Regulation of Water Balance

Question 1

Function of vasa recta

Answer 1

Supply blood to the medulla.
Highly permeable.
Absorb the water and solutes released from tubular lumen.

Question 2

Urea is not reabsorbed at:

Answer 2

TAL of LOH, DCT, cortical CD

Question 3

What transporters allow for urea flow into the medullary interstitium?

Answer 3

UT-A1 and UT-A3

Question 4

Some urea can be:

Answer 4

Recycled

Question 5

What created the medullary interstitial osmotic gradient?

Answer 5

Combination of aquaporin channels and the absence of tight junctions within the thin ascending limb, which provides a pathway for water to move.

Question 6

ADH is synthesized in:

Answer 6

The supraoptic and paraventricular nuclei of the hypothalamus.

Question 7

What causes the release of ADH?

Answer 7

When the SO or PV nuclei arestimulated, nerve impulses travel down the neuron and increase its permeability to Ca. This allows ADH that was stored in vesicles to be released.

Question 8

Osmoreceptors can activate what processes? (2)

Answer 8

Release of ADH

Thirst stimulation

Question 9

Osmoreceptors are sensitive to what level of change of plasma osm?

Answer 9

1-2%

Question 10

What pathway does osmoreceptors activate first? How?

Answer 10

ADH pathway.

Question 11

Why do osmoreceptors activate the ADH pathway first?

Answer 11

In order to increase plasma conc, urine must be concentrated, causing thirst to develop later on.
This allows us to not need to search for water all day.

Question 12

Principle cells function

Answer 12

Reabsorb Na+, Cl- and H2O but secrete K+.
ADH acts on AQP-2 of this type.
W/O ADH, principle cells are mainly impermeable to water.

Question 13

Intercalated cells function

Answer 13

Reabsorb K+ and secrete H+.
Can be acted on by aldosterone to increase H+ secretion.
Important for acid-base balance.

Question 14

Steroid aldosterone acts on:

Answer 14

ENaC channels to increase the reuptake of Na+ from the tubular lumen and secretion of K+ into the lumen.

Question 15

What triggers aldoserone release?

Answer 15

Rise in plasma K+ conc.

Question 16

Where are AQP channels inserted?

Answer 16

Apical membrane of principal cells

Question 17

If AQP channels are present, what happens?

Answer 17

Water will be osmotically pulled from the CD into the surrounding interstitium and creating a more concentrated urine.

Question 18

Overhydration causes what to ADH?

Answer 18

Inhibition of ADH

Question 19

Dehydration causes what to ADH?

Answer 19

ADH released

Question 20

Central “Neurogenic” Diabetes Insipidus

Answer 20

Failure to produce ADH
Urine is very dilute (can be more than 15L/day)
Can cause dehydration

Question 21

Cause of CDI (3)

Answer 21

Head injuries
Infection
Congenital

Question 22

TTM for CDI

Answer 22

Synthetic analog of ADH (desmopressin) which acts on V2 receptors to increase water permeability in late DCT and CD.

Question 23

Nephrogenic Diabetes Insipidus

Answer 23

Kidneys do not respond to ADH.
Normal levels of ADH.
Large volumes of dilute urine formed.

Question 24

What drugs can impair the ability of the distal nephron to respond to ADH?

Answer 24

Lithium (manic-depressive d/o)

Tetracyclines

Question 25

How to dx CDI vs. NDI

Answer 25

Administer desmopressin.

If urine volume does not decrease promptly and increase in osm within 2 hrs, then it is likely NDI.

Question 26

SIADH

Answer 26

Excessive release of ADH.
Causes extensive water retention, disturbing water/electrolyte balance.
Major cause of low Na+ levels.

Question 27

DI overview (6)

Answer 27

High urinary output
Low levels of ADH
Hypernatremia
Dehydrated
Lose too much fluid
Excessive thirst

Question 28

SIADH overview (6)

Answer 28

Low urinary output
High levels of ADH
Hyponatremia
Overhydrated
Retain too much fluid
Excessive thirst

Question 29

Hyponatremia

Answer 29

Most abundant electrolyte disturbance.
Excess of water relative to solutes.
Develops as a result of ADH in the kidney.
Usually stimuli are nonosmotic (drugs, pain, nausea, etc)

Question 30

Hypernatremia

Answer 30

Deficit of water compared to solutes.
Impaired thirst (not enough water intake).

Question 31

Polyuria urine volume

Answer 31

> 2.5 L/day

Question 32

Oliguria urine volume

Answer 32

300-500 ml/day

Question 33

Anuria urine volume

Answer 33

<50 ml/day

Question 34

4 mechanisms that cause Polyuria

Answer 34

Increased intake of fluids
Increased GFR
DM
Inability of kidneys to reabsorb water in DCT (CDI, NDI, CRF)

Question 35

Water diuresis

Answer 35

Increased water excretion w/o corresponding increase in salt excretion.
Caused by increase water intake.

Question 36

Solute diuresis

Answer 36

Increased water excretion concurrent w/ increased salt excretion.
Caused by significant increase in salt present in the tubular fluid.

Question 37

Obligatory urine volume

Answer 37

Minimal amt of solutes that a completely bedridden human must excrete in a day. About 700-1000 mosm for an active individual.

Question 38

Free-water clearance

Answer 38

Rate at which solute-free water is excreted by the kidneys.
(+) excess water is being excreted.
(-) excess solutes are being excreted.

Question 39

Whenever urine osmolarity is greater than plasma osmolarity, what will happen to free water clearance?

Answer 39

It will be negative

Question 40

Uosm:Posm > 1
Uosm:Posm = 1
Uosm:Posm < 1

Answer 40

Kidneys can concentrate urine.
Iso-osmotic urine produced.
Kidneys produce diluted urine.