The Excretory System: Back To Top

11/25/21, 7:07 PM Excretory System
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
www1.biologie.uni-hamburg.de/b-online/library/onlinebio/BioBookEXCRET.html 1/10
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.
Image from W.H. Freeman and Sinauer Associates, used by

permission.

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.
Images from W.H. Freeman and Sinauer Associates, used by

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.
Images from W.H. Freeman and Sinauer Associates, used by

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.

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. Maintain volume of extracellular

fluid
2. Maintain ionic balance in extracellular
fluid
3. Maintain pH and osmotic concentration of the
extracellular fluid.
4. 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.
Text ©1992, 1994, 1997, 1998, 1999, 2000, M.J. Farabee, all
rights reserved. Use for educational purposes is
encouraged.
Back to Table of Contents
Email: mj.farabee@emcmail.maricopa.edu
Last modified: 2000/01/07:13:29:18
The URL of this page is: gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookEXCRET.html

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11/25/21, 7:07 PM Excretory System

THE EXCRETORY SYSTEM

The Human Excretory

Cells produce water and carbon dioxide as

Single-celled organisms have most of their wastes

Not all animals use the same routes or excrete

Nitrogen wastes are a by product of

Ammonia is very toxic and usually is excreted

The excretory system is responsible for regulating

Marine vertebrates, however, have internal

Terrestrial animals use a variety of methods to

1. Collect water and filter body

Many invertebrates such as flatworms use a

Image from W.H. Freeman and Sinauer Associates, used by

Image from W.H. Freeman and Sinauer Associates, used by

Body fluids are drawn into the Malphigian

Images from W.H. Freeman and Sinauer Associates, used by

ALL vertebrates have paired kidneys. Excretion is

The urinary system is made-up of the kidneys,

Images from W.H. Freeman and Sinauer Associates, used by

Image from W.H. Freeman and Sinauer Associates, used by

The nephron has three functions:

1. Glomerular filtration of water and solutes

Nephrons filter 125 ml of body fluid per minute;

1. Filtration in the glomerulus and nephron

Components of The Nephron

Glomerulus: mechanically filters

In some cases, excess wastes crystallize as

Kidneys perform a number of homeostatic

1. Maintain volume of extracellular

Water reabsorption is controlled by the

the urine. When too

Infection, environmental toxins such as mercury,

Back to Table of Contents

Last modified: 2000/01/07:13:29:18

The URL of this page is: gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookEXCRET.html

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