KIDNEY FUNCTION: URINE ANALYSIS AND REGULATION
OF OSMOLARITY

INTRODUCTION

The kidneys are important regulators of homeostasis in the body. They regulate ions and pH as well as water. In addition, kidneys also serve as the principle organ for the elimination of metabolic waste products. The functional unit of the kidney is the nephron and each kidney contains approximately two million of these structures. The nephron is divided into two regions: the glomerulus and the renal tubule. The heart pumps 7000 l/day (32 55 gal drums), and almost1/4 (8 55 gal drums) goes through kidneys. Via the glomerulus to the lumen of Bowman's capsule there is 180 l/day of filtrate (primitive urine) that passes on to the renal tubule. Finally, there is 1 l urine/day.

How is this volume reduced? The primitive urine is similar to the blood's plasma in composition except that the large molecules(>70,000 MW), such as plasma proteins, are excluded. As the filtrate passes through the renaltubule most of the water is reabsorbed and many of the essential substances are actively orpassively reabsorbed into the blood stream. Toxic byproducts of metabolism and substances inexcess are retained in the filtrate or are actively secreted into the filtrate and finally excreted asdefinitive urine. Thus the final composition of the urine is very different from that of theglomerular filtrate and reflects the integrity of kidney function and changes in blood composition.
An analysis of urine can yield valuable information about the health of the body in general. The
volume of urine produced and its specific gravity give information on the state of hydration or
dehydration of the body. Several diseases are characterized by abnormal metabolism and the
abnormal byproducts can be found in the urine. For example, phenylpyruvic acid appears in the
urine of someone suffering from phenylketonuria (PKU), a disease resulting in mental retardation.
Type I diabetes mellitus, a disease resulting from a deficient production of insulin by the pancreatic islets, is characterized by the appearance of glucose in the urine, a condition known as glycosuria. Readings above or below the normal range for specific gravity may indicate a pathological condition. For example, a low reading is found in chronic nephritis and a high one in acute nephritis.

Recent advances in urinalysis techniques have made it possible to perform, in a few seconds, tests which formerly took hours. The Multistix test is a combined test of pH, proteins, glucose, ketones and occult blood. Alkaline urine is found in many conditions. One example is cystitis, a condition in which urine decomposes in the bladder to form ammonia. Typically urine is slightly acidic (around pH 6); however, the pH can be lowered by a diet rich in proteins or citrus fruits. Thus pH by itself is not very informative. An abnormally low pH coupled with high glucose and ketones are characteristics of diabetes mellitus. The presence of proteins or blood in the urine is more useful as a diagnostic tool. Blood and proteins in the urine may indicate nephritis, a disease characterized by damaged glomeruli that allow plasma proteins and erythrocytes to leak into the urine. However, long distance runners sometimes test positive for blood in their urine.

In the first portion of this exercise, you will analyze your urine for some of the more clinically
important constituents. In the second portion of this exercise, the class will be divided into four test
groups and the members of each group will drink one of the following beverages:
(1) tap water
(2) Gatorade
(3) beer
and
(4) coffee

Precautions:

Beer - We card! Do not volunteer if you will be driving later.

Coffee - Do not volunteer to take caffeine if you do not normally have caffeine in your diet and/or you know to be sensitive to caffeine.

PROCEDURE FOR URINE ANALYSIS

For convenience (so the lab proceeds in a timely manner), the schedule will be very different. As stated in the syllabus, you need to come prepared to micturate. Go to the rest room and fill the sample cup, and note the time. We do not need to know the volume, but, so that a volume measurement after drinking is more meaningful, please void completely after filling the cup.

Next, we will take a reading with the urinanalysis strip. I really do not understand how they expect you to make all those readings on the key that is rolled around the jar, so, conveniently, last year's TA, Marie Miller, put keys cleverly in water proog pages that will be provided.

Here's how:
Obtain a Multistix from the front lab bench. Dip the reagent part of the Multistix into the urine so that all reagents are completely covered. Dip quickly for not more than one second! Wipe off the excess urine by dragging the chemstrip across the lip of the urinalysis jar. The specified number of secondsseconds read the strip by comparing it to the colored scale on the Multistix pages provided.

Then we will drink, as fast as possible, noting the time,

Group 1: drinks 710 ml (the equivalent of 2 cans, 12 oz each) of tap water
Group 2: drinks 710 ml of Gatorade
Group 3: drinks 710 ml of coffee
Group 4: drinks 2 cans of beer

Gatorade is an electrolyte replacement with glucose and, in an 8 fl oz serving has 110 mg sodium and 30 mg potassium.

Caffeine and alcohol are famous for their diuretic effects. The latter (perhaps both) inhibit antidiuretic hormone.

This is when we will take the quiz. Maybe a few other business matters after that to kill time.

1 (or maybe 1.5) hr after drinking, each student will empty his/her bladder into a clean beaker or bottle of choice.

Analyze the urine from each collection for Volume--and for everything else, especially specific gravity, and we will tabulate the results.

The normal range of specific gravity for urine is 1.0015 to 1.035.

Reference: Familiarize yourselves with chapters 19 and 20 in Silverthorn

Homework

Use the data below to write a "half lab report" (results and discussion). What would you expect? Is that how it turned out? What can we do better this year?

Data from last year

Why was it so difficult to get good data collection? (Maybe we can do a better job this year!) While we used the strips to test everything that is on the strips (and we will do so again this year), you will note that everything is qualitative, except specific gravity and pH (which are quantitative). So those are the only values I can give you (before and after drinking). We had students come prepared to collect a sample for testing and, at the same time, void for time zero. We tried to get the subjects to drink quickly and hoped that students would need to "go" again in one hour when we could measure volume and test strip values. Instead, you will need to deal with differing times somehow.

Subject #, specific gravity before, s g after, pH before, pH after, volume, time difference (hr:min)

Beer
1,1.03,1.01,6.0.6.0,190,1:12
2,1.03,1.01,6.0,5.0,725,1:32
3,1.015,1.01,6.0,6.0,400,1:34
4,1.02,1.015,5.0,6.5,300,0:50
5,1.01,1.005,6.0,7.5,700,1:00

Gatorade
1,1.025,1.015,5.5,7.0,500,1:13
2,1.015,1.015,6.5,6.3,400,1:10
3,1.015,1.025,6.7,6.0,410,1:15
4,1.03,1.03,6.0,5.0,150,1:00
5,1.03,1.015,6.0,5.0,109,1:00
6,1.003,1.003,6.5,6.3,830,1:00

Water
1,1.015,1.01,6.5,6.5.305,1:17
2,1.025,1.01,6.0,6.0,300,1:24
3,1.015,1.005,6.5,7.5,1020,1:23
4,1.03,1.015,6.0,6.0,240,1:00
5,1.03,1.02,5.0,6.0,80,1:20
6,1.03,1.005,6.0,5.0,920,1:00

Coffee
1,1.02,1.01,6.0,6.5,620,1:00
2,1.01,1.00,7.0,8.0,450,1:32
3,1.015,1.005,6.5,7.0,550,1:22
4,1.015,1.005,6.25,7.0,6.0,1:00
5,1.03,1.005,6.25,5.5,500,0:45
6,1.015,1.015,5.5,5.5,350,1:00

Last year I gave a quiz, and here are the questions and answers

BL A347 - General Physiology Laboratory Spring semester, 2004
Prof. Stark - 7th quiz - April 22

Why would you expect urine output to be higher after beer than an equivalent volume of water?

it inhibits ADH

Why is the blood vessel exiting the glomerulus called an efferent arteriole instead of a vein?

because, like hypothalamus->pituitary and gut->liver, kidney has a portal blood flow

Glucose is filtered into Bowman's capsule then "recovered." Why, then, do patients with untreated diabetes mellitus have glucosuria (glycosuria)?

glucose transport saturates

Why are we comparing the output after Gatorade vs water in the urinanalysis lab?

gatorade has electrolytes

After glomerular filtration, reabsorption and secretion processes recover important molecules and dispose of wastes respectively. If all reabsorption and secretion were blocked, what would that do to the volume of urine?

it would go WAY up

Hopefully your Multistix reading will give a low protein reading. What is the mechanism responsible for low protein in urine?

proteins don't fit through filtration sieve

Why is urine pH, by itself, not useful in diagnosis?

dietary factors can swing pH widely

Rounded off to the nearest digit (e.g. -1, 0, 1), what is the specific gravity of urine in a normal subject?

it's just decimal places above 1

Which part of the nephron goes into the kidney's medulla?

loop of Henle

What is the difference in the use of the words "excretion" and "secretion" in the context of your kidney coverage?

secretion is pumping out of wastes while excretion is the whole proces of urine manufacture

This page was last updated 11/5/04


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