Lecture 15: TBL Free Water Flashcards
How do you determine osmotic load?
Intake of electrolytes + intake of protein = osmotic load
How do you determine minimum urine volume needed to excrete the osmotic load?
Min. urine volume (V dot) = Osm load / Max U(osm)
What is the obligatory water loss?
Minimum urine volume needed to excrete the osmotic load
Define free water clearance and electrolyte-free clearance
Free water clearance =
Filtered water in urine clear of solutes
Electrolyte-free clearance =
Water contains solutes but cleared of electrolytes
What components make up free water clearance?
1) iso-osmotic component
2) distilled water “free water” component
What equation is used to calculate urine flow for free water clearance?
V dot =
C(osm) + C(H2O)
Reabsorption of sodium in which segments is important for generating distilled “free” water component of urine?
Thick AL
DCT
CNT/CD in absence of ADH
How do you calculate iso-osmotic component, C(osm)?
C(osm) =
U(osm) * Vdot / P(osm)
How do you calculate free water component, C(H2O)?
C(H2O) =
Vdot - C(osm)
What does a negative free water clearance mean?
Water that was reabsorbed/returned to the body instead of excreted
Reabsorption of water in which segment of the nephron is important for returning distilled water to the body?
CD + ADH
For electrolyte-free water clearance, urine flow is divided into what two components?
1) iso-electrolyte component
2) electrolyte-free component
What is the equation for urine from electrolyte free water clearance?
Vdot = C(salts) + C(EFW)
What is the equation for C(salts)?
C(salts) =
U(Na + K) * Vdot / P(Na)
If C(EFW) is positive, what is the impact of urinary excretion on plasma sodium?
Positive C(EFW) = inc. P(Na)
If C(EFW) is negative, what impact does urinary excretion have on plasma sodium?
Dec. plasma sodium
If C(EFW) = 0, what is happening to plasma sodium?
Stable
What is ECF?
Volume of fluid that is NOT in cells
Ex: plasma and ISF
What factors can increase ADH secretion?
Dec. stretch of arterial BRs and veno-atrial mechanoreceptors
Inc. plasma osmolality
What are aquaretics prescribed for?
Hypervolemic hyponatremia
Inappropriate fluid retention due to dec. ECV = hyponatremia
What are causes of hypervolemic hyponatremia?
HF
Cirrhosis
(Dissociation of ECF and ECV)
Describe how the C(EFW) concept can be used to predict whether plasma sodium is increasing, decreasing or stable even if the urine flow rate (Vdot) is unknown
U(Na+K)/P(Na) ratio:
If <1 = P(Na) is inc.
If >1 = P(Na) is dec.
If =1 = P(Na) is stable
