#1 - Disorders of Extracellular Fluid Volume and Tonicity Flashcards
ADH release stimulated by
increase in tonicity
Total body water = ___% body weight
60 for men; 50 for women
Total body water distribution
2/3 intracellular, 1/3 extracellular
Of extracellular fluid, ___ is intravascular
1/4
Intravascular volume sensed via
effective arterial volume - sensed by kidney
Volume regulation senses
EABV via baroreceptors, causes change in aldosterone and catecholamine release which affect urine Na+
Osmoregulation senses
Posm via hypothalamic osmoreceptors; stimulates AVP/ADH and thirst
Physical exam indications of decreased EABV:
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
Renal retention of NaCl indicates
low EABV; calculate with FE (fractional excretion) which uses creatinine for comparison as it’s neither secreted nor reabsorbed
FE =
(Ux X Pcreat)/(Ucreat X Px)
Low is <0.5-1%, indicating low EABV
BUN and uric acid
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
Hydration/dehydration related to ______
Salt status related to ____
osmoregulation; volume regulation
Tonicity
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
Osmolality =
2 x [Na+] + (glucose/18) + (BUN/2.8)
Normal = 280-300 mOsm/L
Usually only Na and glucose really needed to calculate it.
3 ways of detecting low effective arterial volume:
1) Postural changes - incr. pulse and decrease BP
2) Na/Cl retention - FE 20) & uric acid levels (>5)
3) Increased BUN/creatinine
Osmolar gap
difference between calculated and measured osmolality. Gap > 10 indicates accumulation of unmeasured substance such as ethanol, methanol, etc.
Ascending loop of Henle
water impermeable, dilutes urine
Collecting duct
ADH-dependent H2O permeability
ADH causes it to become more water permeable and reabsorb water
ADH
=AVP=vasopressin. Synthesized in hypothalamus and stored in post. pituitary in response to increased osmolality or decreased effective arterial volume
ADH levels at very low osmolality
undetectable because not at threshold
At higher osmolality, ADH
increases linearly. Thirst threshold is higher than ADH’s.
ADH also increases when ____
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.
ADH secretion more sensitive to
increases in osmolality than decreases in BP as it takes a large decrease in blood volume to cause ADH secretion
Aquaporin 2
Phosphorylated and inserted into lumen membrane of cells of collecting duct in response to ADH