa Recta Blood Flow Rate. Decreased medullary osmolarity

Answer 60

means less reabsorption of water more urine output

Question 61

Proximal tubule  65% of filtered electrolytes are reabsorbed along with proportional amount of water  Filtrate flow goes from

Answer 61

125 mls/minute to 44 mls/minute

Question 62

Descending Loop tubular flow

Answer 62

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

Question 63

Thin Ascending Loop

Answer 63

``` 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 ```

Question 64

Thick Ascending Loop

Answer 64

 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)

Question 65

Changes in Osmolarity Through Nephron | Early Distal Tubule

Answer 65

 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)

Question 66

Changes in Osmolarity Through Nephron | Late Distal Tubule / Cortical Collecting Tubules

Answer 66

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

Question 67

Changes in Osmolarity Through Nephron | Medullary Collecting Tubules

Answer 67

 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

Question 68

Kidneys can produce concentrated urine that contains little sodium or chloride even though under normal conditions

Answer 68

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

Question 69

Kidneys can produce large quantities of dilute urine without changing

Answer 69

sodium excretion | Changing [ADH] which changes water reabsorption in later segments of nephron without changing sodium reabsorption

Question 70

Obligatory urine volume dictated by

Answer 70

max ability to concentrate the urine