AP BIOLOGY NOTES ON CHAPTER 44 – OSMOREGULATION AND EXCRETION
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OSMOREGULATION BALANCES THE UPTAKE AND LOSS OF WATER AND SOLUTES
Osmoregulation – is the process by which animals control solute concentrations and
balance water gain and loss
Review what happens with cells when they are put into hypotonic, hypertonic and isotonic
solutions.
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Osmoregulation is a homeostatic process
Review homeostasis
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Most metabolic wastes must be excreted from the body because they are poisonous for us.
One of the most important types of waste is nitrogenous waste from the breakdown of
proteins and nucleic acids.
Excretion includes the removal of nitrogenous wastes and other harmful materials from the
body.
Osmoregulation is largely based on the intake and release of water, because water follows
the solute by osmosis. (So water always flows from the less salt concentrated area to the
more salt concentrated area).
Animals can maintain their water balance in two ways:
o Osmoconformers – are isotonic with their environment. These are marine animals
that have the same concentration of salts and other dissolved substances as the sea
water. As a result, they can easily keep their water concentration. However, they
need a different concentration of various ions (Na, Ca, Cl, Mg etc.) than the water
has, they must actively transport these ions into their body to maintain
homeostasis.
o Osmoregulators – some marine bony fish (cod), fresh water fish and terrestrial
animals must regulate their water concentration in various ways:
 Marine osmoregulators – bony fish – they constantly lose water by osmosis
into the sea water. They balance their water by drinking lots of sea water
and using their gills and kidneys to get rid of the excess salt. The gills for
example have special chloride cells that actively transport chlorine out and
Na ions passively follow.
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II.
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Freshwater animals – These animals are more concentrated than their
environment (hypertonic animals). As a result, they constantly gain water
by osmosis and lose ions by diffusion. As a solution, freshwater fish drinks
almost no water at all and excretes lots of very dilute urine. Salts are
replenished by eating or taking in salts through their gills. To take in salts,
they use chloride cells to actively transport Cl ions and sodium ions follow
passively.
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Land animals – Water loss (dehydration) is a major issue for land animals.
They must come up with a wide range of evolutionary adaptations to
preserve water and reduce water loss – water resistant outer coverings,
nocturnal lifestyle, lungs are inside of the body unlike gills, specialized
excretion methods. Despite these adaptations, land animals need to drink
water and eat moist food or producing water metabolically.
In most animals, osmotic regulation and metabolic waste disposal rely on different types of
transport epithelium – layers of specialize epithelial cells that regulate solute movement.
These cells generally have large surface area and are arranged into complex tubular
networks, connected to the outside environment directly or by channels.
ANIMALS ‘ NITROGENOUS WASTES REFLECT THEIR PHYLOGENY AND HABITAT
When amino acids and nucleotides break down, ammonia (NH3) is produced from the amino
group and other nitrogen containing functional groups.
Ammonia is very toxic, because it can change the pH of blood, interfere with oxidative
phosphorylation, and disturb the osmotic balance of the body.
Depending on their origin and lifestyle, animals excrete the nitrogenous waste in different
forms:
o Ammonia – because it is water soluble, aquatic animals can release ammonia with
lots of water, without being hurt by the increased water loss during the process.
This also requires less energy because the waste ammonia does not need to be
converted to any other substance in the body. In fish, most of the ammonia is
released through the gills.
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III.
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Urea – mammals, most adult amphibians, sharks, some marine body fishes, turtles
excrete their nitrogenous waste as urea. It is produced in the liver from ammonia
+CO2. Because it is not very toxic, urea can be transported through the circulatory
system in higher concentrations and excreted with less water. This way, dry land
organisms can preserve water. The disadvantage of excreting urea is that it an
energy requiring process to produce it.
o Uric acid – land snails, reptiles, birds excrete nitrogenous waste in the form of uric
acid. It is not toxic and does not dissolve in water. As a result, the excretion of
these organisms is paste-like. However, making uric acid is even more energy
requiring.
Humans also release some uric acid, especially if their food contains too much animal tissue
– gout is a painful inflammation of the joints when uric acid crystals deposit there.
In general, the form of waste excreted is related to the evolutionary history of the
organisms and the environment that they live in, mostly the water availability and the
reproductive strategy are the defining factors.
DIVERSE EXCRETORY SYSTEMS OF ANIMALS – VARIATIONS ON THE TUBULAR THEME
The excretory system of animals is responsible for maintaining water balance, proper fluid
composition and getting rid of metabolic wastes.
A. General Overview of The Process of Excretion:
 The fluid waste that is released from most animals is urine. To produce it, four basic
processes occur:
o Filtration – The excretory tubule filters the blood and collects a filtrate. Water and
various dissolved substances are forced through a selectively permeable membrane
of a cluster of capillaries by the blood pressure. This filtrate is collected into the
excretory tubule.
o Reabsorption – In the excretory tubule the transport epithelium takes back valuable
substances from the filtrate into the blood stream or other body fluids. The type
and volume of reabsorbed substances changes depending on the needs of the
organism.
o Secretion – Other unnecessary substances such as toxins and excess ions are
extracted from the body fluids and added to the filtrate in the excretory tubule.
o Excretion – The altered filtrate leaves the system and the body.
B. Survey of Excretory Systems of Animals:
 Simpler animals: Don’t need an excretory system. Waste, ions, water are all released by
diffusion throughout the body.
 Protonephridia – excretory system found in flatworms (Platyhelminthes). This is a set of
dead-end tubules that branch throughout the body. Cellular bodies called flame bulbs cap
the branches of each protonephridium. During filtration the cilia on the flame bulb beats
and with this it draws water and solute into the bulb from the interstitial fluid (fluid
between the cells). The filtrate then travels along a tube system, some substances get
reabsorbed from the tube and the rest is excreted as urine into the environment.
Protonephridia are also found in some annelids and mollusk larvae.
http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf
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Metanephridia – Most annelids have this excretory system. Metanephridia start open in the
coelom with a ciliated funnel around the opening. As the cilia beat, fluid from the coelom flows
into a collecting tubule which ends in a storage bladder. The storage bladder opens to the
outside. Every segment of an annelid has a pair of metanephridia. Because earthworms and
other annelids that live in damp soil, constantly take in water, metanephridia excrete dilute
urine to compensate for the water intake.
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Malphigian tubules – Found in terrestrial arthropods, used to remove nitrogenous waste and
act in osmoregulation. Start with a branch of dead-end tubes that are immersed into
hemolymph (“blood” of organisms with open circulation). Once these tubes absorb water and
waste, it is released into the digestive system. Reabsorption occurs from the digestive system.
Because most of the water is reabsorbed, their urine is paste-like. With this process arthropods
preserve lots of water – adaptation to dry land.
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Kidneys – Used in most chordates and all vertebrates. These organelles function in both
osmoregulation and excretion. See next section for details.
IV.
THE STRUCTURE OF THE MAMMALIAN EXCRETORY SYSTEM:
 The human excretory system has the following organs:
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Kidneys – the main organ of excretion (all processes of filtration, secretion and
reabsorption takes place here)
Renal artery – carries waste, water, salts and O2 to the kidneys. The filtrate forms from
the blood in this vessel.
Renal vein – carries the reabsorbed materials and the rest of the blood away – CO2 rich
blood
Ureter – a pair of tubes, urine exits the kidneys through them
Urinary bladder – stores urine until it is expelled. Sphincter muscle closes it to the
urethra.
Urethra – shorter single tube that empties urine into the environment.
The structure of the kidneys:
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Composed of the outer renal cortex and the inner renal medulla
The renal pelvis connects the kidneys to the ureter
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The functional unit of excretion is the nephron which is microscopic:
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/an/m9/s3/index.htm -- slide show
http://www.youtube.com/watch?v=aQZaNXNroVY – animation
You need to be able to draw and label the structure of the nephron
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The nephron is made up of the a single ball of capillaries called the glomerulus, a blind end of a
set of tubules that surrounds the glomerulus called the Bowman’s capsule and a single long
tubule.
Each kidney has about a million nephrons with a total tubule length of about 80 km.
Filtration occurs, as blood pressure forces fluid from the blood in the glomerulus into the lumen
of the Bowman’s capsule. The epithelium of the glomerulus is porous, so it allows salts, amino
acids, glucose, vitamins, nitrogenous waste, water and other small molecules to leave the blood.
The filtrate is very similar to blood plasma.
Each nephron is supplied by an afferent arteriole that is an offshoot of the renal artery. This
arteriole pushes the blood into the nephron before divides up to capillaries in the nephron. The
efferent arteriole collects the blood from each glomerulus (because this is always narrower than
the afferent arteriole, the blood plasma is squeezed through the capillaries into the Bowman's
capsule).
After the filtrate has formed, it passes into the proximal tubule, the first major region of the
nephron. The next is the loop of Henle, which is the hairpin-shaped turn of the nephron. The
last region of the nephron is called the distal tubule, which empties its content to the collecting
duct. The collecting duct flows into the renal pelvis than into the ureter.
Daily about 1600 L of blood is filtered through the kidneys, 180 L of filtrate forms, but this only
becomes about 1.5 L of urine a day.
http://argosymedical.com/Urinary/samples/animations/Urine%20Formation/ --- the entire process
http://interactivehuman.blogspot.com/2008/06/animation-kidney-parts-of-nephron.html -- more detailed.
V. FROM BLOOD FILTRATE TO URINE -- DETAILED LOOK
 In this section we will follow the filtrate along its path and examine what gets reabsorbed and
secreted at each section of the nephron.
A. Proximal Tubule:
 Reabsorption here recaptures important ions, water and nutrients such as amino acids and
glucose. Na+ ions are reabsorbed by active transport, while Cl- ions passively follow, as well as
water follows the sodium ions by osmosis. Glucose and amino acids are also reabsorbed by
active transport, while K+ ions are reabsorbed by passive transport
 Secretion also occurs here. Cells of the transport epithelium secrete H+ ions (by active
transport) and ammonia, to regulate the pH of body fluids.
B. The Loop of Henle:
 The descending limb of the loop of Henle reabsorbs mostly water by osmosis with the help of
aquaporins. This process makes the filtrate very concentrated. The most concentrated filtrate
is found in the elbow (bottom) of the loop.
 The ascending (upward) limb of the loop of Henle does not contain aquaporins but contains a
high concentration of ion channels. As a result, this part of the loop is impermeable to water
but is very permeable to Na+ ions and Cl- ions. Which are absorbed first by passive than by
active transport. This process makes the filtrate more dilute.
 The flow of filtrate in the loop of Henle is an example of a countercurrent system, this allows
the kidney to form a concentrated urine with minimal water loss. As the filtrate moves down in
the descending loop, more water can be reabsorbed because the surrounding blood vessels
have higher and higher ion concentrations, but moving up in the ascending loop, the
surrounding vessels are less concentrated in ions so some can be reabsorbed without using
much energy to active transport. However, in the upper sections of the ascending loop, active
transport of Na+ is necessary.
C. Distal Tubule:
 In the distal tubule, the amount of NaCl reabsorbed and K+ ions secreted depends on the needs
of the body. Also various ions can be reabsorbed or secreted to regulate the pH again
depending on the needs and H+ levels in the body.
D. Collecting Duct:
 Salt and water can live the duct depending on the needs of the body (hormonal control). This
duct leads the urine into the renal pelvis.
VI. HORMONAL CIRCUITS LINK KIDNEY FUNCTION, WATER BALANCE, AND BLOOD PRESSURE:
 The mammalian kidney is regulated by a combination of nervous and hormonal controls.
A. Antidiuretic Hormone (ADH)
 Antidiuretic hormone is a protein hormone produced by the hypothalamus and stored in the
posterior pituitary gland. Osmoreceptors in the hypothalamus monitor the osmotic
concentration in the blood and make the endocrine cells of the hypothalamus release ADH
when necessary.
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If we ingest salty food or lose too much water, the blood becomes more concentrated. At this
point, ADH is released into the bloodstream and moves to the kidneys. As a result, more water
gets reabsorbed from the distal tubules and from the collecting ducts and the blood becomes
more dilute, the urine becomes more concentrated.
Mutations that result in not producing ADH or not having the right receptors in the kidneys to
detect ADH result in diabetes insipidus -- this disorder can cause severe dehydration and a large
volume of very dilute urine.
B. The Renin-Angiotensin-Aldosterone System (RAAS)
 This system involves a specialized tissue called juxtaglomerular apparatus, located in the
kidneys next to the afferent arterioles of nephrons. When the blood pressure drops in the
afferent arterioles, the juxtaglomerular apparatus releases and enzyme called renin.
 Renin activates a hormone called angiotensin II.
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Angiotensin II causes arterioles (small arteries) to constrict. This increase the blood pressure
back to normal. Angiotensin II also causes the adrenal glands (small hat-like structures on the
tip of the kidneys) to release another hormone called aldosterone.
Aldosterone (a steroid hormone) acts on the distal tubules and make them reabsorb more
sodium and more water. This increases the blood volume and as a result increases the blood
pressure.
THIS IS THE END OF THE UNIT