CHAPTER 50: Temperature, Osmotic Regulation, and the Urinary
System
WHERE DOES IT ALL FIT IN?
Chapter 50 builds on the foundations of Chapter 32 and provides detailed information about animal
form and function Students should be encouraged to recall the principles of eukaryotic cell structure
and evolution associated with the particular features of animal cells. Multicellularity should also be
reviewed. The information in chapter 50 does not stand alone and fits in with the all of the chapters
on animals. Students should know that animals and other organisms are interrelated and originated
from a common ancestor of all living creatures on Earth
SYNOPSIS
The cell specialization found in complex vertebrates like humans requires extensive integrated
control systems. These homeostatic processes are involved with nearly every bodily function and
regulate such things as blood glucose levels and body temperature. Homeostasis occurs through
various kinds of feedback loops. Negative loops attempt to return a system above or below a
certain setpoint to that normal value and include regulation of blood glucose and body
temperature. Positive loops accelerate changes and are exemplified by the process of childbirth.
Most marine invertebrates are simple osmoconformers; they maintain an internal concentration
of ions equal to that of their environment. On the other hand, osmoregulators maintain an
osmolality independent of their environment. Freshwater vertebrates have a greater internal salt
concentration and must constantly exclude water. The internal salt concentration of marine
specimens is roughly one-third that of the environment. They must retain water to prevent
dehydration. Terrestrial animals tend to lose body water to the air through dehydration and must
also conserve water.
Animals utilize various strategies to regulate water balance. Simple protists and sponges possess
contractile vacuoles to exclude water. Flatworms draw fluids from their bodies into flame cells,
water and metabolites are reabsorbed, excreted substances are expelled through pores in the body
wall. In earthworms, the internal fluid is collected in the nephrostomes and filtered, NaCl is
reabsorbed by active transport. The Malpighian tubules of insects secrete potassium ions into the
gut which pull body fluids and waste past their filtering apparatus. Vertebrates use the pressuredriven filtration system of the kidney, possible only with their closed circulatory system. Certain
small molecules are selectively reabsorbed in the kidneys to help conserve water.
Freshwater fish kidneys simply remove excess water that is absorbed. Marine bony fish combat
dehydration by drinking enormous quantities of seawater as very little water is reabsorbed in the
kidneys. Elasmobranchs reabsorb waste urea to make their blood isotonic with the sea, thus
preventing excessive water loss and reducing the need to remove large amounts of ions.
Amphibian kidneys operate much the same as those of freshwater fishes. Reptile systems are
quite variable as are their habitats. Mammal and bird kidneys are able to produce urine that is
more concentrated than their blood plasma, efficiently reabsorbing water from the filtrate.
Nitrogenous wastes must be eliminated from the body. Bony fish excrete ammonia, but other
vertebrates eliminate the less toxic urea and uric acid.
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Mammalian kidneys are paired organs with functionally and structurally distinct outer cortex and
inner medulla. The mammalian nephron begins in the renal cortex where a mass of glomerular
capillaries are surrounded by the Bowman’s capsule. Glomerular filtrate passes from the capsule
to the proximal convoluted tubule and into the distal convoluted tubule. One of the key
characteristics of the mammalian nephron is the hair-pin-curved loop of Henle. Several nephrons
empty into a single collecting duct that empties into the funnel-shaped renal pelvis. Most of the
water filtered through the kidney is reabsorbed, a consequence of salt reabsorption. Glucose and
amino acids are reabsorbed through active transport carriers. Other foreign molecules and waste
products are actively secreted from the nephron.
Humans filter vast quantities of liquid through their kidneys. Two-thirds of the water and NaCL
is immediately reabsorbed in the proximal tubule, the final third of water is reabsorbed in the
collecting duct. The loop of Henle descends deeply into the hypertonic renal medulla. A
countercurrent flow results where the filtrate passing through the descending limb loses water to
the renal medulla because of the salts transported out of the ascending limb. The collecting
tubule contains a very concentrated filtrate especially high in urea. The collecting duct is
permeable to urea, which passes into the renal medulla followed by more water.
The overall solute concentration of the blood is regulated by altering the hypothalamic output of
antidiuretic hormone. ADH causes the collecting ducts to become more permeable to urea. An
increase in the osmolality of the blood plasma triggers a sensation of thirst and stimulates ADH
secretion. The extracellular fluid in the renal medulla becomes hyperosmotic to the filtrate in the
collecting duct. Therefore, even more water leaves the filtrate via osmosis and reenters the
bloodstream. This dilutes the blood and returns its osmolarity to normal. Plasma salt balance is
regulated by production of aldosterone by the adrenal gland. Aldosterone stimulates reabsorption of
sodium in the distal convoluted tubule, decreasing the amount lost in the urine. As a result, whole
body sodium increases, as does overall water retention. Atrial natriuretic hormone promotes
excretion of salt and water in the urine, opposing the action of aldosterone.
LEARNING OUTCOMES
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Understand why homeostasis is a vital function of the human body.
Differentiate between positive and negative feedback loops. Give examples of each.
Pick one example of a negative homeostatic regulating process and explain how this system
maintains physiological normalcy.
Compare osmoconformer and osmoregulator water balance.
Compare the excretory systems of flatworms, earthworms, and insects.
Describe the structure of a typical vertebrate nephron, the process of filtration and indicate which
regions are associated with reabsorption or secretion.
Compare the efficiency of water reabsorption in freshwater, bony marine, and cartilaginous
fishes; amphibians, reptiles, birds, and mammals.
Describe the internal structure of a typical mammalian kidney.
Understand the process of filtration in the human kidney especially as it relates to the materials
that enter and leave proximal tubule and the loop of Henle.
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Compare the osmotic condition of the renal cortex to the renal medulla and understand how it
affects reabsorption of water.
Discuss hypothalamic and adrenal regulation of kidney function
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 50 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students believe that all animals have kidneys
Students are unaware of the distinction between nephridia and kidneys
Students are not aware of the effects of temperature on the rate of metabolic reactions
Students do not understand the principles of diffusion and osmosis related to osmotic
regulation in fresh water and salt water
Students think that all animals evolved at about the same time
Students believe that most animals are vertebrates
Students do not equate humans with being animals
Students believe that all animals have identical organ system structures
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
This is a difficult topic for beginning students to understand. Set the stage, introduce the
characters, then begin the play. Describe the gross anatomy of a mammalian kidney including the
osmotic differences between the cortex and the medulla. Then indicate the location of the various
parts of the nephron: capsule, proximal and distal convoluted tubules, descending and ascending
limbs of the loop of Henle, collecting duct, and associated blood vessels. Finally put everything
together by following a drop of blood and its filtrate through the capsule, into the nephron and
out the collecting duct.
Most students have experienced the effects of caffeine and ethanol on their urine output and will
be interested in the physiology that regulates urine output. Compare the operation of a dialysis
machine to that of a normal kidney. Discuss why artificial dialysis is necessary relative to
increased levels of blood urea nitrogen (BUN) and creatinine.
The kidneys excrete many metabolic toxins and drugs, penicillin and tetracycline for example.
Excess vitamin C is also dumped here (and turns the urine bright orange). Many of these
compounds can cause damage to the renal tubule when used for too long.
Explain the absence of blood in the urine of a healthy individual.
Explain the need for providing a urine specimen to your doctor.
HIGHER LEVEL ASSESSMENT
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Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 50.
Application
Analysis
Synthesis
Evaluation
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Have students explain why heating lamps are needed to for raising reptiles
in captivity.
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Have students explain why an organism controls body temperature to
prevent its temperature from raising or lowering 5oC.
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Ask students to describe the path of sodium from the blood through the
kidney.
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Have students explain how an osmoregulator would adapt to being placed
in a saltwater environment.
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Have compare and contrast the energy needs of ectotherms and
endotherms.
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Ask students to describe how the diet of an organism affects urinary
system function.
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Ask students to design an experiment to test relationship between
environmental temperature and efficiency of digestive system function in
ectotherms.
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Have students come up with commercial application of knowledge about
Q10 effects on enzymes.
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Ask the students come up with an agricultural application of a chemical
that elevates an organisms body temperature.
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Ask students evaluate the value of letting people monitor their health by
measuring urea content of their urine.
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Ask students and value and safety of drugs called thermogens that elevate
the body’s temperature.
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Ask students evaluate the accuracy of studying amphibians as a model for
human metabolism.
VISUAL RESOURCES
Obtain various samples of vertebrate kidneys from the butcher or slaughter house. Few kidneys
resemble the “kidney bean” structures possessed by those animals a student typically dissects.
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Set up a simple dialysis experiment to proceed as you lecture.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Digestive System Histology
Introduction
This demonstration uses urinary system histology images to help students understand the
cellular composition of urinary system. It is a good demonstration for reinforcing histological
form and function.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Boston University histology website at
http://www.biology.ualberta.ca/facilities/multimedia/index.php?Page=252
Procedure & Inquiry
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5.
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7.
Ask the class what they know about ion and water balance in fish.
Load up the University of Alberta website and click on the Fish Gill Models.
Select Fresh Water
Go through the animations of the two models.
Have students describe the activities of the models.
Select the Salt Water
Have students explain the strategies of ion balance in fresh water and salt water fish
USEFUL INTERNET RESOURCES
1. The University of Alberta has a valuable homeostasis animations website for
supplementing lectures on the urinary system. The site is available at
http://www.biology.ualberta.ca/facilities/multimedia/index.php?Page=252
2. A discussion about the biomechanics of an artificial kidney is a way to promote an
interest in the urinary system. A website with valuable information about artificial
kidney research is available at:
http://cape.uwaterloo.ca/che100projects/organs/Kidney/kidney.htm.
3. Faculty and students will find value in websites that simplify animal anatomy and
physiology concepts. The information can be used for projects that educate children and
civic groups about animals. Biology-4-kids is a model website for animal education. The
website can be found at http://www.biology4kids.com/files/systems_digestive.html.
4. Case studies are an effective tool for stimulating interest in a lesson on animals. The
University of Buffalo has a case study called “The Campus Coffee Shop: Caffeine
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Conundrums” which has students investigating the impact of caffeine on the overall
internal environment of the body. The case study can be found at
http://www.sciencecases.org/caffeine/caffeine.pdf.
LABORATORY IDEAS
A. Urine as an Indicator of Kidney Function
This activity has students has students investigate the chemistry of urine as an indicator
of urinary system function and health.
a. Explain that they will be looking for endocrine organs in a fish and frog using a human
endocrine system chart as a guide.
b. Provide students with the following materials:
a. Urine test strips (Multistix or equivalent) sliced lengthwise in half
b. Test tubes
c. Test tube rack
d. “Artificial Urine” Samples
i. Sample 1
1. Dissolve 3g sodium chloride, 5g urea, 1g glucose powder and 1g
albumin powder in 100ml water. Add 1 drop of 2M hydrochloric
acid.
ii. Sample 2
1. Dissolve 3g sodium chloride, 5g urea and 1g glucose powder in
100ml water. Add 3 drops of 2M hydrochloric acid.
iii. Sample 3
1. Dissolve 3g sodium chloride and 1g glucose powder in 100ml
water. Add 3cm3 of 1M ammonia solution.
iv. Sample 4
1. Dissolve 3g sodium chloride, 5g urea and 1g albumin powder in
100ml water.
v. Sample 5
1. Dissolve 3g sodium chloride, 1g glucose powder and 1g albumin
powder in 100ml water.
e. Charts of normal urine results
c. Tell students to test the different urine and investigate the reasons why they are testing
the selected urine components.
d. Ask them to note the differences and similarities of the urine samples.
e. Then have them compare the samples to the measurements of normal urine and have
them look up the probable causes of any variation from the normal.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
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in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students do a lesson do a hands-on program exercise and the cardiovascular system
for elementary school students.
2. Have students tutor high school students studying animal anatomy and physiology.
3. Have students volunteer for a local heart association group.
4. Have students volunteer at the educational center of a zoo or marine park.
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