Lecture 15: TBL Free Water

Question 1

How do you determine osmotic load?

Answer 1

Intake of electrolytes + intake of protein = osmotic load

Question 2

How do you determine minimum urine volume needed to excrete the osmotic load?

Answer 2

Min. urine volume (V dot) = Osm load / Max U(osm)

Question 3

What is the obligatory water loss?

Answer 3

Minimum urine volume needed to excrete the osmotic load

Question 4

Define free water clearance and electrolyte-free clearance

Answer 4

Free water clearance =
Filtered water in urine clear of solutes

Electrolyte-free clearance =
Water contains solutes but cleared of electrolytes

Question 5

What components make up free water clearance?

Answer 5

1) iso-osmotic component

2) distilled water “free water” component

Question 6

What equation is used to calculate urine flow for free water clearance?

Answer 6

V dot =

C(osm) + C(H2O)

Question 7

Reabsorption of sodium in which segments is important for generating distilled “free” water component of urine?

Answer 7

Thick AL

DCT

CNT/CD in absence of ADH

Question 8

How do you calculate iso-osmotic component, C(osm)?

Answer 8

C(osm) =

U(osm) * Vdot / P(osm)

Question 9

How do you calculate free water component, C(H2O)?

Answer 9

C(H2O) =

Vdot - C(osm)

Question 10

What does a negative free water clearance mean?

Answer 10

Water that was reabsorbed/returned to the body instead of excreted

Question 11

Reabsorption of water in which segment of the nephron is important for returning distilled water to the body?

Answer 11

CD + ADH

Question 12

For electrolyte-free water clearance, urine flow is divided into what two components?

Answer 12

1) iso-electrolyte component

2) electrolyte-free component

Question 13

What is the equation for urine from electrolyte free water clearance?

Answer 13

Vdot = C(salts) + C(EFW)

Question 14

What is the equation for C(salts)?

Answer 14

C(salts) =

U(Na + K) * Vdot / P(Na)

Question 15

If C(EFW) is positive, what is the impact of urinary excretion on plasma sodium?

Answer 15

Positive C(EFW) = inc. P(Na)

Question 16

If C(EFW) is negative, what impact does urinary excretion have on plasma sodium?

Answer 16

Dec. plasma sodium

Question 17

If C(EFW) = 0, what is happening to plasma sodium?

Answer 17

Stable

Question 18

What is ECF?

Answer 18

Volume of fluid that is NOT in cells

Ex: plasma and ISF

Question 19

What factors can increase ADH secretion?

Answer 19

Dec. stretch of arterial BRs and veno-atrial mechanoreceptors

Inc. plasma osmolality

Question 20

What are aquaretics prescribed for?

Answer 20

Hypervolemic hyponatremia

Inappropriate fluid retention due to dec. ECV = hyponatremia

Question 21

What are causes of hypervolemic hyponatremia?

Answer 21

HF
Cirrhosis

(Dissociation of ECF and ECV)

Question 22

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

Answer 22

U(Na+K)/P(Na) ratio:

If <1 = P(Na) is inc.
If >1 = P(Na) is dec.
If =1 = P(Na) is stable