#1 - Disorders of Extracellular Fluid Volume and Tonicity

Question 1

ADH release stimulated by

Answer 1

increase in tonicity

Question 2

Total body water = ___% body weight

Answer 2

60 for men; 50 for women

Question 3

Total body water distribution

Answer 3

2/3 intracellular, 1/3 extracellular

Question 4

Of extracellular fluid, ___ is intravascular

Answer 4

1/4

Question 5

Intravascular volume sensed via

Answer 5

effective arterial volume - sensed by kidney

Question 6

Volume regulation senses

Answer 6

EABV via baroreceptors, causes change in aldosterone and catecholamine release which affect urine Na+

Question 7

Osmoregulation senses

Answer 7

Posm via hypothalamic osmoreceptors; stimulates AVP/ADH and thirst

Question 8

Physical exam indications of decreased EABV:

Answer 8

increases in pulse, decreases in BP, “orthostatic hypotension” (a “tilt”), especially decreased diastolic pressure when standing
Most sensitive finding is BPM increase of 8/min or more

Question 9

Renal retention of NaCl indicates

Answer 9

low EABV; calculate with FE (fractional excretion) which uses creatinine for comparison as it’s neither secreted nor reabsorbed

Question 10

FE =

Answer 10

(Ux X Pcreat)/(Ucreat X Px)

Low is <0.5-1%, indicating low EABV

Question 11

BUN and uric acid

Answer 11

blood urea nitrogen - increased in low EABV, as is uric acid (>20 and >5 respectively); protein metabolism or purine metab. can affect them
BUN/creatinine ratio >20 suggests low EABV

Question 12

Hydration/dehydration related to ______

Salt status related to ____

Answer 12

osmoregulation; volume regulation

Question 13

Tonicity

Answer 13

Effective osmolality - only counts particles that do not penetrate cell membranes such as Na, glucose.
Ineffective osmoles include urea, ethanol, ethylene glycol, acetone. These don’t affect fluid distribution

Question 14

Osmolality =

Answer 14

2 x [Na+] + (glucose/18) + (BUN/2.8)
Normal = 280-300 mOsm/L
Usually only Na and glucose really needed to calculate it.

Question 15

3 ways of detecting low effective arterial volume:

Answer 15

1) Postural changes - incr. pulse and decrease BP
2) Na/Cl retention - FE 20) & uric acid levels (>5)
3) Increased BUN/creatinine

Question 16

Osmolar gap

Answer 16

difference between calculated and measured osmolality. Gap > 10 indicates accumulation of unmeasured substance such as ethanol, methanol, etc.

Question 17

Ascending loop of Henle

Answer 17

water impermeable, dilutes urine

Question 18

Collecting duct

Answer 18

ADH-dependent H2O permeability

ADH causes it to become more water permeable and reabsorb water

Question 19

ADH

Answer 19

=AVP=vasopressin. Synthesized in hypothalamus and stored in post. pituitary in response to increased osmolality or decreased effective arterial volume

Question 20

ADH levels at very low osmolality

Answer 20

undetectable because not at threshold

Question 21

At higher osmolality, ADH

Answer 21

increases linearly. Thirst threshold is higher than ADH’s.

Question 22

ADH also increases when ____

Answer 22

effective arterial volume decreased. This is inefficient because only 1/3 of retained water will be in ECF so only used in emergency states such as hemorrhage with >10% volume loss so it’s a backup system. ADH also causes release of vWF and pro-coagulation.

Question 23

ADH secretion more sensitive to

Answer 23

increases in osmolality than decreases in BP as it takes a large decrease in blood volume to cause ADH secretion

Question 24

Aquaporin 2

Answer 24

Phosphorylated and inserted into lumen membrane of cells of collecting duct in response to ADH

Question 25

Marker for hypotonicity is

Answer 25

hyponatremia

Question 26

Pseudohyponatremia

Answer 26

Lab artifact due to increases in TGs or protein which makes Na appear low when it’s actually normal

Question 27

Polydipsia as cause of hypotonicity

Answer 27

Drinking more water (20+ L/day) than kidney can excrete, usually due to CNS problem or psychosis
- urinary osmolality appropriately dilute, less than 100 mOsm/L

Question 28

Hyponatremia caused by renal defect shows

Answer 28

urinary osmolality greater than 100 mOsm/L

ADH secretion may be appropriate due to decreased EABV in pts. with low volume or edematous disorders

Question 29

SIADH

Answer 29

Syndrome of inappropriate ADH - EABV normal, serum hyponatremia, concentrated urine and high ADH
- usually associated with CNS problems such as hypothalamus or carcinomas secreting ADH, certain drugs, glucocort. deficiency

Question 30

Primary defense against hypernatremia is

Answer 30

thirst

Question 31

Hypertonic challenges leading to hypernatremia

Answer 31

• decreased volume from renal loss or extra renal loss (skin burns, vomiting, diarrhea)
• isovolemic DI - renal water loss
• Increased volume from iatrogenic administration of hypertonic solution

Question 32

Enhanced renal water loss presents with

Answer 32

polyuria and polydipsia. Can be DI, osmotic diuresis

Question 33

Diabetes insipidus (DI)

Answer 33

inappropriate water loss by kidney in absence of osmotic diuresis

Question 34

Central DI

Answer 34

ADH secretion impaired; urine concentration improves with ADH administration. Fluid restriction may help if not complete central DI.

Question 35

Nephrogenic DI

Answer 35

ADH resistance due to urinary obstruction, hypokalemia, hypercalcemia, amyloidosis, sickle cell, X-linked disorder.
- urine concentration does not improve with fluid restriction nor ADH admin.

Question 36

Hyperglycemia can cause increased

Answer 36

serum osmolality despite low Na. Glucose draws water into ECF in presence of somatostatin especially, diluting Na. Increased tonicity stim. thirst and AVP secretion.

Question 37

IV gamma globulin can cause

Answer 37

hyponatremia and hyperkalemia as it often contains much sucrose which draws water and K+ from cells in to ECF

Question 38

Principal danger of correcting hypo- or hypernatremia is

Answer 38

effects on CNS function due to brain size changes

If acute change, can change fast, if chronic need to change more gradually

Question 39

Hyponatremia leads to ___ in CNS

Answer 39

swelling, cerebral edema if faster onset.
Must be treated accordingly as sudden return to increased ECF osmolality can cause cell shrinkage and Central pontine myelinolysis
- Don’t want to correct too quickly. May use furosemide to excrete some Na in urine and excrete water, with hypertonic saline.
- Fluid restriction will correct it

Question 40

Hypernatremia causes brain

Answer 40

shrinkage initially. Correcting too quickly can cause cerebral edema with cell swelling as the cells have compensated by increasing their osmolality
- Should do half correction in 24 hours to avoid this