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Topic 18-Honors Biology-Mr.Dans
Excretory Systems: Excretion is the removal of any product
of metabolism (carbon dioxide, oxygen, ammonia, water
etc….)
 Unicellular Organisms & Hydra
Why is the picture above considered excretion?
 Earthworm:
Nephridia & Nephrostomes
 Grasshopper: Malpighian Tubules
Human Excretory System:
 Are human lungs part of the excretory system? Why
or why not?
 Organs/Glands of the excretory system: Kidneys,
lungs, skin (sebaceous/sudoriferous/ceruminous
glands), goblet cells (these are considered glands
themselves), lacrimal glands, intestinal glands, liver,
pancreas, salivary glands. Also all cells in the body
have excretory functions.
 Accessory Organs of Excretion: ureters, urinary
bladder, urethra, gall bladder
 Ammonia, Uric Acid and Urea: What are they and
where do they come from?
 Functions of Human Excretory System:
Removal of protein metabolism waste, regulates
level of water in body, electrolytes (substances
whose solution conducts electricity…think ions) in
blood, elimination of foreign substances like drugs,
maintenance of proper pH, releasing of certain
hormones, temperature regulation (by sweat).
Structure of Urea
Urea is the chief nitrogenous waste of mammals.
Most of our nitrogenous waste comes from the breakdown of
amino acids.
This occurs by deamination.
Deamination of amino acids results in the production of
ammonia (NH3).
Ammonia is an extremely toxic base (why is it a base?) and its
accumulation in the body would quickly be fatal.
However, the liver converts the ammonia (and carbon dioxide) into
urea.
This is called the urea cycle.
Although our bodies cannot tolerate high concentrations of urea, it is
much less poisonous than ammonia.
Urea is removed efficiently by the kidneys.
Uric Acid
Uric acid
Humans also excrete a second nitrogenous waste, uric acid. It is the
product of nucleic acid, not protein, metabolism.
Uric acid is only slightly soluble in water and easily precipitates out of
solution forming needlelike crystals. These
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contribute to the formation of kidney stones;
produce the excruciating pain of gout when deposited in the joints.
Curiously, our kidneys reclaim most of the uric acid filtered at the
glomeruli. Why, if it can cause problems?
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Uric acid is a potent antioxidant and thus can protect cells from
damage by oxygen free radicals.
The concentration of uric acid is 100-times greater in the cytosol
than in the extracellular fluid. So when lethally-damaged cells
release their contents, crystals of uric acid form in the vicinity.
These enhance the ability of nearby cells to "present" any antigens
released at the same time to T cells leading to a stronger immune
response.
So the risk of kidney stones and gout may be the price we pay for these
protections.
(NOT ON TEST: Most mammals have an enzyme — uricase — for
breaking uric acid down into a soluble product. However, during the
evolution of great apes and humans, the gene encoding uricase became
inactive. A predisposition to gout is a human legacy.)
Uric acid is the chief nitrogenous waste of
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insects
lizards and snakes (reptiles)
birds
(It is the whitish material that birds leave on statues.)
These animals convert the waste products of protein metabolism — as
well as nucleic acid metabolism — into uric acid.
Because of its low solubility in water, these animals are able to eliminate
waste nitrogen with little loss of water and be able to allow for the
development of the embryo within an egg without the urea or ammonia
killing the embryo.
The formation of uric acid does involve more energy but it is
advantageous.
 The Kidney &
 the Nephron: a. Anatomy of b. Physiology of
 The Effective Glomerular Filtration Rate:
Approx. 1.2 liters of blood passes through the kidneys per
minute. If the average total blood volume is approx. 5
liters how many times does your entire blood volume get
filtered per day?
Answer: A day contains 1440 minutes. Each minute is equivalent to 1.2 liters for a grand
total of 1728 liters a day.
1728/5=345 times a day
 Reabsorption:
 Secretion:
 What hormones play a role in excretion and the
regulation of blood pressure(see further on)?
 Examine the information concerning “skin” in your
pamphlet.
The Countercurrent Concentration of Urine:
As taught in class, countercurrent means when two
things are flowing in opposite directions. In
thermoregulation a countercurrent heat exchange
exists in organisms whose blood vessels lie at close
proximity to each other.
Notice in the diagram above that cold blood from
extremities is warmed by the warm blood coming from
the core of the body (especially the liver).
A countercurrent mechanism is also at work at the
kidneys and provides an explanation in which the kidneys
form an osmotically concentrated urine.
The mechanism depends on:
1. The permeability of the tubules and capillaries
2. The overall structure of the loop of Henle
3. The active transport of sodium ions
4. The concentration gradient in the renal medulla
5. The fact that fluid flows “down” the descending loop
and then “up” the ascending loop (this fact is how
the name received it’s ‘countercurrent name”)
The function of the loop in the nephron is to
generate a concentration gradient. The descending
loop does NOT actively transport any substance out
of it, including sodium ions. However sodium ions
may enter it through passive transport.
The descending limb is highly permeable to water,
which moves out of the loop by osmosis (due to the
hypertonic environment outside the loop in the
renal medulla).
As the filtrate moves down this loop it loses water
passively and therefore becomes more and more
concentrated.
The filtrate suddenly makes a turn upward through
the ascending portion of the loop (which runs
parallel to the descending loop). Water should have
a tendency to enter the ascending loop by osmosis
but it can’t because the ascending loop is
impermeable to water.
The ascending limb actively transports Na+ out. The
filtrate then becomes less and less concentrated as it
moves “up” the ascending loop approaching the
distal convoluted tubule.
The Na+ and water in the medulla is reabsorbed by
the vasa recta (peritubular capillaries).
Hormones/Enzymes:
Aldosterone(made by cortex of adrenal glands): increases
the amount of sodium that’s reabsorbed into the blood by
the distal tubule. When sodium is reabsorbed, water
follows, so water is reabsorbed as well.
ADH (antidiuretic Hormone, made by posterior pituitary,
also called vasopressin): causes the walls of the collecting
duct/tubule to become permeable to water which is then
taken up by osmosis into the blood (remember that as water
leaves the duct through into the blood the urine becomes
more concentrated) If low levels of ADH then urine becomes
more dilute. In the absence of ADH the walls are
impermeable to water so that water can’t leave the
collecting duct. ADH levels are high when the body is
dehydrated.
Renin (made by kidney): is really an enzyme that causes
vasoconstriction through a series of steps when blood
pressure is low. Renin causes the production of a local
hormone called angiotensin II. Vasoconstriction causes
blood pressure to rise.
Angiotensin II (made throughout body by cells): causes the
adrenal cortex to release aldosterone. Aldosterone causes
reabsorption of sodium (see above) which then increases
blood volume. An increase in blood volume causes an
increase in blood pressure (as a general rule; when blood
volume increases blood pressure has a tendency to
increase).
Please refer to class notes/textbook for further details.