Fluid Balance

Objective:

To explain in short essays or diagrams the control of fluid and electrolyte balance following water and/or salt consumption, at the level of 85% proficiency for each student.

 

In order to achieve this objective, you will need to be able to:

  1. Measure urine volume and composition response to various water and salt intake.

Materials:

Group Supplies:

2 - 500 mL of bottled water

2 - packages of pretzels weighed for 1 gm Na (use pretzels with a high sodium content)

5 - paper cups for urine collection

1 paper cup for drinking solutions

2 - 4 test tubes

20% potassium chromate solution (in dropper bottle)

2.9% silver nitrate (in dropper bottle)

Lab Supplies

[Vapor Pressure Osmometer optional]

[Pipetter with 10 micoliter pipette tips optional]

100 mL Graduated cylinders

Orange Biological waste disposal bag

 

Methods:

Students should work in groups of four. Each student in the group should be the subject for one of four conditions of water and salt ingestion.

Initially, each student voids, emptying the bladder as completely as possible. The urine is saved, labeled, and used for the control.

 

The conditions of water and salt ingestion are:

  1. Drink 1000 mL of water as rapidly as comfortable.
  2. Drink 1000 mL of water and eat pretzels (weighed to contain 1 gm of Na) as rapidly as comfortable.
  3. Eat pretzels (weighed to contain 1 gm of Na) as rapidly as comfortable.

Each student voids, emptying the bladder as completely as possible every half-hour for 2 hours after obtaining the control urine. The urine is collected each time and all urines, including the control urine are analyzed. No other fluids or food should be ingested during the experimental period.

If a student is unable to void at any of the half-hour intervals, the urine is retained in the bladder until the next half-hour collection time.

Determine the following information for each sample of urine including controls

  1. volume
  2. Rate for formation mL/min.
  3. appearance
  4. specific gravity or osmolarity
  5. Sodium chloride concentration

Determination of Sodium Chloride Concentration:

  1. Measure 10 drops of urine into a test tube using a standard medicine dropper. Add 1 drop of 20% potassium chromate solution to the urine.
  2. Add 2.9% silver Nitrate solution 1 drop at a time using the dropper in the bottle. Vigorously swirl the test tube after each drop of silver nitrate added.
  3. Count the drops of silver nitrate solution required to turn the solution from a bright yellow to an orange brown, color.
  4. Each drop of 2.9% silver nitrate required to produce the color change represents approximately 1 gram/liter of NaCI.

 

Osmolarity [optional]:

Osmolarity will be measured with a Vapor-Pressure Osmometer according to instructions given in the lab. Only 10 microliters (µL) of volume is needed for each measurement.  Therefore, only a few drops of urine from each sample needs to be saved to measure osmolarity. Record all data on the Data Sheet.

Results

DATA SHEET: Effects of Water & Salt Load on Kidney Function

Solution Imbibed

Time

Volume Produced

mL/min

Appearance

Osmolarity

NaCl
conc.

Comments

1000 mL H2O

0

 

 

 

 

 

 

½ hr

 

 

 

 

 

 

1 hr

 

 

 

 

 

 

1 ½ hr

 

 

 

 

 

 

2 hr

 

 

 

 

 

 

1000 mL H2O
and
Pretzels 1 gm Na

0

 

 

 

 

 

 

½ hr

 

 

 

 

 

 

1 hr

 

 

 

 

 

 

1 ½ hr

 

 

 

 

 

 

2 hr

 

 

 

 

 

 

Pretzels 1 gm Na

0

 

 

 

 

 

 

½ hr

 

 

 

 

 

 

1 hr

 

 

 

 

 

 

1 ½ hr

 

 

 

 

 

 

2 hr

 

 

 

 

 

 

Nothing

0

 

 

 

 

 

 

½ hr

 

 

 

 

 

 

1 hr

 

 

 

 

 

 

1 ½ hr

 

 

 

 

 

 

2 hr

 

 

 

 

 

 

 

 

Discussion:

  1. Explain how the osmotic gradient of the renal medulla is created by the transport mechanisms of the renal tubules and loop of Henle.
  2. What is the function of ADH? Outline a cybernetic system that causes a concentrated or dilute urine to be formed.
  3. What is the function of aldosterone? Outline a cybernetic system that causes natriuresis or sodium retention.
  4. What factors determine the amount of urine produced? How is the effect of each brought about?
  5. According to your group data how does NaCl, water and coffee intake influence urine production?

 

© David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006