Lab 10
Renal Physiology
Laboratory 10
Renal Physiology
The primary function of the kidney is to maintain a constant level of blood solute concentration (or
blood osmolarity). Additionally, the kidneys are responsible for excretion of metabolic wastes
(urea, ketones, and metabolic acids) and reabsorption of essential molecules (i.e. glucose,
calcium, amino acids).
The way the kidney maintains blood osmolarity is complex and requires both an understanding of
kidney microanatomy and of basic principles of osmosis.
The following three pictures outline the salient features of kidney anatomy.
Collectively this is the
NEPHRON
The physiological process that generates URINE starts in the CORTEX. Urine then passes to the
MEDULLA (the collecting ducts of the pyramids) then to the renal pelvis then to the URETER
then it is stored in the URINARY BLADDER (not pictured) until you MICTURATE, then urine gets
outside through your URETHRA!
The physiology of Urine production
Focus on the third picture (the Nephron) for the following discussion.
The picture of the NEPHRON contains three basic parts
1) Glomerulus
2) The Renal Tubule (with many named parts)
3) The Peritubular capillary system (unlabelled but it is the black tube -circulatory
system- wrapped around the renal tubule!)
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Lab 10
Renal Physiology
The GLOMERULUS is a tightly woven capillary bed stuffed within the RENAL CAPSULE. Thus,
the glomerulus is the interface between the renal arterial supply and the renal tubule where urine
is formed. An extreme amount of pressure in the glomerulus results in the squeezing of blood
FROM THE CAPILLARY TO THE CAPSULE! The liquid/fluid entering the capsule consists of
water and dissolved solutes; plasma proteins and blood cells remain in the capillaries because
they are too big to fit through the capillary walls.
The liquid/fluid entering the capsule is called FILTRATE and is isotonic to normal blood (300
mOsm/L)!
The filtrate moves down the length of the renal tubules where the NEPHRON allows reabsorption
of some molecules and secretion of others. Make sure you understand the difference between
reabsorption and secretion!
Reabsorption occurs when a molecule moves FROM the renal tubule TO the peritubular
capillaries!
Secretion occurs when molecules move FROM the interstitial fluid (located between the
peritubular system and the renal tubule) INTO the renal tubule!
When the filtrate reaches the end of the renal tubule and collecting tubule, it changes names to
URINE and then flows into renal pelvis and on from there! Because both reabsorption and
secretion occur along the length of the renal tubule the osmolarity of the FILTRATE is very,
very different from the osmolarity of the URINE.
Which molecules are reabsorbed, how are they reabsorbed, and why does this result in
concentrated urine?
Many different ions and molecules are reabsorbed during the process of urine formation;
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however, those with a major role include Sodium (Na ), Chloride (Cl ), and Water (H2O).
The following diagram shows how ions/molecules move during urine formation. Focus
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on how NaCl (actually moving as Na and Cl !) and water move!
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Lab 10
Renal Physiology
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As the filtrate (300 mOsm/L) moves into the proximal convoluted tubule, Na ions are actively
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transported into the interstitial fluid. Three things happen as a consequence of this active Na
+
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transport: 1) Cl follows Na due to electrical attraction; 2) With Na and Cl movement, the
interstitial fluid becomes more concentrated and 3) Water molecules leave the renal tubule and
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enter the interstitial fluid via Osmosis. Thus, active transport of Na and the following of Cl
dfiffussion create a hypertonic interstitial fluid that helps the body reabsorb water (passively via
OSMOSIS). When the filtrate reaches the descending limb of the loop of Henle, 70% of the
filtrate volume was reabsorbed in the proximal convoluted tubule and the filtrate is isosmotic to
the interstitial fluid at this point!
Since water continues to move out as the filtrate travels through the descending limb of the loop
of Henle the filtrate becomes more concentrated as it approaches the bent portion (the “loop”) of
the Loop of Henle.
Next, the concentrated filtrate (up to 1200 mOsm/L) moves into the ascending limb of the loop of
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Henle. Since the filtrate is SO concentrated here, Na diffuses from the ascending limb of the
loop of Henle into the interstitial fluid (Cl follows due to electrical attraction) and both ions are
then reabsorbed by the peritubular capillary! Movement of the filtrate through the loop of
Henle ensures that ions are reabsorbed and water is conserved! Desert animals and
human from arid environments have a REALLY REALLY long loop of Henle to increase water
and ion reabsorption!
The ascending limb of the loop of Henle is impermeable to water, so water does not follow the
ions into the interstitial fluid. Therefore, by the end of the loop of Henle the filtrate is dilute (100
+
mOsm). So why is urine so concentrated? The interstitial fluid is concentrated with lots of Na and
Cl . As the dilute filtrate passes through the distal convoluted tubule and into the collecting tubule,
water moves into the interstitial fluid by OSMOSIS. The interstitial fluid is hypertonic to the
filtrate in the tubule….thus by Osmosis water will move towards the hypertonic solution! This
makes the end product of the collecting tubule SUPERCONCENTRATED URINE!
The body has internal mechanisms to alter water and ion balance in response to dehydration
and/or exercise. We will not focus on these in detail. However, diuretics (such as Caffeine!) are
drugs that mimic the effects of water balance within the body. Specifically, diuretics enter the
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proximal tubule and PREVENT Na FROM BEING REABSORBED! Think about the cascading
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effects of this diuretic. Increasing Na solute concentration in the proximal tubule will
DECREASE the movement of water to the interstitial fluid (less water and Cl will follow the active
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transport of Na , thus more will be retained in urine). This will result in less concentrated urine
and a more dehydrated internal environment of the body (less water reabsorbed). This is why
you NEVER NEVER NEVER drink pop when you are dehydrated!
When you drink different fluids, you body releases hormones which alter the normal function of
the kidney! In this lab, you will examine how different fluids alter normal kidney function by
examining 1) amount of urine output and 2) the characteristics of urine output. By seeing how
urine changes when consuming different beverages you should be able to work back to the level
of the nephron and figure out how normal kidney function was altered!
Objectives:
Based on these results you should:
1) know how caffeine and NaCl alters urine output, and
2) understand why changes in renal function GENERATE the observed changed in urine output
For example, when you drink Gatorade, you should see a decrease in urine output…….based on
your understanding of renal function you should be able to reason HOW RENAL FUNCTION
CHANGES TO RESULT IN DECREASED URINE OUTPUT and changes in chloride clearance!!!
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Lab 10
Renal Physiology
Each group of 4 will take test beverages 1-4.
1)
2)
3)
4)
These beverages contain the following
DIET Caffeinated Pop (I think you southerners call this stuff Soda?)
DIET Decaffeinated Pop
Water
Gatorade
At your bench you should have four graduated cups. You will urinate into these!
In addition, you should have a rack of test tubes. You will transfer urine into these, as necessary,
to examine different properties.
Procedure:
1) You will urinate at 0, 30, 60, and 90 minutes.
0 minutes will be urination BEFORE you drink your test beverage. Each group member
will drink ONLY ONE test beverage. You MUST drink it ALL during the five minutes after
you come back from the restroom with your sample (time 0).
Then you will collect as much urine as possible at each of these times!
2)
You should measure the following for each time
a.- Color
Describe the color you see when placing the graduated cup on a white
piece of paper.
b.- Urine Volume
Use the graduated cylinder to measure the volume of urine output
Make sure to respect the MENISCUS!
c.- Specific Gravity with Urinometer and Float
1) Fill urinometer cylinder ~ 2/3 full, remove foam with filter paper
2) Figure out how to read the marking on the float!
3) Insert the float into the urine and make sure it is free floating
4) Read the marking on the float at the level of the meniscus
d.- pH
1) Dip the pH paper in the urine 3 times and then shake away any excess urine
2) Let sit for ~ 1 minute and compare color on wheel to color of paper
e.- Chloride concentration
1) Save ~ 10 drops in a test tube of urine from each test time!
2) Add 1 drop of Potassium Chromate to each tube
3) Gently Swirl
4) Add silver nitrate drop by drop to each tub while swirling
5) Count the number of drops required to get a BRICK RED PRECIPITATE. The
number of drops is equal to the grams of NaCl per Liter of Urine!
f.- Clearance
Use the following equation to measure clearance of Chloride:
Plasma Chloride Concentration is normally 3.5 g/L
(Urine Volume)(Chloride Concentration)
Clearance of Chloride = ---------------------------------------------------------(Plasma Chloride Concentration)(Time)
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Lab 10
Renal Physiology
Clearance tells you how much plasma can be cleared of the substance over time! Its an
important clinical tool to test for normal renal function!!!!!!!!
Make sure all you units Jive! Liters, grams, and minutes!
I drank _________________
0 min
and this is what I got:
30 min
60 min
Color
Volume
Specific
Gravity
pH
Chloride Conc
(g/L)
Chloride
Clearance
Graph Volume and Chloride Clearance VS. time
5
90 min
Lab 10
Renal Physiology
Results of each Treatment, note how each variable changed over time in each treatment!
Diet Caffeinated
Diet Decaff.
Color
Volume
Specific
Gravity
pH
Chloride Conc
(g/L)
Chloride
Clearance
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Water
Gatorade