Kidney Function and Urinalysis
The excretory system consists of paired kidneys, a ureter draining each, a
urinary bladder for accumulating urine and the urethra through which urine
is voided. (W&D pg 182)
Of the structures listed, only the kidney contribute to the production of
urine.
Blood is supplied to the kidney via renal vessels.
(The term excretion refers to the release of urine from the kidney to the
ureter.
Voiding of urine from the body thru the urethra is termed micturition.)
The kidney is a vital organ and humans cannot survive without at least one
kidney.
The critical functions of the kidney include:
1] the regulation of fluid volume and osmotic pressure;
2] the regulation of the chemical composition of iterstitial fluid,
plasma and lymph
a] metabolic wastes
b] electrolytes
c] pH
These functions are performed by nephrons, about 1 million/kidney, which
are the functional units of the kidney. (See text)
The kidney utilizes three processes to form urine:
1] glomerular filtration
2] tubular reabsorption
3] tubular secretion
Filtration is accomplished due to the hydrostatic pressure of blood.
Since glomerular capillaries are very permeable and the surface area is
large, the glomerular filtration rate (GFR) of both kidneys averages
180L/day (45 gallons)! Obviously most of the water must be reabsorbed and
returned to the vascular system to maintain blood volume and pressure. In
addition to water, the glomerular filtrate contains all small molecules
found in plasma in essentially the same concentration (glucose, urea,
amino acids, salts, water).
Large molecules, such as proteins, and cells are not filtered.
As the filtrate travels through the nephron, most of its components are
reabsorbed to varying degrees, depending on the need to adjust blood
chemistry and the kidney's ability to reabsorb nutrients and other
molecules. The cells of the nephron may secrete ions and drugs as well.
Since urine output is only 1 - 2 L/day, it is apparent that most of the
water filtered, as well as required nutrients and electrolyes are returned
to circulation. The amount of urine produced varies with dietary intake,
exercise and pregnancy.
The liter+ of urine produced by a person contains about 60g of solids, 15g
of which is NaCl. Various organic and inorganic molecules contribute to
the remaining 15g,
including creatine, amino acids, uric acid, ammonia, sulfates, potassium,
and magnesium. Occasionally and in pathological conditions, substances
normally absent or present in trace amounts (such as glucose, proteins,
hemoglobin, ketone bodies, cells, may be detected in the urine.
Objectives:
1] To become familiar with the organization of the kidney
2] To examine urine samples for constituents
3] To acquire an appreciation of urinalysis as a diagnostic tool
4] To become familiar with the concepts of glomerular filtration rate and
renal clearance
Pg 2
I. Kidney structure
Examine a prepared slide of mammalian kidney. Utilize W&D Fig 25.1
and illustration
on chalkboard. Identify:
Gross features - cortex, medulla and medullary
pyramid, pelvis
Microscopic features - renal glomerulus, Bowman's
capsule,
x-sec of
convoluted tubules in cortex,
long.-sec of
collecting tubules in medulla
Answer the questions on the Lab Response Sheet.
-----------------------------------------------------------------------------------------------------------
II. Urinalysis W&D pg 182 - 185 no human subjects
BioKit simulations and analyses.
Because of the increased incidence of chemical abnormalities and
health risks in human
populations, we will not sample our own body fluid. Instead
simulated specimens will be
provided.
Follow instructions on the worksheet and especially pay attention to
any modifications given
by your instructor. Use liquids sparingly.
Construct your own copy of the tables and fill in the data. Answer
questions from the
worksheet.
Read the material in W&D on these topic. Attempt to reason through
the answers to W&D's
questions.
------------------------------------------------------------------------------------------------------------
III. Glomerular Filtration Rate
To calculate GFR: 125 ml/min X 60 min/hr X 24 hr/day
daily GFR = 180 L/ day
A 70Kg person has a blood volume equal to about 7% body weight. If we
assume a hematocrit of 0.45, we can calculate the plasma volume filtered
each day.
1 L/Kg X 70 Kg X 0.07 (1 - 0.45) = plasma volume = 2.695 L plasma
If the kidneys filter 180 L of plasma per day and the plasma volume is 2.7
liters,
the total plasma volume is filtered ____________ per day.
Answer question on Lab Response Sheet.
-----------------------------------------------------------------------------------------------------------
IV. Renal Clearance
To characterize the combined functions of filtration, reabsorption and
secretion, the concept of renal clearance is employed. If you know the
volume of plasma that would contain the amount of substance appearing in
the urine per unit time you can measure kidney function. This volume of
plasma is known as the renal clearance of that substance.
To assess this value, we can choose a substance that can be readily
determined quantitatively, is completely filtered by the glomerulus, and
is physiologically inert (neither absorbed not secreted).
Inulin is polysaccharide that satisfies these conditions.
So, Pin = plasma conc. of inulin (mg/ml) ,
Uin = urine conc. of inulin (mg/ml),
V = urine flow ml/min, and
Cin = plasma clearance of inulin
The clearance is related to these parameters as:
Cin = V x Uin / Pin and also equals GFR.
The clearance value for inulin is about 135 ml/min.
The comparison of renal clearances of several substances can be made to
that of inulin.
For example we find that the renal clearance of p-aminohippuric acid (PAH)
[widely used as diagnostic drug in renal disease] is 650 ml/min. Since
this is well above the GFR and renal clearance value for inulin, it is
reasonable to conclude that PAH is being secreted into the tubule as well
as being filtered at the glomerulus.
On the other hand, glucose normally has a clearance of zero. If plasma
concentration of glucose exceeds the capability of the reabsorption system
of the nephron, then glucose would appear in the urine.
Since urine production is only a fraction of the filtration rate, it
should be apparent that the bulk of the glomerular filtrate is reabsorbed
by the nephron. Experiments show that 85% of the glomerular filtrate is
absorbed by the proximal convoluted tubule. The remaining fluid may be
further modified to produce urine. An impressive fact - humans can
maintain salt and water balance on less than a half a gram of salt and a
liter of water a day.
W&D pg 182 - 185 no human subjects
Read the material in W&D on this topic.
Think about the questions posed by W&D. Answer the questions on the
LRS.
Lab Response Sheet
I. Kidney Structure.
1] Where are most of the glomeruli located in the kidney?
2] What is the function of the glomeruli?
3] Using your text (homework) list, in sequence, the parts of the
nephron through which the
filtrate passes.
II. Urinalysis - construct your own tables for your group's data; answer
the questions
III. GFR
4] A "normal" woman's kidney filters, on average, 115 ml/min and
hematocrit is 38%.
Ms. Muffit weighs 70 Kg but suffers from hypertension. Due to
her high blood pressure,
her kidneys are filtering 135 ml/min.
Calculate the normal GFR for women.
Calculate Ms. Muffit's GFR and plasma volume filtered per day.
Adjusting for gender, how much more work, as % normal, is Ms.
Muffit's kidneys doing?
IV. Renal Clearance
5] Define secretion and reabsorption
6] If we had done the comparison among your classmate described in
W&D, what variables
could have affected the values obtained for different
individuals?
7] What is the main solute in blood?
Also in urine?
Notes to TAs:
I will make 15 copies of the handout. Please do not let them "walk."
Students may download the handout from my web site. If you want them to
turn in anything at the END of lab then make copies of the Lab Response
sheet.
I will put 1 or 2 diagrams on the wall chalk board. You may wish to use
them in your introduction.
Kits have been ordered and materials to supplement kits or to replace a
theses are utilized. Please be frugal so that we don't run out at end of
wk.
Although work with the kits is the main focus, students should look at
kidney and you should talk students thru W&D.