6_HST110 Osmoregulation and Disorders of Water Balance 2017

Question 1

What percentage of the total body weight is water in healthy adults?

Answer 1

60%

Question 2

Positive water balance is when there is more (X) than (Y). Negative is the opposite

Answer 2

X = input
Y = output

Question 3

What percentage of TBW is extracellular, plasma, and arterial (effective), in order of decreasing quantities

Answer 3

Extracellular: 1/3
Plasma: 1/12
Arterial: 1/60

Question 4

Plasma and interstitial Na+ concentration is (X) than inside the cell, where (Y) dominates

Answer 4

X = higher
Y = K+

Question 5

(X) determines the distribution of body water between the different fluid compartments. Water moves from low (Y) to high (Y)

Answer 5

X = Osmotic pressure
Y = osmolality

Question 6

(X) determine movement of water between plasma and interstitial compartments across a capillary wall.

Answer 6

X = Starling forces

Question 7

Changes in plasma osmolality or plasma (X) reflect changes in water balance

Answer 7

X = [Na+]

Question 8

(X) of all fluid compartments is essentially equal, so we can use plasma (X) to estimate (X) of the total body water

This is the parameter that is sensed and tightly regulated to maintain water balance

What is its normal range?

Answer 8

X = Osmolality

Normal range is 275-290 mOsm/kg

Question 9

What is the expression for plasma osmolality (mOsm/kg)

Answer 9

P_osm ~ 2* Na+ (major component)

Question 10

What is the human form of ADH?

Answer 10

Arginine Vasopressin (AVP)

Question 11

ADH is produced by neuroendocrine cells in supraoptic and paraventricular nuclei of the (X). It is packaged in granules, transported down axons, and stored in nerve terminals located in the (Y). Acts on collecting ducts to regulate (Z)

Answer 11

X = hypothalamus
Y = posterior pituitary
Z = water reabsorption

Question 12

The absence or presence of (X) is the major physiologic determinant of urinary water excretion or retention

Answer 12

X = ADH

Question 13

What are the actions of ADH in the kidney (V2 receptors)

Answer 13

Increases permeability of the collecting ducts to water (AQP)

Increases permeability of the medullary collecting ducts to urea

Stimulates reabsorption of NaCl by the thick ascending limb, distal tubule, and collecting duct

Question 14

What are the actions of ADH on the vasculature (V1 receptors)

Answer 14

Vasoconstriction

Question 15

What are the 2 major stilmuli that regulates ADH release?

Answer 15

Plasma osmolality (Changes in plasma osmolality play the MOST important role in regulating ADH secretion)

Volume depletion

Question 16

Osmolality is sensed by (X) in the anterior hypothalamus, which shrink or swell in response to changes in osmolality

Answer 16

X = osmoreceptors

Increase in Posm -> osmoreceptors shrink -> stimulates ADH secretion

opposite effect for decrease in Posm

Question 17

Volume is sensed by baroreceptors
Carotid sinus, aortic arch, atria, afferent arteriole.

Over the physiologic range of effective circulating volume, ADH levels are determined almost entirely by plasma osmolality

However, volume depletion of (X) or greater significantly influences ADH levels (shifts ADH-osmolality curve to the left, resetting osmostat)

Answer 17

X = 10%

Question 18

Name a few other stimuli for ADH release

Answer 18

Nausea
Stress (pain, emotion, exercise)
Drinking
Acute hypoglycemia (>20% decrease)
Drugs (Nicotine, Narcotics, Cyclophosphamide, Tricyclic antidepressants, Phenothiazine antipsychotic agents

Question 19

Increase in plasma osmolality or decrease in volume stimulates (X). Osmotic threshold for triggering (X) is higher than that for ADH secretion (295 vs 285 mOsm/kg)

Answer 19

X = thirst

Question 20

The differences between descending and ascending limbs of the (X) are essential to the process of concentrating or diluting the urine to conserve or remove water

Answer 20

X = LOH

Question 21

Countercurrent Exchange occurs primarily in the juxtamedullary nephrons with (X) loops of Henle

Answer 21

X = long

Question 22

What is the goal of the countercurrent exchange mechanism?

Answer 22

To establish a hyperosmotic medullary interstitium that will promote the reabsorption of water in the collecting duct

Question 23

What are the 3 components of the countercurrent exchange mechanism?

Answer 23

Active NaCl transport by TAL (loop of Henle)

Medullary vasa recta

Urea recycling

Question 24

Countercurrent exchange mechanism: Active NaCl transport occurs at thick ascending limb of the loop of Henle. Mediated by (X) and removes solute without water, diluting tubular fluid and Establishes interstitial (Y)

Answer 24

X = Na+-K+-2Cl- symporter
Y = osmotic gradient

Question 25

At the tip of the LOH, what is the osmolality? What is this due to?

Answer 25

1200 mOsm/kg Countercurrent multiplier

Question 26

Tubular fluid leaving the ascending limb and entering the CCD is hypoosmotic to plasma (usually (X) mOsm/kg). With No ADH, dilute urine. With ADH, concentrated urine

Answer 26

X = 100

Question 27

Final osmolality and volume of the urine is determined by water permeability in the (X), not by the events in the loop of Henle

Answer 27

X = collecting ducts

Question 28

Countercurrent exchange mechanism: Descending and ascending vasa recta run near each other in parallel but in opposite directions. They return NaCl and water reabsorbed in LOH and MCD to systemic circulation What is this called?

Answer 28

Countercurrent

Question 29

Countercurrent exchange mechanism: Vasa Recta maintains the hyperosmotic interstitial gradient. Vasa recta exchanges water and solutes with surrounding interstitium and equilibrates osmotically with the surrounding interstitium. (X) permits solutes to accumulate in the medullary interstitium.

Answer 29

X = low blood flow

Question 30

Countercurrent exchange mechanism: Urea recycling. Urea is (X) filtered. (Y)% is reabsorbed in the PCT and 50% secreted into LOH. It is "trapped" in tubule from distal tubule to OMCD and then reabsorbed in (Z)

Answer 30

``` X = freely Y = 50 Z = IMCD (Intermedullary collecting duct) ```

Question 31

In the presence of ADH: Urea concentration in the collecting duct rises Permeability of IMCD to urea is (X) This favors urea reabsorption into the interstitium (via UT1) Contributes ~50% to the hyperosmostic interstitial gradient at the (Y) tip

Answer 31

``` X = increased Y = papillary ```

Question 32

Urine osmolality can range from (X) (maximally dilute) to (Y) mOsm/kg (maximally concentrated)

Answer 32

``` X = 50 Y = 1200 ```

Question 33

Urine volume can range from (X) to as low as (Y) L/day

Answer 33

``` X = 18 Y = 0.5 ```

Question 34

Disorders of water balance are detected on the basis of abnormally low or high (X) concentration in the blood. These are disorders of WATER, not sodium

Answer 34

X = Na+

Question 35

What is the formula for plasma osmolality?

Answer 35

(2*Na+ + 2*K+)/TBW

Question 36

What is the formula for plasma sodium concentration?

Answer 36

(Na+ + K+)/TBW

Question 37

What is the threshhold for hyponatremia?

Answer 37

Plasma [Na+] < 135 mEq/L (normal 136-145) Usually associated with hypoosmolality

Question 38

Theoretically hyponatremia can result from either of two situations. What are they?

Answer 38

Loss of Na+ and K+ Increase in total body water (retention of ingested or infused water)

Question 39

Nearly all causes of hyponatremia are due to excessive (X). This generally only occurs if there is a defect in renal water excretion, specifically the inability to suppress ADH secretion, which can be appropriate or inappropriate.

Answer 39

X = water retention relative to solute

Question 40

Name the 3 categories of decreased plasma osmolality (hyposmolar hyponatremia) categorized based on the patient's total body volume status

Answer 40

Hypovolemic (Diarrhea, vomiting, diuretics) Euvolemic (Adrenal insufficiency, SIADH) Hypervolemic (Congestive heart failure, cirrhosis, nephrotic syndrome)

Question 41

In the setting of volume depletion: Since most fluid losses are isoosmotic, (X) does not typically occur unless you replace those lost fluids with ingested water (hypotonic)

Answer 41

X = hyponatremia

Question 42

Treatment for volume depletion hyponatremia: Treatment involves correcting the volume depletion with (X) fluids (e.g. (X) saline) Correction of the volume deficit will also suppress the stimulus for (Y) release and prevent further water retention

Answer 42

``` X = isotonic Y = ADH ```

Question 43

In patients with SIADH (Syndrome of (X) antidiuretic hormone), there is (X), non-osmotic secretion of ADH, which stimulates renal water retention

Answer 43

X = inappropriate

Question 44

What is the treatment for SIADH?

Answer 44

Restrict free water intake to minimize further water retention If possible, remove the stimulus for ADH secretion -Stop causative medications -Treat nausea and/or pain -Treat pulmonary or CNS diseases

Question 45

What is the threshhold for hypernatremia?

Answer 45

Plasma [Na+] > 146 mEq/L (normal 136-145) Associated with hyperosmolality

Question 46

Theoretically hypernatremia can result from either of two situations. What are they?

Answer 46

Administration/ingestion of hypertonic fluids (too much sodium) Increased water losses (more likely/common)

Question 47

Normally, an increase in plasma osmolality activates ADH secretion and thirst mechanisms, which promote (X) and increase water intake Therefore, in order for hypernatremia to occur, (Y) function or thirst must be impaired

Answer 47

``` X = water retention Y = ADH ```

Question 48

Name common causes of Hypernatremia from water loss

Answer 48

Renal Loss: Diabetes insipidus (central, nephrogenic) and osmotic diuresis) Non-renal loss: Insensible losses (sweating, burns, respiratory infections), GI losses, etc.

Question 49

Name common causes of Hypernatremia from increased sodium

Answer 49

Administration of hypertonic NaCl or NaHCO3 | Ingestion of sodium

Question 50

Diabetes Insipidus: Complete or partial failure in either: ADH secretion ((X) DI) Renal response to ADH ((Y) DI) Impaired renal water reabsorption results in excretion of (Z) urine (3-20 L/day)

Answer 50

``` X = central Y = nephrogenic Z = dilute A = polyuria and polydipsia ```

Question 51

Diabetes insipidus (hypernatremia) presents with (X). Does not cause hypernatremia as long as (Y) mechanism is intact

Answer 51

``` X = polyuria and polydipsia Y = thirst ```

Question 52

What is Central Diabetes Insipidus characterized by?

Answer 52

Inability to produce or secrete ADH ``` 75% of cases are due to: Idiopathic DI Autoimmune Familial Neurosurgery Head trauma Neoplasm Infiltrative diseases Histiocytosis X Sarcoidosis ```

Question 53

What is Nephrogenic Diabetes Insipidus characterized by?

Answer 53

Impaired renal response to ADH ADH production is normal Causes: Congenital V2 receptor mutations AQP2 mutations Acquired Drugs (eg, lithium) Hypercalcemia Osmotic diuresis

Question 54

Treatment of Hypernatremia

Answer 54

Replace water losses (Oral, Intravenous (as 5% dextrose in water)) Replace both the existing water deficit and any ongoing water losses In patients with DI: - Exogenous ADH (if central DI) - Induce mild volume depletion (thiazide, low-salt diet)

Question 55

What is "Free Water?"

Answer 55

Solute-free (e.g. Na+ free) water, created when Na+ is separated from water Occurs in the diluting segments of the nephron (TAL, DCT) that are impermeable to water but carry out active Na+ reabsorption

Question 56

What is the equation for free water clearance?

Answer 56

C_H2O = V' - (U_osm/P_osm)*V'