Lect 12: Regulation of Plasma Osmolarity

Question 1

Body Balance

Answer 1

gain of water = loss of water

Urine excretion is Regulated

Question 2

How does the kidney preserve normal ECF osmolarity?

Answer 2

it is achieved by the ability of the kidney to maximize (concentrate) or minimize (dilute) the osmolarity of the urine in response to increases and decreases in ECF osmolarity

Question 3

When the ratio of urine to plasma osmolarity (U/P) = 1

Answer 3

the urine is isotonic and ECF osmolarity is isotonic?

Question 4

When the ratio U/P osmo > 1

Answer 4

the urine is hyperosmotic and ECF osmolarity is also hypertonic.
==>The kidney compensates for this by excreting a urine that is much more hypertonic than the plasma

Question 5

When the ratio of U/P osmo < 1

Answer 5

the urine is hypotonic and ECF osmolarity is hypotonic? But the urine is much more hypotonic (dilute) than the plasma

Question 6

When you are dehydrated and are volume contracted with high ECF osmo, the way that the kidney corrects for this is

Answer 6

excreting a urine that is more conc than plasma (higher osmolarity) …..and vice versaa

Question 7

The formation and excretion of a hypertonic urine indicates

Answer 7

the kidney is responding to minimize plasma hyperosmolarity by retaining water in excess of solutes or “free water” which is water not osmotically obligated to remain in the tubular fluid due to the presence of solutes.

Question 8

The formation of a hypotonic urine indicates

Answer 8

the kidney is responding to minimize and correct plasma hypoosmolarity by eliminating water in excess of solutes

Question 9

increased plasma osmolarity does what to ADH?

Answer 9

it will induce an increase in ADH , which results in retention of free water by the kidney (water pulled out of the fluid and rreturned to the circ) and a hypertonic/concentrate urine.

Question 10

decreased plasma osmolarity does what to ADH

Answer 10

decreases ADH which results in the elimination of free water by the kidney and a hypotonic urine.

Question 11

The challenge of the kidney is to maintain plasma and ECF osmolarity within the normal range (280-300) is met by regulating the excretion of

Answer 11

water

Question 12

Where water intake is high the solutes were excreted in large volumes of dilute urine (600/30 = 20mOsm/L). Lots of water consumed, lots of water excreted.

Answer 12

Normally: Osmolar excretion (E) = Uosm x V
Uosm=600
V = 1.5L
So typically E= 600/1.5L/day ==400mOsm. So our typical urine osmolarity is 400mOsm.
Based on how much you consume this number can go up or down. Let’s say that you increase water intake and your Uosm reduces to 30mOsm. How much would your new osmolar volume (V) be? 600/30 = 20L/day…this is it max that your urine volume can become.

Question 13

Where water intake is low the solutes were excreted in small volumes of concentrated urine. Little water taken in, little water excreted (600/1200)

Answer 13

Normally: Osmolar excretion (E) = Uosm x V
Uosm=600
V = 1.5L
So typically E= 600/1.5L/day ==400mOsm. So our typical urine osmolarity is 400mOsm.
Based on how much you consume this number can go up or down. Let’s say that you reduce your water intake, the kidney can increase Uosm to as high as 1200mOsm. So therefore your new volume would be 1200/600 = 1.5L/day (Notice that Uosm DOES NOT CHANGE. It was the volume of urine that changed based on your water intake)

Question 14

Where water intake was high the solutes were excreted in large volume of dilute urine

Answer 14

and where water intake was low, the solute were excreted in a small volume of concentrated urine

Question 15

A deficit of water consumption concentrates and increases plasma osmolarity

Answer 15

The most effective way to minimize this increase in plasma osmolarity is to minimize the excretion of the water without a change in the excretion of solute.
==>A decrease in solute excretion (increased solute reabsorption) would increase plasma osmolarity.

Question 16

When the kidney excretes a concentrated urine and the urine osmolarity is greater than the plasma osmolarity, the kidney is taking water in excess of solutes (free water) from

Answer 16

the TF and returning it to plasma. This dilutes the plasma osmolarity and decreases hyperosmolarity, raising the volume.

So i response to a hyperosmotic plasma, the kidney has made a concentrated urine with a much higher osmolarity than plasma. It can take it to 1200mOm/L

Question 17

The formation of a hypotonic urine indicates the kidney is responding to minimize and correct plasma and ECH hypoosmolarity by (resorbing or eliminating???)

Answer 17

eliminating water in excess of solutes

Question 18

Changes in ADH affects the ECF and plasma osmolarity. An increase in osmolarity will induce an increase in ADH, which results in

Answer 18

retention of free water by the kidney and a hypertonic urine

Question 19

A decrease in osmolarity will induce a decrease in ADH, which results in an elimination of free water by the kidney

Answer 19

and a hypotonic urine. In both instances, gain or loss of free water serves to normalize ECF and plasma osmolarity

Question 20

When the kidney excretes a dilute urine and the urine osmolarity is less than the plasma osmolarity, the kidney is

Answer 20

taking water in excess of solutes FROM the plasma and adding it to the tubular fluid. So the kidney has made a more dilute urine with a lower osmolarity than plasma

Question 21

Free water clearance,

Answer 21

I understand this…it is a lot of equations so see notes p. 1319

Question 22

Effects of solute consumption on free water clearance

Answer 22

positive and negative free water clearance is limited by the magnitude of solute consumed.

Question 23

When you consume a diet low in solute, this will increase NEGATIVE free water clearance. Why?

Answer 23

low solute diet means U/P ratio is >1 because the denominator is smaller than the numerator, so overall the U/P ratio is larger.

plugging in this to Cwater = V- Cosm
makes V minus a very larger number, making the whole thing negative overall. So free water clearance is negative. Consuming a diet low in solute makes you increase negative free water clearance.

This means that the amt of water obligated to remain in the TF fluid is reduced and more water is avalaiblefor reabsorption.

Question 24

When you consume a diet high in solute, this will increase POSITIVE free water clearance. Why?

Answer 24

high solute diet means U/P ratio is <1 because the denominator is larger than the numerator, so overall the U/P ratio is smaller.

plugging in this to Cwater = V - Cosm
makes V minus a very smaller number, making the whole thing positive overall. So free water clearance is positive. Consuming a diet high in solute makes you increase positive free water clearance.

So basically the amt of free water obligated to remain in the TF is increased and less water is available for reabsorption

Question 25

When you double you consumption or excretion of solute, your urine volume doubles as well, in order to maintain the same overall osmolarity

Answer 25

So if you consume and excrete a 600mOsm of solute (as in, you are in osmolar balance, the largest volume of urine excreted will be 20L/day for a 30mOsm urine (the most dilute the urine can be). The smallest volume for this consumption/excretion is 0.5L, for a urine osmolarity that is 1200mOsm

E= Uosm x Volume

Question 26

Why doesn’t the kidney simply add solutes back into the plasma when the plasma osmolarity decreases? Why does it eliminate water instead?

Answer 26

the kidney does not corrcet a decrease in the plasma osmolarity by increasing the reabsorption of solute in excess of watter from the TF. While this would correct the decrease in osmolarity, it would have the effct of increasing plasma volume and cause isotonic fluid retention (hypervolemia), easily substititing hypervolemia for hypoosmolarity. By excreting water in excess of solute, the kidney correct hypoosmolarity without a significant change in plasma volume

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Question 27

Why doesn’t the kidney simply excrete solutes back into the plasma when the plasma osmolarity increases? Why does it add water instead?

Answer 27

the kidney does not corrcet an increase in the plasma osmolarity by secreting solute in excess of water from plasma to the TF or by decreasing solute reabsorption. While this would correct the decrease in osmolarity, it would have the effect of decreasing plasma volume and cause hypovolemia), easily substititing hypovolemia for hyperosmolarity. By absorbing water in excess of solute, the kidney correct hyperosmolarity without a significant change in plasma volume

Question 28

Anti-diuresis

Answer 28

refers to restricted water intake when the kidneys maximally concentrate and excrete a minmal volume of urine because of increased reabsorption of water in excess of solutes (water diuresis is the ingestion of water and the kidneys are pumping out the most dilute urine)

Question 29

Hypoosmotic TF is generated in the distal nephron, regardless of whether the plasma is hyperosmotic or hypoosmotic. A hypertonic meddullary interstitium is generated by surorunding the medullary nephron segments. In effect the kidney is ready and waiting for the command to excrete more or less water in excess of solutes to reverse plasma hyper or hypoosmolarity.

Answer 29

command comes from ADH which opens the door of the collecting duct allowing return of not allowing return of water from the hypoosmotic TF to the plasma

Question 30

As the TF flows from cortex to medulla through the collecting duct osmotic equilibration results in the progressive absorprtion of water, osmosis. In the presence of ADH, osmotic equilibration results in the progressive reabsorpion of water

Answer 30

the absorption of water in excess of solute concenytaes the fluid remaining in the tubule and a concentrated, hypertonic urine is excreted in antidiuresis (negative free water clearance). in the absence of ADH, no osmotic equilibration occurs and a dilute, hypotonic urine is excreted in diuresis. Need aquaporins

Question 31

High circulating levels of ADH cause vasoconsitriciton and maintain BP and blood flow when

Answer 31

blood volume is severely reduced (hemorrhagic shock)

Question 32

The proximal tubule is higher in water permeability than the collecting tubules

Answer 32

because it allows the massive reabsorption of glomerular filtrate back into plasma. In the cortex

Question 33

Water movement occurs passively from high to low water conc

Answer 33

Water channels are not required and they reside constituitively in the basolateral membrane.

Question 34

When plasma osmolarity is WNL, ADH is low

Answer 34

For a given increase in plasma osmolarity, more ADH/ AVP will be secreted when plasma volume is contracted and less ADH/AVP will be secreted when plasma volume is expanded.

Question 35

The response to correct an increased plasma osmolarity and a decreased effectve circulating volume is a decrease in free water excretion and stimulation of thirst and water consumption. The renal reabsorption of water in excess of solute as well as the increased

Answer 35

consumption of water serves to reduce plasma osmolarity and increase the effective circulating volume, in turn, reduces release of ADH from the post pit. (the opposit is true)