Fluid & Electrolyte Balance

Human Renal Physiology




The mammalian kidney plays a major role in waste excretion and in the balance of water and electrolytes. Human beings, like other osmoregulators, must maintain their cells in osmotic and ionic conditions that are within acceptable limits.Changes in interstitial fluid composition and volume are detected and corrected, mainly through feedback control mechanisms.The control of water retention by the human kidney is a well-known example of one of these feedback mechanisms.When a large volume of water is lost from the body, dehydration can occur. Unless the lost water is replaced through ingestion of water, plasma osmolarity will increase.

Changes in plasma osmolarity are sensed by osmotically- sensitive neurons located in the hypothalamus.These neurons respond to increased plasma osmolarity by increasing the rate at which they fire.The impulses are carried down the axons of these cells to terminals located in the posterior lobe of the pituitary gland.The result is the release of ADH (Antidiuretic Hormone) into the circulatory system.ADH travels through the blood and eventually finds its way to the epithelial cells that make up the walls of the kidneyís collecting ducts.As ADH levels increase in the blood, the walls of the collecting duct become more permeable to water and thus more water is drawn out of the urine.As a result, the body retains more water and concentrated urine is created. Water conserved by ADH re-enters the circulatory system through the capillary network that surrounds the entire tubular system of the kidney.†† Plasma osmolarity decreases as a result of the reabsorbed water, which dilutes solute concentrations in the blood.Eventually, the lower plasma osmolarity is sensed by the neurons in the hypothalamus and the firing rate of these cells decrease.As a result, secretion of ADH decreases and the permeability of the collecting duct to water is reduced.

Feedback regulation of blood osmolarity by action of ADH


Another feedback mechanism involves the synthesis and release of Aldosterone, a steroid hormone produced by the adrenal cortex in response to increased extracellular potassium (low extracellular Na+).Release of Aldosterone results in sodium reabsorbtion in the distal convoluted tubule (DCT) and the collecting ducts of the mammalian kidney.As sodium is reabsorbed from the kidney tubule, extracellular osmolarity increases (sodium is the main extracellular ion in human interstitial fluid).Eventually, extracellular osmolarity reaches a value that inhibits further release of Aldosterone.In the absence of Aldosterone, sodium is permitted to pass through the DCT/collecting ducts and finally is excreted in the urine.


The following lab exercise is designed to illustrate the nature of fluid and electrolyte balance in the human kidney. You will measure the output of your own urine and determine the effect of imposed conditions on urine volume and osmolarity.†††


1.   Do not drink anything but water during the 2-hour period before lab. You may drink as much water as you wish.

2.   Urinate one hour before coming to lab. Do not urinate again until the first collection at the beginning of laboratory.

3.   If you have circulatory problems, poor kidney function or have any medical condition related to diet, do not volunteer as a subject for this experiment.

Equipment Required

Drinking cup

Urine collecting cups

Test tube

Test tube rack






Exercise 1: Physical Analysis of Urine

Goal: Measure the volume and specific gravity (relative solute concentration) of a urine sample.



1.       At the start of laboratory, you will be asked to clear your bladder by urinating into a graduated specimen cup (provided by your TA).Return to lab with the cup at least half filled (you may dispose of the rest of your urine in the bathroom (toilet) after approximating the total volume of urine produced).

2.       In your laboratory notebook, record the time when this collection was made and the time of your last urination (before lab).

3.       Note the total volume of urine collected (in milliliters), the color (light yellow to amber) and transparency (transparent to cloudy).

4.       Calculate the rate of urine production (ml/min) by dividing the urine volume (ml) by the number of minutes since your last urination.

5.       Use a Refractometer to determine the specific gravity of your urine.Your TA will assist you if you do not have experience working with a Refractometer.††††

**Note:the specific gravity of water is 1.000 **

6.       Do not dispose of your urine sample until you are told to do so by your TA




Exercise 2: Chemical Analysis of Urine

Goal:Use a diagnostic tool (Chemstick) to measure for the presence of: leukocytes, red blood cells, nitrites, proteins, glucose, ketones, urobilinogen, and bilirubin.Determine urine pH.



1.       Use a pipette (same as above, unless contaminated) to fill a CLEAN, DRY test tube ĺ full with a sample of your urine.

2.       Obtain a prepared Chemstick from your TA.Do not touch the reagent section of the stick with your fingers!

3.       Completely immerse the reagent areas (colored squares) of the strip in your urine and remove immediately (the Chemstick should not remain in the urine for more than one second!).

4.       While removing the strip from the test tube, run the edge of the strip against the rim of the container to remove any excess urine.

5.       Hold the strip in a horizontal position and lay on a paper towel to avoid mixing of chemicals.

6.       Use the indicator key on the Chemstick box to measure the pH of the urine sample and to determine the presence (or absence) of: urinary leukocytes, red blood cells, nitrite, protein, glucose, ketones, urobilinogen and bilirubin.All reagent areas should be analyzed within two minutes!

7.       Record all observations in your laboratory notebook.†† Look up the MEANING/SIGNIFIGANCE of your test results (Chemstick test) using the Internet while conducting Exercise 3 (below).Record the EXPECTED/NORMAL TEST RESULTS next to your own in your laboratory notebook!!!

8.       Dispose of your urine sample and rinse your test tube.Keep the test tube for future tests.


Exercise 3: Excretion Analysis: Fluid and Electrolyte Balance

Goal:Measure urine output (total volume), specific gravity, and urine production rate as a function of time and solution/solute intake.



1.       For this exercise, the class will be divided into five treatment groups and one control group (see table below).Each individual assigned to a treatment will eat or drink as instructed by the TA.†† If you have any medical condition that prohibits you from participating in a particular treatment group, inform the lab staff.You can pick another treatment or be excused from being a test subject.






7.5 ml per kg of body weight


7.5 ml per kg of body weight

Caffeine-free Cola

7.5 ml per kg of body weight

†† Diet Cola

7.5 ml per kg of body weight

†† Potato Chips

Single serving


Drink and eat nothing





* There are approximately 2.2 lbs per kilogram*




2.       If you are assigned a solution to drink, determine the appropriate volume you should ingest using the table above.Drink the entire volume of the solution you are assigned as quickly as possible.Do not ingest any fluids or eat any food for the rest of the laboratory period.

3.       If you are assigned a substance to eat, eat as quickly as possible.Do NOT ingest any other food or fluids until the lab period comes to an end.

4.       For all treatments, record the time you finish drinking/eating.

5.       Collect urine samples at 30, 60 and 90 minutes after ingestion of the treatment.If you need to urinate more frequently, keep a record of the total volume of urine collected during each 60-minute interval.

**Use the Internet to look up the EXPECTED diagnostic test results for Exercise 2 (Chemstick Test) when you are not collecting or analyzing your data for Ex 3. Record the expected/normal results in your laboratory notebook, next to your own test results.

6.       During each collection, determine the total volume of urine produced using a graduated specimen cup (re-use from Exercise 1).Retain enough urine to determine the specific gravity in the lab.Also calculate your urine production rate (Exercise 1). Record these data in your laboratory notebook.

7.       Repeat these measurements (total urine volume, specific gravity and urine production rate) at 30-minute intervals until 90 minutes is reached.

Clean Up

1.       Wash all glassware with soap (antibacterial).Rinse repeatedly.

2.       Wash all containers and implements used, including those to be discarded.

3.       Discard all disposables (after rinsing) in the autoclave bag provided.

4.       Rinse out sink where urine was disposed.

5.       Wipe off lab bench with alcohol.

6.       You will not be permitted to leave until your TA has checked your station.

Data Analysis

1.       After you have collected your data individually, meet with all other individuals assigned to your particular treatment group.Determine the average urine volume, average specific gravity and average urine production rate for each 30-minute interval.Discard any obviously erroneous results.

2.       Place your data (averages) on the dry erase board for the rest of the class.Record the data from the other treatment groups.

3.       Make a line graph (Excel) of your groupís (average) specific gravity as a function of time (time after treatment).

4.       Make a line graph (Excel) of your groupís urine output (average total volume) as a function of time.

5.       Examine the results and graphs from other treatment groups in the class.


1.       How did the results of your Chemstick Test compare to the normal/expected values you located on the Internet?Were any of your test results abnormal?If so, give a logical explanation as to why (for ex: women who are menstruating may test positive for red blood cells in their urine).


2.       Name the treatment to which you were assigned in Exercise 3.Describe ALL ingredients (within the treatment) that may have influenced your results (Ex: Na+, glucose, caffeine).Explain how the intake of EACH ingredient (describe each separately!) should influence urine output (volume) and osmolarity.


3.       Did your groupís urine production rate change over time? Why or why not? Explain!


4.       Did your group experience a change in specific gravity over time?If so, in which direction?Explain the results of your group with respect to the particular treatment you were assigned to (does the change in specific gravity fit with what you would predict given your particular treatment?).


5.       Which treatment group had the highest average specific gravity? Is this what you would expect?Why or why not?


6.       Which group had the lowest average specific gravity? Why?


7.       During the experiment, which group(s) probably had the highest concentration of ADH in their bodies?Which had the least?Explain your answers.











Biology 305 Laboratory




Observation††††† †††† Normal Value††††††† Student Urine Specimen

Color††††††††††††††††††††††††††††† Pale Yellow†††††††††††† ___________________



Transparency††††††††††††††† Transparent††††††††††††††† †† ______________________




Odor†††††††††††††††††††††††††††††† Characteristic††††††††††††† ______________________




Volume††††††††††††††††††††††††† Depends†††††††††††††††††††† ______________________




Specific Gravity††††††††††† 1.001 Ė1.030†††††††††††††† ______________________



















Organic††††††††††††††††††††††††† Results†††††††††††††† †††††Expected Value††††††††††††††††††††††††

††††††††† Component††††††††††††††††††††††††††††††††††††††††††††† ††(Internet Search)



†† WBCs




††† Nitrite




††† Glucose



†† Albumin (protein)




††† Ketone




†† Urobilinogen




††† Bilirubin




††† RBCs††††††††††














Treatment Group (Solution Ingested):†††††††††


†† Name:


†††† Student

††††† Initials††

Volume Excreted 30 min


60 min


90 min

Specific Gravity 30 min


60 min


90 min


































































†††† Average