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Osmoregulation and Disposal of Metabolic Wastes
Lecture Outline
I.
Excretory systems help maintain homeostasis
A. Body fluids are collected by excretory systems, and the composition is
adjusted by selective excretion of products
B. Elimination is the process of passing materials that have never been absorbed
C. Excretion is the process of passing materials that have been part of a metabolic
process
II.
Metabolic waste products are water, carbon dioxide, and nitrogenous wastes
A. Nitrogenous wastes include ammonia, uric acid, and urea
1. Ammonia is a result of deamination of amino acids
2. Ammonia is very toxic and must be excreted rapidly or converted into a
less toxic form such as uric acid or urea
B. Uric acid forms crystals and is excreted in a relatively dehydrated form
1. Uric acid is the primary excretory compound in terrestrial animals such as
insects, some reptiles, and birds
2. Uric acid is important in egg-laying terrestrial vertebrates
C. Urea is excreted by amphibians and mammals
1. The liver synthesizes urea in the urea cycle
2. Urea is highly soluble, and a relatively high amount of water is needed for
its storage and excretion
III.
Invertebrates have adaptations for osmoregulation and metabolic waste disposal
D. Most marine invertebrates are osmotic conformers
1. The marine environment is stable, and the salinity does not vary
significantly in most areas
2. Simple organisms pass wastes by diffusion and have no specialized
structures
3. Organisms that live in estuaries are often osmotic regulators with
specialized excretory structures
4. Terrestrial organisms have excretory structures that conserve water
E. Nephridial organs are specialized for osmoregulation and/or excretion
1. Nephridia are tubules that open to the outside of the animal
2. Flatworms, nemerteans, rotifers, some annelids, and lancelets have
protonephridia, which are blind tubes with ciliated flame cells
3. Metanephridia are seen in the majority of annelids and mollusks
a) Metanephridia are open at both ends, have cilia, and are highly
vascularized
b)
Metanephridia process coelomic fluid and can produce a concentrated
waste product
F. Malpighian tubules are an important adaptation for conserving water in
insects
1. Insects and spiders have Malpighian tubules, which are blind tubes that
process the hemolymph and dump into the intestine
2. Uric acid is the primary waste product, which conserves water
IV.
Osmoregulation and metabolic waste disposal in vertebrates
G. Freshwater animals must rid themselves of excess water
1. The body fluids of freshwater fish are more concentrated than their
medium
2. Freshwater vertebrates must actively transport salts from the water into the
body
a) Fish have cells in their gills that transport salts into the body
b) Freshwater amphibians have cells in their skin that transport salts into
the body
H. Marine vertebrates must replace lost fluid
1. Bony marine fish are hypotonic to their medium
a) Bony fish have cells in their gills that excrete salts, and they have very
few or no glomeruli in their kidneys
2. Chondrichthyean fish have high concentrations of urea and are hypertonic
to their medium
a) Chondrichthyean fish have well-developed kidneys
b) A specialized rectal gland excretes excess salts
3. Marine mammals produce a very concentrated urine, particularly
important in carnivorous marine mammals
I. Terrestrial vertebrates must conserve water
1. Adult amphibians excrete urea and reabsorb some water from the urinary
bladder
2. Amniotes have a skin that minimizes water loss, and many excrete uric
acid, which requires little water
3. Birds conserve water by excreting uric acid
4. The skin, lungs, and digestive system also aid in fluid balance and
excretion of wastes
V.
The kidneys, urinary bladder, and their ducts make up the urinary system
J. The outer portion of the kidney is the cortex; the inner is the medulla
1. The medulla is composed of 8-10 pyramids, each with a renal papilla at the
apex, where the collecting ducts empty
2. The urine flows from a papilla to the renal pelvis
3. The ureters carry urine to the urinary bladder, which is a distensible,
muscular sac
4.
During urination, urine flows through the urethra
a) In males, the urethra also carries semen
K. The nephron is the functional unit of the kidney
1. Human kidneys consist of several million nephrons
2. The Bowman’s capsule collects fluid from the glomerulus
3. The renal tubule can be divided into the proximal convoluted tubule, loop
of Henle, and distal convoluted tubule
4. The distal convoluted tubule empties into the collecting duct
5. Blood from the arterial system of the kidney enters the nephron via the
afferent arterioles
a) The glomerulus is a capillary bed within the Bowman’s capsule
6. The efferent arterioles carry blood away from the glomerulus and is
smaller in diameter than the afferent arterioles, producing a higher
pressure within the glomerulus
7. The efferent arteriole branches into a second set of capillaries, the
peritubular capillaries, which surround the renal tubules
a) The peritubular capillaries receive reabsorbed materials from the
tubules and return them to the venous circulation
8. The peritubular capillaries unite to form small veins, which ultimately lead
to the renal vein
L. Urine is produced by filtration, reabsorption, and secretion
1. Filtration is not selective with regard to ions and small molecules
a) Small molecules are forced out of the glomerular capillaries into the
lumen of the Bowman’s capsule
b) The capillaries of the glomerulus are fenestrated
c) The cells of the surface of the Bowman’s capsule in contact with the
glomerulus are permeable podocytes, which are separated by gaps
called filtration slits
d) The glomerular capillary walls and podocytes make up the filtration
membrane
e) The filtration membrane is impassable to cells in the blood and most
proteins
f) The amount of fluid filtered per day is 4.5 x the amount of fluid in the
entire body; therefore most is reabsorbed
2. Reabsorption is highly selective
a) 99% of the filtrate is reabsorbed in the renal tubules
b) The epithelia of the renal tubules have microvilli, which increase the
surface area, and abundant mitochondria to provide energy for active
transport of materials
c) The proximal convoluted tubule is most important in reabsorption
d) Some materials are actively transported into the tubule cells; others
follow by passive diffusion
e)
f)
The tubular transport maximum (Tm) is the maximum rate at which a
substance may be reabsorbed
The renal threshold is the level above which it will not be reabsorbed,
and is therefore excreted in the urine
3.
Some substances are actively secreted from the blood into the filtrate
a) Some ions and drugs are secreted into the filtrate, particularly by the
cells of the distal convoluted tubule
b) Controlled secretion of hydrogen ions allows regulation of blood pH
c) Aldosterone, produced by the adrenal cortex, is involved in regulation
of tubular secretion
M. Urine becomes concentrated as it passes through the renal tubule
1. Cortical nephrons have small glomeruli and short loops of Henle
a) Cortical nephrons produce a relatively dilute urine
2. Juxtamedullary nephrons have large glomeruli and long loops of Henle
a) The loop of Henle is specialized to produce a concentrated urine
b) The descending loop has walls that are permeable to water but
impermeable to sodium and urea
c) The interstitial fluid surrounding the loop has a high concentration of
sodium
d) The ascending loop has walls that are permeable to salt but less
permeable to water
e) Filtrate is concentrated as it moves down the descending loop, and
diluted as it moves up the ascending loop
f) This countercurrent mechanism maintains the high salt concentration
in the interstitial fluid
(1) The collecting ducts lose water due to the high salt concentration,
concentrating the urine (hypertonic urine)
g) The vasa recta, a set of capillaries surrounding the loop of Henle in
juxtamedullary nephrons, absorbs water
N. Urine is composed of water, nitrogenous wastes, and salts
1. Healthy urine is sterile, but the urea rapidly decomposes to form ammonia
2. Urinalysis is the physical, chemical, and microscopic examination of urine
a) Urinalysis is useful in drug testing
O. Kidney function is regulated by hormones
1. Urine volume is regulated by the hormone ADH
a) Antidiuretic hormone is produced by the posterior pituitary and
targets the collecting ducts, making them more permeable to water,
resulting in a more concentrated urine
b) Secretion of ADH is stimulated by the hypothalamus
c) Receptors that are stimulated by osmotic changes cause production of
ADH, and a thirst receptor causes increased fluid intake
d) Diabetes insipidus is a condition caused by a lack of ADH, and in
uncontrolled situations, results in a great volume of urine being lost
2.
Sodium reabsorption is regulated by the hormone aldosterone
a) Aldosterone is produced by the adrenal cortex and stimulates the
distal convoluted tubules and collecting ducts to increase sodium
reabsorption
b) Aldosterone secretion is stimulated by a decrease in blood pressure
causing the cells of the juxtaglomerular apparatus to produce renin,
which activates the renin-angiotensin pathway
c) Atrial natriuretic peptide (ANP) is produced by the walls of the atria
of the heart, and inhibits aldosterone secretion and renin release
Research and Discussion Topics

Compare the functioning of the kidneys of a desert mammal, a human, and a
freshwater mammal, like the beaver. What would be the expected variation in
concentration of urine produced? What about the length of the nephron tubules in
these respective animals?

The kangaroo rat (Dipodomy spp.) has a variety of adaptations to allow it to live in a
habitat with minimal water. The classic studies done decades ago illustrate the
amazing efficiency of their kidneys, respiratory system, and behavioral adaptations
as well. Research the physiological adaptations of this little rodent.

Some current research indicates that the dinosaurs may have been inertial
homeotherms, or actually endotherms. Investigate the evidence for these
hypotheses.