KIDNEY FUNCTION: URINE ANALYSIS AND REGULATION
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
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
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
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
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
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
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)
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
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
This page was last updated 11/5/04
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