Septal Nephridia
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These lie attached to both sides of the septa behind the 15 th segment.
Septal Nephridia
Pharyngeal Nephridia
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These are located only in the 4th, 5th and 6th segment.
The septal nephridia may be considered as a typical nephridia for detailed description.
Parts of a Typical Nephridium
The Anatomy and Physiology of Animals/Excretory System
Worksheet/Worksheet Answers
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1.Some of these are not examples of homeostasis: (circle those that are not)
a. Sweating, erection of hairs and capillary dilation/contraction to control the body temperature
b. Adjusting the rate of breathing to remove carbon dioxide from the blood
c. Production of concentrated or dilute urine to maintain the concentration of water and salts in the blood
within a narrow range
d. Blood clotting to prevent loss of blood
e. Reproduction to produce the next generation. Note: this is the only option in this list that is not a
mechanism to maintain homeostasis.
f. The action of the hormone insulin to keep the level of glucose in the blood within a narrow range
g. White cells engulfing bacteria
h. Eating to supply the body with essential nutrients
i. Sense organs that allow the animal to sense and then remove itself from unsafe conditions
j. Various mechanisms to keep the pH of the blood within a narrow range
2. Fill in the left hand column with the terms/values from the list below.
66%
The proportion of the body’s water found within the cells.
Hypothalamus
The part of the brain concerned with controlling water balance.
15%
The proportion of the body’s water an animal will not survive losing.
Thirst
This is the main mechanism for diluting the blood.
80%
The proportion of an animal’s body that consists of water.
Kidneys
These organs are important for controlling water balance.
3. Add the following labels to the diagram below of the urinary system of a mammal.
4. These are functions of the kidney: (Circle 4)
b. Controlling the concentration of water in the blood
c. Removing urea from the blood
f. Keeping the blood at the right pH (acidity/alkalinity)
h. Controlling the concentration of salts like sodium and potassium chloride in the blood
5. Match the organ with the function in the table below.
Organ
Function
Urethra
Carries urine from the bladder to the outside of the body
Renal vein
Carries deoxygenated blood away from the kidney
Medulla
The inner region of the kidney
Sphincter
Muscle that opens to allow urine to be removed from bladder
Cortex
The outer region of the kidney
Renal artery
Carries oxygenated blood to the kidney
Renal pelvis
The part of the kidney that collects the urine before it passes down the ureter
Capsule
The tough fibrous coat around the kidney
Bladder
Stores urine before it is removed from body
Ureter
The tube that carries urine away from the kidney
Kidney
Converts blood to urine
6. Add the following labels to the diagram of a kidney below. If you like you can also colour in the diagram as
indicated. :capsule- turquoise; renal artery – red; renal vein – blue; cortex - brown; medulla - pink; pelvis
- yellow; ureter – green; pyramids - purple
7. Add the following labels to the diagram below of a kidney tubule or nephron.
collecting duct; branch of renal artery; loop of Henle; distal convoluted tubule; glomerulus;
proximal convoluted tubule; Bowman’s capsule
8. Arrange these parts of the kidney tubule in the order in which the fluid that is being converted into urine
passes through them.
Glomerulus(G)
Bowman’s
capsule
(BC)
Proximal
convoluted
tubule (PCT)
Loop of
Henle
(LH)
Distal
convoluted
tubule (DCT)
Collecting
duct (CD)
9. Indicate whether these parts of the kidney tubule are found in the cortex "C" or the medulla "M" of the
kidney.
Bowman’s capsule C
Collecting duct M
Proximal convoluted tubule C
Distal convoluted tubule C
Glomerulus C
Loop of Henle M
10. Match the term with its function.
Term
Function
Renal artery
Carries blood to the kidney
Antidiuretic hormone or
ADH
The hormone that is involved in producing concentrated urine
Bowman’s capsule
Cup shaped structure through which the fluid part of the blood is filtered
Collecting duct
Where the majority of water is extracted from the urine
Loop of Henle
Looped portion of the tubule. Important for helping concentrate the urine
Distal convoluted tubule
Where hydrogen and potassium ions are secreted into the urine
Proximal convoluted tubule
Glucose, salts, water and amino acids are reabsorbed into the blood
capillaries here
Glomerulus
Tuft of capillaries carrying high pressure blood
11. Circle the substances in the list below that are NOT found in the fluid that has filtered through into the
Bowman’s capsule of a healthy animal?
Red blood cells; proteins; white blood cells These are large and do not pass through the holes in the
walls of the glomerulus and Bowman's capsule
12. Circle the substances in the list below that are NOT found in the urine of a healthy animal?
Antidiuretic hormone; red blood cells; glucose; proteins; white blood cells
13. Fill in the blanks in the statements below about antidiuretic hormone (ADH).
a. ADH is produced by the posterior pituitary gland/hypothalamus
b. ADH acts on the walls of the collecting ducts of the kidney tubule.
c. When no ADH is produced an animal is said to have the condition known as diabetes insipidus
Now cross out the option that does not apply in each of the statements below:
d. ADH makes the walls of the collecting ducts permeable to water.
e. ADH is secreted when the blood becomes too concentrated.
f. When an animal is dehydrated lots of ADH is produced.
g. When ADH is secreted the urine produced is concentrated.
h. The main symptom of a failure to produce ADH would be lots of dilute urine.
14. Normally all the glucose filtered into the kidney tubule is absorbed further down the tubule. If glucose is
found in the urine what might one suspect to be the cause? Problems with production of insulin by the pancreas
i.e. diabetes mellitus.
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_Answers"
THE EXCRETORY SYSTEM
Table of Contents
Regulation of Extracellular Fluids | Nitrogen Wastes | Water and Salt Balance
Excretory System Functions | Invertebrate Excretory Organs | Vertebrates Have Paired Kidneys
The Human Excretory System | Kidney Function | Hormone Control of Water and Salt
Disruption of Kidney Function | Links
Cells produce water and carbon dioxide as by-products of metabolic breakdown of sugars, fats, and
proteins. Chemical groups such as nitrogen, sulfur, and phosphorous must be stripped, from the
large molecules to which they were formerly attached, as part of preparing them for energy
conversion. The continuous production of metabolic wastes establishes a steep concentration
gradient across the plasma membrane, causing wastes to diffuse out of cells and into the
extracellular fluid.
Single-celled organisms have most of their wastes diffuse out into the outside environment.
Multicellular organisms, and animals in particular, must have a specialized organ system to
concentrate and remove wastes from the interstitial fluid into the blood capillaries and eventually
deposit that material at a collection point for removal entirely from the body.
Regulation of Extracellular Fluids | Back to Top
Excretory systems regulate the chemical composition of body fluids by removing metabolic wastes
and retaining the proper amounts of water, salts, and nutrients. Components of this system in
vertebrates include the kidneys, liver, lungs, and skin.
Not all animals use the same routes or excrete their wastes the same way humans do. Excretion
applies to metabolic waste products that cross a plasma membrane. Elimination is the removal of
feces.
Nitrogen Wastes | Back to Top
Nitrogen wastes are a by product of protein metabolism. Amino groups are removed from amino
acids prior to energy conversion. The NH2 (amino group) combines with a hydrogen ion (proton) to
form ammonia (NH3).
Ammonia is very toxic and usually is excreted directly by marine animals. Terrestrial animals
usually need to conserve water. Ammonia is converted to urea, a compound the body can tolerate at
higher concentrations than ammonia. Birds and insects secrete uric acid that they make through
large energy expenditure but little water loss. Amphibians and mammals secrete urea that they form
in their liver. Amino groups are turned into ammonia, which in turn is converted to urea, dumped
into the blood and concentrated by the kidneys.
Water and Salt Balance | Back to Top
The excretory system is responsible for regulating water balance in various body fluids.
Osmoregulation refers to the state aquatic animals are in: they are surrounded by freshwater and
must constantly deal with the influx of water. Animals, such as crabs, have an internal salt
concentration very similar to that of the surrounding ocean. Such animals are known as
osmoconformers, as there is little water transport between the inside of the animal and the isotonic
outside environment.
Marine vertebrates, however, have internal concentrations of salt that are about one-third of the
surrounding seawater. They are said to be osmoregulators. Osmoregulators face two problems:
prevention of water loss from the body and prevention of salts diffusing into the body. Fish deal
with this by passing water out of their tissues through their gills by osmosis and salt through their
gills by active transport. Cartilaginous fish have a greater salt concentration than seawater, causing
water to move into the shark by osmosis; this water is used for excretion. Freshwater fish must
prevent water gain and salt loss. They do not drink water, and have their skin covered by a thin
mucus. Water enters and leaves through the gills and the fish excretory system produces large
amounts of dilute urine.
Terrestrial animals use a variety of methods to reduce water loss: living in moist environments,
developing impermeable body coverings, production of more concentrated urine. Water loss can be
considerable: a person in a 100 degree F temperature loses 1 liter of water per hour.
Excretory System Functions | Back to Top
1. Collect water and filter body fluids.
2. Remove and concentrate waste products from body fluids and return other substances to
body fluids as necessary for homeostasis.
3. Eliminate excretory products from the body.
Invertebrate Excretory Organs | Back to Top
Many invertebrates such as flatworms use a nephridium as their excretory organ. At the end of each
blind tubule of the nephridium is a ciliated flame cell. As fluid passes down the tubule, solutes are
reabsorbed and returned to the body fluids.
Excretory system of a flatworm. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Excretory system of an earthworm. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Body fluids are drawn into the Malphigian tubules by osmosis due to large concentrations of
potassium inside the tubule. Body fluids pass back into the body, nitrogenous wastes empty into the
insect's gut. Water is reabsorbed and waste is expelled from the insect.
Excretory system of an ant. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Vertebrates Have Paired Kidneys | Back to Top
ALL vertebrates have paired kidneys. Excretion is not the primary function of kidneys. Kidneys
regulate body fluid levels as a primary duty, and remove wastes as a secondary one.
The Human Excretory System | Back to Top
The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an
evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from
the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and collects in
the bladder. The bladder can distend to store urine that eventually leaves through the urethra.
Human excretory system and the details of the kidney. Images from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
The Nephron
The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long
renal tube. Blood flows into the kidney through the renal artery, which branches into capillaries
associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter
into the capsule. Fluid flows through the proximal tubule, which include the loop of Henle, and
then into the distal tubule. The distal tubule empties into a collecting duct. Fluids and solutes are
returned to the capillaries that surround the nephron tubule.
Filtration of the blood in the fine structure of the kidneys. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
The nephron has three functions:
1. Glomerular filtration of water and solutes from the blood.
2. Tubular reabsorption of water and conserved molecules back into the blood.
3. Tubular secretion of ions and other waste products from surrounding capillaries into the
distal tubule.
Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times
each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are
reabsorbed. The remaining 1.5 liters forms urine.
Urine Production
1. Filtration in the glomerulus and nephron capsule.
2. Reabsorption in the proximal tubule.
3. Tubular secretion in the Loop of Henle.
Components of The Nephron





Glomerulus: mechanically filters blood
Bowman's Capsule: mechanically filters blood
Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids
Loop of Henle: Countercurrent exchange, which maintains the concentration gradient
Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.
Kidney Stones
In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful
irritant that may require surgery or ultrasound treatments. Some stones are small enough to be
forced into the urethra, others are the size of huge, massive boulders (or so I am told).
Kidney Function | Back to Top
Kidneys perform a number of homeostatic functions:
1.
2.
3.
4.
Maintain volume of extracellular fluid
Maintain ionic balance in extracellular fluid
Maintain pH and osmotic concentration of the extracellular fluid.
Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.
Hormone Control of Water and Salt | Back to Top
Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback. ADH is
released from the pituitary gland in the brain. Dropping levels of fluid in the blood signal the
hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to increase water
absorption in the kidneys. This puts more water back in the blood, increasing the concentration of
the urine. When too much fluid is present in the blood, sensors in the heart signal the hypothalamus
to cause a reduction of the amounts of ADH in the blood. This increases the amount of water
absorbed by the kidneys, producing large quantities of a more dilute urine.
Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron
to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing
more sodium to pass from the nephron to the blood. This causes water to flow into the blood by
osmosis. Renin is released into the blood to control aldosterone.
Disruption of Kidney Function | Back to Top
Infection, environmental toxins such as mercury, and genetic disease can have devastating results
by causing disruption of kidney function. Many kidney problems can be treated by dialysis, where
a machine acts as a kidney. Kidney transplants are an alternative to dialysis.
Links | Back to Top


Urea Molecular model and information about the inorganic synthesis of this molecule.
The Fluid Regulation System NASA article (with illustrations) about fluid regulation and how it relates
to space.