CHAPTER
38 BODY FLUID REGULATION AND EXCRETORY
SYSTEMS
This chapter presents a study of the mechanisms by which animals regulate body fluid concentrations. Included are
sections of aquatic animals and terrestrial animals, and the forms of nitrogenous waste produced by prototype
animals. The various organs of excretion in animals are characterized. The human urinary system is described in
detail.
Chapter Outline
38.1 Body Fluid Regulation
1. The excretory system regulates body fluid concentrations by regulating the water and ions in body
fluids.
-
This regulation depends on the concentration of mineral ions (i.e., Na+, CI-, K+, and HCO3 ).
Body fluids gain mineral ions when animals eat food and drink fluids; the body then loses ions by
excretion.
4. Water enters animals via ingestion (eating and drinking) and by metabolism (cellular respiration
produces water).
5. Water is lost by evaporation from skin and lungs, through feces, and by excretion.
6. To be in fluid balance, the water entering the body must equal the water lost.
7. If osmolarity (i.e., solute concentration) differs between two regions, water moves into the region with
the higher solute concentration.
8. Marine environments are high in salt and promote the loss of water and the gain of ions by drinking
water.
9. Fresh water promotes a gain of water by osmosis and a loss of ions as this excess water is excreted.
10. Terrestrial animals tend to lose both water and ions to the environment.
A. Aquatic Animals
1. Only cartilaginous fishes (sharks, rays) and marine invertebrates (molluscs, arthropods) have body
fluids that are isotonic to seawater, yet those fluids do not contain the same amount of salt as seawater.
a. Their blood has high concentrations of urea to match the tonicity of seawater; for some unknown
reason, this is not toxic to the fishes.
2. Bony Fishes
a. Marine bony fishes have a moderate salt level compared to seawater; their common ancestor
probably evolved in fresh water.
1) They are prone to water loss and could become dehydrated, therefore they constantly drink
seawater.
2) They swallow water equal to 1% of their weight every hour to counteract dehydration.
3) To remove excess salt, they actively transport Na+ and CI- ions (salt) at the gills.
b. The body fluids of freshwater bony fish are hypertonic to freshwater; they passively gain water.
1) Freshwater fishes never drink water.
2) They take in salts at the gills and pass large quantities of dilute, hypotonic urine.
3) They discharge a quantity of urine equal to one-third their body weight each day.
4) Because this causes them to lose salts, they actively import Na+ and CI- ions into the blood at
the gills.
c. Some fish can move between marine and freshwater environments.
1) Salmon, for example, begin their life in freshwater streams, mature in ocean and return to
freshwater to breed.
2) Salmon alter their behavior and gill and kidney functions in response to osmotic changes.
B. Terrestrial Animals
1. Some terrestrial animals that live near oceans are able to drink seawater despite its high osmolarity.
2. Such birds and reptiles have a salt gland that excretes concentrated salt solution.
2.
3.
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3.
Water Loss Prevention
a. Some animals excrete a rather insoluble nitrogenous waste.
b. Animal skin is adapted to moist (moist, thin, permeable) or dry (dry, thick, impermeable)
environments.
c. Some animals have other unique adaptations to prevent water loss; e.g., some nematodes literally
dry up under adverse conditions—this is called anhydrobiosis.
d. The camel and kangaroo rat have convoluted nasal passages that absorb moisture from exhaled air.
4. Most terrestrial animals must drink fresh water often; however, the kangaroo rat completely avoids
drinking water.
a. It forms a very concentrated urine.
b. It defecates fecal matter that is almost completely dry.
c. It meets its water requirements with the metabolic water derived from aerobic respiration.
38.2 Nitrogenous Waste Products
1. The breakdown of nucleic acids and amino acids results in nitrogenous wastes.
2. Amino acids derived from protein synthesize body proteins or nitrogen-containing molecules.
3. Unused amino acids are oxidized to generate energy or are stored as fats or carbohydrates.
4. In both cases, amino groups (—NH2) must be removed.
5. Nitrogenous wastes are excreted as ammonia, urea, or uric acid, depending on the species.
6. The removal of amino groups requires a fairly constant amount of energy that differs for each
conversion.
A. Excretion of Nitrogenous Wastes
1. Amino groups removed from amino acids form ammonia (NH3) by adding a third hydrogen ion (H+).
a. This requires little or no energy.
b. Ammonia is quite toxic but it is water soluble; it requires the most water to wash it away from the
body.
c. Bony fishes, aquatic invertebrates, and amphibians excrete this ammonia through gills and skin
surfaces.
2. Terrestrial amphibians and mammals usually excrete urea as their main nitrogenous waste.
a. Urea is much less toxic than ammonia; excreted in a moderately concentrated solution, it also
conserves body water.
b. Production of urea requires energy; it is produced in the liver as a product of the energy-requiring
urea cycle.
c. In the urea cycle, carrier molecules take up carbon dioxide and two molecules of ammonia, finally
releasing urea.
3. Insects, reptiles, and birds excrete uric acid as their main nitrogenous waste.
a. Uric acid is not very toxic and is poorly soluble in water; uric acid is readily concentrated for
water conservation.
b. In reptiles and birds, a dilute solution of uric acid passes from the kidneys to the cloaca, a
common reservoir for products of the digestive, urinary, and reproductive systems.
c. After any water is absorbed by the cloaca, the uric acid passes out with the feces.
d. Reptiles and bird embryos are enclosed in eggshells; the uric acid that builds up is nontoxic in
storage.
e. Uric acid is synthesized by enzymatic reactions using even more ATP than urea synthesis.
f.
Therefore, there is a trade-off between water conservation and energy expenditure.
38.3 Organs of Excretion
• Most animals have tubular organs to regulate salt-water balance and excrete metabolic wastes.
A. Flame Cells in Planarians
1. Planaria have two strands of branching excretory tubules that open to the outside through excretory
pores.
2. Located along the tubules are flame cells (photonephridia) containing tufts of cilia that appear to
flicker.
3. The cilia beat back and forth, propelling a hypotonic fluid through canals emptying at the body surface.
4. This system functions in the excretion of excess water and wastes.
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B. Nephridia in Earthworms
1. An earthworm’s body is divided into segments and nearly every segment has a pair of nephridia.
2. A nephridium is a tubule with a ciliated opening, the nephridiostome, and an excretory nephridiopore.
3. Fluid from the body cavity is propelled through this tubule by cilia.
4. Nutrients are reabsorbed and carried away by the network of capillaries surrounding the tubules.
5. The nephridia form urine that contains only metabolic wastes, salts, and water.
6. Each day, an earthworm produces urine equal to 60% of its body weight, enough water that it can
safely excrete ammonia.
C. Malpighian Tubules in Insects
1. Insects have a unique excretory system consisting of long, thin Malpighian tubules attached to the
gut.
2. The Malpighian tubules take up metabolic wastes and water from the hemolymph—these follow the
salt gradient established by active transport of K+ ions.
3. At the rectum, water and other useful substances are reabsorbed.
4. Uric acid remains and eventually passes out the anus.
5. Insects that live in the water or that eat large quantities of moist food reabsorb very little water.
6. Insects in dry climates reabsorb most of the water and excrete a dry, semisolid mass of uric acid.
7. Crayfish (an arthropod) have antennal glands (green glands) in the head which perform selective
secretion of certain salts.
8. In shrimp and pillbugs, the excretory organs, called maxillary glands, are located in the maxillary
segments.
9. Arachnids (spiders, scorpions, etc.) have coxal glands used in excretion near the joint of one or more
appendages.
38.4 Urinary System in Humans
•
The human urinary system is an organ system with four parts.
1. The human kidneys are two bean-shaped, reddish brown organs, each about the size of a fist.
a. They are located on either side of the vertebral column, below the diaphragm, and partly protected
by the lower rib cage.
b. The kidneys are the sites of urine formation.
2. Each kidney is connected to a ureter; each conducts urine from a kidney to the urinary bladder.
3. The urinary bladder stores urine until it is voided from the body through the urethra.
4. A single urethra conducts urine from the urinary bladder to the exterior of the body.
5. The male urethra runs through the penis and also carries semen; in females, it opens the ventral to the
vaginal opening.
A. Kidneys
1. If a kidney is sectioned longitudinally, three major regions can be distinguished.
a. The renal cortex is the thin, outer region and it appears granular.
b. The renal medulla consists of the striped, pyramid-shaped regions that lie on the inner side of the
cortex.
c. The renal pelvis is the innermost hollow chamber.
2. Each human kidney is composed of about one million tiny tubules called nephrons.
3. Some nephrons are located primarily in the cortex but others dip down into the medulla.
B. Nephrons
1. Each nephron is comprised of several parts.
2. The end of a nephron pushes in to form a cuplike structure called the glomerular capsule.
a. The outer layer is composed of simple squamous epithelium.
b. The inner layer is made of specialized cells that allow easy passage of molecules.
3. Nearest the glomerular capsule is the proximal convoluted tubule, lined by cells with many
mitochondria and tightly packed microvilli.
4. Simple squamous epithelium forms the loop of the nephron, the middle portion of the nephron tubule
with a descending and ascending limb.
5. The distal convoluted tubule is the distal portion of the nephron tubule; several join to deliver the
urine into collecting ducts.
6. The loop of nephron and the collecting duct give pyramids of the medulla their striped appearance.
7. Each nephron has its own blood supply.
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a.
C.
D.
E.
F.
G.
H.
The renal artery branches into smaller arteries, which then branch into afferent arterioles, one for
each nephron.
b. Each afferent arteriole divides to form a capillary bed or glomerulus.
c. The glomerular capillaries drain into an efferent arteriole which branches into a peritubular
network.
d. The peritubular capillaries drain into a venule; the venules from many nephrons drain into a small
vein; small veins join to form the renal vein, a vessel that enters the inferior vena cava.
Urine Formation
1. Urine production requires three distinct processes.
2. Glomerular filtration occurs at the glomerular capsule.
3. Tubular reabsorption occurs at the proximal convoluted tubule.
4. Tubular secretion occurs at the distal convoluted tubule.
Glomerular Filtration
1. When blood enters the glomerulus, blood pressure moves small molecules from the glomerulus across
the inner membrane of the glomerular capsule into the lumen of the glomerular capsule; this is
glomerular filtration.
2. The glomerular walls are 100 times more permeable than the walls of most capillaries.
3. Molecules that leave the blood and enter the glomerular capsules are the glomerular filtrate.
4. Plasma proteins and blood cells are too large to be part of the glomerular filtrate.
5. Failure to restore fluids would soon cause death from loss of water, nutrients, and low blood pressure.
Tubular Reabsorption
1. Tubular reabsorption of fluids from the nephron back to the blood occurs through the walls of the
proximal convoluted tubule.
2. Reabsorption recovers much of the glomerular filtrate.
a. The osmolarity of the blood equals the filtrate so osmosis of water does not occur.
b. Sodium ions are actively reabsorbed; chlorine ions follow passively.
c. This changes the osmolarity of the blood so that water moves passively from the tubule back to the
blood.
d. About 60–70% of salt and water are reabsorbed at the proximal convoluted tubule.
3. Cells of the proximal convoluted tubule have numerous microvilli, increasing the surface area for
absorption, and numerous mitochondria, which supply the energy needed for active transport.
4. Only molecules with carrier molecules for them are reabsorbed.
5. If there is more glucose, for example, than carriers, excess glucose will appear in the urine.
6. In diabetes mellitus, there is a too much glucose because the liver fails to store glucose as glycogen.
Tubular Secretion
1. Tubular secretion moves substances from the blood to the tubular lumen by other than glomerular
filtration.
2. Secretion back into the filtrate is primarily associated with the distal convoluted tubule.
3. This helps rid the body of potentially harmful compounds that were not filtered into the glomerular
capsule.
4. Uric acid, hydrogen ions, ammonia, and penicillin are eliminated this way.
Urine Formation and Homeostasis
1. The kidneys regulate the water balance of the blood, thereby maintaining the blood volume and blood
pressure.
Maintaining the Salt-Water Balance
1. The long loop of nephron is comprised of a descending limb and an ascending limb.
2. Salt (NaCl) passively diffuses out of the lower portion of the ascending limb, but the upper, thick
portion of the limb actively transports salt out into the tissue of the outer renal medulla.
3. Less salt is available for transport from the tubule as fluid moves up the thick portion of the ascending
limb; the ascending limb is impermeable to water.
4. Urea leaks from the lower portion of the collecting ducts, causing the concentration in the lower
medulla to be highest.
5. Because of the solute concentration gradient within the renal medulla, water leaves the descending
limb of the loop of nephron along its length.
6. The decreasing water concentration in the descending limb encounters an increasing solute
concentration; this is a countercurrent mechanism.
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7.
I.
Fluid received by a collecting duct from the distal convoluted tubule is isotonic to the cells of the
cortex; as this fluid passes through the renal medulla, water diffuses out of the collecting duct into the
renal medulla.
8. The urine finally delivered to the renal pelvis is usually hypertonic to the blood plasma.
9. Antidiuretic hormone (ADH) is released from the posterior lobe of the pituitary.
a. ADH acts on the collecting ducts by increasing its permeability to H2O, thereby increasing H2O
retention.
b. When ADH is released, more water is reabsorbed and there is less urine.
c. When ADH is not released, more water is excreted and more urine forms.
d. If an individual does not drink much water, the pituitary releases ADH; if hydrated, ADH is not
released.
e. Diuresis means increased urine; antidiuresis means a decreased amount of urine.
10. Reabsorption of Salt
a. Usually more than 99% of the sodium filtered out at the glomerulus is returned to the blood.
b. Most is reabsorbed at the proximal tubule, 25% is extruded by the ascending limb of the loop of
nephron and the rest is from the distal convoluted tubule and collecting duct.
c. Aldosterone is a hormone secreted by the adrenal cortex.
1) It acts on the distal convoluted tubules to increase the reabsorption of Na+ and the excretion of
K +.
2) Increased Na+ in the blood causes water to be reabsorbed, increasing blood volume and
pressure.
3) If the blood pressure is insufficient to promote glomerular filtration, the afferent arteriole cells
secrete renin.
4) Renin catalyzes the conversion of angiotensinogen (a protein produced by the liver) into
angiotensin I.
5) Later, angiotensin I is converted to angiotensin II by angiotensin-converting enzyme.
6) Angiotensin II stimulates cells in the adrenal cortex to produce aldosterone.
7) Angiotensin II increases the blood pressure as a vasoconstrictor.
11. The Atrial natriuretic hormone (ANH) is produced by the atria of the heart when cardiac cells
stretch.
a. When blood pressure rises, the heart produces ANH to inhibit the secretion of renin and the
release of ADH.
b. Therefore, this hormone decreases blood volume and pressure.
Maintaining the Acid-Base Balance
1. The bicarbonate buffer system and breathing work together to maintain blood pH.
2. The excretion of H+ ions and NH3, and reabsorption of bicarbonate ions (HCO3 - ) is adjusted.
a. If the blood is basic, fewer hydrogen ions are excreted and fewer sodium and bicarbonate ions are
reabsorbed.
H+ + HCO3- ⇔ H2CO3 ⇔ H2O + CO2
b. If the blood is acidic, H+ ions are excreted with ammonia, while Na+ and HCO3- ions are
reabsorbed; Na+ ions promote formation of hydroxide ions and bicarbonate takes up H+ ions when
carbonic acid is formed.
NH3 + H+ → NH4+
c. Ammonia is produced in the tubule cells by deamination of amino acids.
3. Reabsorption or excretion of ions by the kidneys is a homeostatic function that maintains the pH of the
blood and osmolarity.
Lecture Enrichment Ideas
Experience Base: Students may know the location of kidneys from the boxing rules about kidney punches. Some
international students, especially from British Commonwealth countries may be familiar with kidney structure from
its more common use in food. Visuals are critical in explaining the nephron system. Using models or media to
demonstrate the gross anatomy of the kidney, bladder, etc. is also important.
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1.
Describe the role of the excretory system in excretion and why it is so important that nitrogenous wastes be
removed from the body; only the American bear can recycle amino acid metabolic wastes to usable
proteins during hibernation; all other organisms must rid themselves of these toxins. Since bears in
hibernation still undergo metabolism although at a slower rate. Students may be able to postulate the
problems that must be overcome.
2.
A freshwater fish placed in the ocean, or a marine fish placed in fresh water are under physiological stress.
A contrast of these two systems helps emphasize the modifications found in organisms.
3.
Compare the countercurrent mechanism in fish gill oxygenation with the same concept found in the human
kidney.
4.
There is an overall principle in excretory systems of pushing out most fluids and then reabsorbing the
useful nutrients and water. Use this as a “running theme” as flame cells, Malpighian tubules, and nephrons
are described.
5.
Students may recoil at old concepts of using warm urine to melt earwax or trapped miners surviving on
urine. This is a place to emphasize that, in contrast to the digestive system, the urinary system is a filtered
and sterile system unless there is a urinary tract infection. The first human trials on the antibiotic penicillin,
produced in small quantities, involved extracting the penicillin in the patient’s urine and recycling it.
“Smelly-urine” restrooms are the result of bacterial action on urine waste molecules after the urine has
exited the body.
6.
Birds must remain lightweight and cannot store large quantities of heavy water. Lecture question: What is
the likely system in birds? Note the variation in toxicity and solubility of ammonia, urea, and uric acid
pastelike wastes.
7.
The variations in length of the urethra is related to the ease of entry of bacterial infections; students should
be able to speculate on whether males or females are more prone to bladder/kidney infections.
8.
Students may confuse “ureter” and “urethra” due to their similarity; a phonics distinction is that “ureters”
sounds correct in the plural but “urethras” does not.
9.
Textbook and overhead outlines of sagittal sections of the pelvic organs usually illustrate the substantially
smaller space allotted for the female urinary bladder, thus accounting for a very real difference in mileage
between bathroom stops during long-distance trips. Lecture question: What is likely to happen in “bladder
space” after a hysterectomy?
10.
On occasion, a person develops four kidneys in the same space as two; however, this means there are twice
as many renal arteries and veins. Ask what the consequence would be in processing time for a soft drink,
for instance. This is a common enough anomaly that a large lecture class is very likely to have one person
with four kidneys, although they may not know it. Why is this so common, from an evolutionary
perspective? But, if it is in fact advantageous, why do we not all have four kidneys?
11.
Lecture question: Why was the kidney one of the first organs to be successfully transplanted? (Answer: It
is encapsulated and has a limited number of connections (artery, vein, ureter). )
12.
Have students keep an approximated record of their fluid intake and urinary output for a 24-hour period and
explain any discrepancies between the two values.
13.
Albumin is a large protein molecule in blood plasma; students will relate to it and its “thickness” as a
molecule in simple raw egg white.
14.
Lecture question: Why must a person continue to take tablets or capsules for a medication to remain in the
blood at levels to be effective? The rate at which a medication is removed is one of the critical items in
drug research, and is the basis for the prescription line: twice-a-day, once-a-day, etc. The faster the kidney
removes the drug from the blood, the more we have to re-dose.
15.
The concept of “countercurrent” is not readily apparent because there are no general visible experiences
that operate as countercurrents in daily life. Therefore, it is necessary to walk through the two reversed
gradients and show the effect with and without the countercurrent process.
16.
Many blood pressure medicines act through changing the blood volume and amount of urine produced. At
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the end of this section, students should be able to deduce why. A diagram showing the kidneys tapping into
a major portion of the systemic circulation may also help—the kidneys intercept approximately 20% of
blood flow.
17.
Hemodialysis involving artificial mechanisms is not effective in removing all metabolic waste molecules.
Lecture question: Why do patients on dialysis machines have to adhere to limited diets?
18.
Chapter concepts here should now make evident what a physician is looking for with urine tests for sugar,
protein, etc.
Critical Thinking
Question 1. Why do birds produce a white paste in bird feces, and why is it so damaging to car paint?
Answer: Birds have a common opening for digestive, excretory and reproductive systems. The white pastelike
material is uric acid, an excretory product that is combined with feces before it is eliminated. The uric acid allows
nontoxic storage and conserves water for the bird that cannot store large reserves of heavy water and fly. Uric acid is
acidic, and therefore caustic to paint.
Question 2. If a diuretic such as tea decreased reabsorption of water from 99% to 98%, what would happen to urine
volume?
Answer: Urine volume would double. Previous urine represented 1% after 99% is reabsorbed; a drop to 98%
leaves 2% or double the product.
Question 3. How are kidneys related to blood pressure?
Answer: Nephrons regulate the water-salt balance of the blood. The more salts in the blood, the greater the
diffusion of water into the blood; the greater blood volume requires the heart to work harder and causes greater
blood pressure.
Question 4. Tea is an excellent diuretic; if you drink one cup of tea, you will probably urinate several times that in
water. Why does a small change in the percentage of water reabsorbed produce a large change in the volume of
urine produced?
Answer:
If 99% of the water is being reabsorbed, the urine constitutes the 1% not reabsorbed. If reabsorption
drops 1% to 98% the urine will constitute 2%, or double the urine production.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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