Maintaining a Balance
Topic 16: The Kidneys
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Explain why the removal of wastes is essential for continued
metabolic activity
 Distinguish between active and passive transport and relate
these to processes occurring in the mammalian kidney
 Explain why the processes of diffusion and osmosis are
inadequate in removing dissolved nitrogenous wastes in some
organisms
Removing Wastes
Any accumulation of wastes may be
toxic to cells and so metabolic
wastes must be removed from the
body to maintain homeostasis. If
wastes are not continuously
removed, their levels in the body
will increase and alter the
conditions in the internal
environment. This in turn inhibits
enzyme functioning and prevents
cells from undergoing normal
metabolic activity.
pettalk.com.au
Removing Wastes
The build up of nitrogenous wastes such as ammonia causes an
increase in pH in cells. This results in them becoming more
alkaline. The build up of carbon dioxide lowers pH which results
in the internal environment becoming more acidic. These
changes can slow down or inhibit enzyme functioning in
metabolism.
students.cis.uab.edu
The Excretory System
The excretory system is a group of organs that function together
to remove metabolic wastes from the tissues of an organism and
expel them to the outside. The kidneys are the main excretory
organs responsible for removing nitrogenous wastes from the
bodies of vertebrate animals, including fish and mammals.
emc.maricopa.edu
The Excretory System
The function of the kidney in
excretion is to filter the blood
that enters it, removing wastes
(in solution) from the
bloodstream so that they can be
excreted from the body.
This filtration is carried out in
millions of tiny units called
nephrons.
emc.maricopa.edu
The Excretory System
Urine is the excretory solution finally produced by these
microscopic tubules and it drains out of the kidneys, carrying
wastes out of the kidney via ducts, the ureters. In vertebrates,
the ureters lead to a urine storage organ, the bladder, which
passes urine via the urethra to the outside.
lurvely.com
The Excretory System
In other vertebrates, the ureters carry urine directly to a
chamber, the cloaca (the common opening of the urinary,
digestive and reproductive tracts), which empties to the outside.
backyardchickens.com
The Excretory System
Oxygenated blood arrives at the
kidney via the renal artery and
carries nitrogenous wastes. These,
together with water and other
solutes are filtered to form urine.
The kidney is then drained of its
fluids by two vessels:
 The renal vein, which carries
purified blood back into the
general circulation
 The ureter, which carries urine
out of the kidney.
kidney.org.au
The Excretory System
Animals use water to flush
metabolic wastes out of the body.
Excretion therefore brings with it
the problem of regulating the
amount of water lost, while
ensuring that wastes are removed.
The role of the kidney includes both
the excretion of nitrogenous wastes
and osmoregulation – maintaining
the water and salt (electrolyte)
balance in animals.
flickrhivemind.net
The Excretory System
The maintenance of a stable blood volume, blood pressure and
pH depends upon a stable water balance within a narrow range
in the body. The importance of conserving water varies in
different animals, depending on the environment which each
inhabits.
abc.net.au
The Excretory System
The main environments inhabited by living organisms are
freshwater, marine and terrestrial habitats. Each habitat presents
its own problems for organisms in terms of maintaining their
internal water and salt balance.
tolweb.org
The Excretory System
The kidneys of animals have
evolved to function in both
excretion and the removal or
conservation of water and salts,
maintaining optimal levels of
these substances within the
bodies of animals that live in
different environments.
fanpop.com
Active and Passive Transport
(Diffusion and Osmosis)
Movement of materials into and out of cells takes place either
passively or actively.
 Passive movement includes the processes of diffusion and
osmosis. These types of movement require no energy input
from the cell, since molecules move along a concentration
gradient.
 Active transport requires an
input of cellular energy to actively
move molecules against a
cumbavac.org
concentration gradient.
Passive Transport in the Kidney
Diffusion and osmosis results from the random movement of
particles, called Brownian motion, whereby particles continually
collide and move randomly. When they are in higher
concentration in one region, this constant movement slowly
results in an overall spreading out from the most concentrated
point, finally brining about their even distribution within a space.
okc.cc.ok.us
Passive Transport in the Kidney
The main limitations of passive movement are:
 it depends on the presence of a difference in concentration of
substances (a concentration gradient) between two regions
 it is relatively slow, especially when the concentration gradient
is not steep
bbc.co.uk
Passive Transport in the Kidney
Diffusion is the movement of any molecule from a region of high
concentration to a region of low concentration of that substance,
until equilibrium is reached. This does not require an energy
input.
notesforpakistan.blogspot.com
Passive Transport in the Kidney
Osmosis is the movement of water molecules from a region of
high water concentration to a region of low water concentration
through a selectively permeable membrane. This also does not
require energy.
hartnell.edu
Passive Transport in the Kidney
Within the kidney, the movement of substances between the
bloodstream and excretory fluid in the microscopic tubules
(called nephrons) involves both active and passive transport.
umm.edu
Passive Transport in the Kidney
A balance in the optimal
concentrations of blood
chemicals is maintained by the
selective excretion of wastes, as
well as any excess water and
salts in urine. Therefore the
ability of the kidney to alter the
urine concentration plays a vital
role in homeostasis.
urologyhealth.org
Passive Transport in the Kidney
Within the kidney tubules, there is a
two-way movement of substances:
 Waste substances pass from the
bloodstream into the kidney
tubules, to be excreted in urine
(filtration and secretion)
 Substances required by the body are
removed from the urine in the
kidney tubules (before it is
excreted) and returned to the
bloodstream (reabsorption)
cumbavac.org
Passive Transport in the Kidney
Passive transport moves water
(by osmosis), and some
nitrogenous wastes such as urea
and ammonia (by diffusion) in
the kidneys of mammals.
lupusqld.org.au
Passive Transport in the Kidney
In the kidney, only excess water
and salts are excreted.
Homeostasis requires that
sometimes water and salts
should be conserved to
maintain the required levels
within the body and at other
times they should be excreted.
Salt movement is via active
transport and this in turn draws
water by osmosis.
naturalguidetohealth.com
Active Transport in the Kidney
Active transport is the movement of molecules from an area of
low concentration to a region of high concentration, requiring an
input of energy.
cumbavac.oraustincc.edug
Active Transport in the Kidney
Sometimes in living organisms there is a need to move a
chemical against the concentration gradient, and to do this,
active transport occurs. This involves a carrier protein that spans
the membrane and this carrier molecule can actively move
chemicals from low to high concentration, utilising cellular
energy.
hyperphysics.phy-astr.gsu.edu
Active Transport in the Kidney
Active transport moves mainly sodium ions, glucose, amino acids
and hydrogen ions across the wall of the nephron.
 All glucose and amino acids are reabsorbed by kidney cells so
that they are not lost in urine and so they move against a
concentration gradient.
 Additional nitrogenous wastes (eg: uric acid) and hydrogen
ions (H⁺) are added to urine (from blood capillaries) in the
kidney tubules
Active Transport in the Kidney
 Additional nitrogenous wastes (eg: uric acid) and hydrogen
ions (H⁺) are added to urine (from blood capillaries) in the
kidney tubules
wikis.lib.ncsu.edu
Active Transport in the Kidney
 A ‘sodium pump’ mechanism
operates in the tubules of the
kidneys, actively transporting
ions (salts) from the urine back
into the kidney cells. Besides
conserving salts, this process
also brings about the
conservation of water within
the body – the active transport
of salts draws water out of the
urine, because water follows by
osmosis (passive transport).
Water is drawn by the osmotic
pull of the salts in solution.
cumbavac.org
How Effective are These Processes?
Diffusion:
The rate of movement is to slow to just rely on diffusion.
Nitrogenous wastes and toxins such as drugs and alcohol must be
dissolved in water to be removed. If their removal by the kidney
was dependent on diffusion only, wastes would be able to move
only if they were concentrated inside the cells or the
bloodstream than in the fluids outside. As their concentrations
begin to equalise, their movement would slow down and
eventually stop.
guardian.co.uk
How Effective are These Processes?
Diffusion:
Not all wastes can be removed
by diffusion. Since nitrogenous
wastes are toxic, it is essential
they are all removed. If
concentrations within the
blood and urine equalised and
no further wastes were
removed, their accumulation
would change the pH of cells
and become toxic.
leavingbio.net
How Effective are These Processes?
Osmosis:
Too much water may be lost in
urine. If urine contains a large
number of nitrogenous wastes in
solution, water will be drawn into
the urine by osmosis to dilute the
wastes and try to equalise the
concentration of fluid inside the
urine and in the surrounding
kidney. Excretion of dilute urine
means the loss of large amounts of
water from the body – something
many terrestrial animals cannot
afford.
modernmom.com
How Effective are These Processes?
Osmosis:
Movement of water may make waste too dilute for excretion by
diffusion. Organisms that live in freshwater environments have a
different problem. Osmosis results in water moving into the
body tissues from the surrounding environment. Although this
dilutes the toxic wastes in the body, it slows down their
excretion by diffusion (lowers the concentration gradient).
Therefore a mechanism is essential to remove wastes against a
concentration gradient.
australian-animals.net
Solution to the Problems
Active transport, which requires energy is quicker and more
effective than diffusion as it removes most wastes, even against a
concentration gradient. It can also be used to pump salts from
urine back into the kidney tissue and these in turn will draw
water with them (by osmosis), ensuring in this way that the
amount of water lost in urine does not affect the body’s water
balance.
humanphisiology.wikispaces.com
Activity
-Students to use ALARM to answer the following DOT Points:
 Explain why the removal of wastes is essential for continued
metabolic activity
 Distinguish between active and passive transport and relate
these to processes occurring in the mammalian kidney
 Explain why the processes of diffusion and osmosis are
inadequate in removing dissolved nitrogenous wastes in some
organisms