40.1 Body
Fluid Regulation
A. Water and Ion Balance
1. Excretory system regulates body fluid concentrations by
regulating water and ions in body fluids.
2. Regulation depends on
concentration of mineral ions (i.e., Na+, CI-, K+, and HCO3-).
3. Body fluids gain
mineral ions when animals eat food and drink fluids; body loses ions by
excretion.
4. Water enters
animals by eating foods, metabolism (cellular respiration produces water) and
drinking.
5. Water is lost by
evaporation, through feces, and by excretion.
6. To be in balance,
water entering the body must equal water lost.
7. If osmolarity
differs between two regions, water moves into region with higher amount of
solutes.
8. Marine environments,
high in salt, promote loss of water and gain of ions by drinking water.
9. Fresh water
promotes a gain of water by osmosis and a loss of ions as excess water is
excreted.
10. Terrestrial animals
tend to lose both water and ions to the environment.
B. Aquatic Animals
1. Cartilage Fishes and Marine Invertebrates
a. Sharks
and rays are isotonic with sea water.
b. Yet they
do not contain the same amount of salt as seawater.
c. Their
blood has high concentrations of urea to match tonicity of sea; this is not
toxic to them.
2. Marine Bony Fishes
a. Marine
bony fishes have a moderate salt level compared to seawater, common ancestor
probably
evolved from freshwater fishes.
b. They
constantly drink sea water; they are prone to water loss and could become
dehydrated.
c. Marine
bony fishes swallow water equal to 1% of their weight every hour to counteract
dehydration.
d. This
provides the marine fish with water and also salt.
e. To remove
excess salt, they actively transport Na+ and CI- ions (salt) at gills.
3. Freshwater Bony Fishes
a. Body
fluids of freshwater bony fish are hypertonic to freshwater; they passively
gain water.
b.
Freshwater fishes never drink water.
c. They take
in salts at gills and pass large quantities of dilute, hypertonic urine.
d. They
discharge a quantity of urine equal to one-third their body weight each day.
e. Because
this causes them to lose salts, they actively import Na+ and CI- ions into
blood at gills.
4. Some fish can move between marine
and freshwater environments.
a. Salmon
begin their life in freshwater streams, mature in ocean and return to
freshwater to breed.
b. Salmon
alter their behavior and gill and kidney functions in response to osmotic
changes.
5. Terrestrial Animals
a. Some terrestrial
animals near oceans are able to drink seawater despite its high osmolarity.
b. Such
birds and reptiles have a nasal salt gland that excretes
concentrated salt solution.
6. Water loss prevention
a. Some
animals excrete a rather insoluble nitrogenous waste.
b. Animal
skin is adapted to moist (moist, thin, permeable) or dry (dry, thick,
impermeable) environments.
c. Some
animals have other unique adaptations to prevent water loss.
d. Camel and
kangaroo rat have convoluted nasal passages that absorb moisture from exhaled
air.
7. Most terrestrial animals must
drink fresh water often; however, kangaroo rat avoids drinking water.
a. It forms
a very concentrated urine.
b. It
defecates fecal matter that is almost completely dry.
c. It meets
its water requirements with the metabolic water derived from aerobic
respiration.
40.2.
Nitrogenous Waste Products
A. Eliminating Nitrogenous Wastes
1. Breakdown of nucleic acids and amino acids results in nitrogenous
wastes.
2. Amino acids derived from protein
synthesize body proteins or nitrogen-containing molecules.
3. Un-used amino acids are oxidized
to generate energy or are stored as fats or carbohydrates.
4. In both cases, amino groups
(-NH2) must be removed.
5. Nitrogenous wastes
are excreted as ammonia, urea, or uric acid,
depending on animal.
6. Removal of amino groups requires
a fairly constant amount of energy that differs for each conversion.
B. Excreting Ammonia
1. Amino groups removed from amino acids form ammonia (NH3) by
adding a third hydrogen ion (H+).
2. This requires little or no energy
3. Ammonia is quite toxic but water
soluble; it requires the most water to wash it from body.
4. Bony fishes, aquatic
invertebrates, and amphibians excrete ammonia through gills and skin surfaces.
C. Excreting Urea
1. Terrestrial amphibians and mammals usually excrete urea.
2. Urea is much less toxic than
ammonia; excreted in a moderately concentrated solution, it conserves body
water.
3. Production of urea requires
energy; it is produced in liver as product of energy-requiring urea cycle.
D. Excreting Uric Acid
1. Insects, reptiles, and birds excrete uric acid as their main
nitrogenous waste.
2. Uric acid is not very toxic and
is poorly soluble in water; it is concentrated for water conservation.
3. In reptiles and birds, a dilute
solution of uric acid passes from kidneys to cloaca, a common
reservoir
for products
of the digestive, urinary, and reproductive systems.
4. After water is absorbed by
cloaca, uric acid passes out with feces.
5. Reptiles and bird embryos are
enclosed in egg shells; uric acids is nontoxic in storage.
6. Uric acid is synthesized by
enzymatic reactions using even more ATP than urea synthesis.
7. There is a trade-off between
water conservation and energy expenditure.
40.3. Organs
of Excretion
A. Tubular organs function in excretion and osmotic
regulation
B. Flame Cells in Planaria
1. Planaria have two strands of branching excretory tubules that open outside
through excretory pores.
2. Located along tubules are flame
cells containing tufts of cilia that appear to flicker.
3. Cilia beat back-and-forth
propelling hypotonic fluid through canals emptying at body surface.
4. System functions in water
excretion, osmotic regulation, and excretion of wastes.
C. Nephridia in Earthworms
1. Earthworm's body is divided into segments; nearly every segment has a pair
of nephridia.
2. Nephridium is a
tubule with a ciliated opening, nephridiostome, and excretory nephridiopore.
3. Fluid from the body cavity is
propelled through tubule by cilia.
4. Certain substances are reabsorbed
and carried away by the network of capillaries surrounding tubules.
5. Nephridia form urine that
contains only metabolic wastes, salts, and water.
6. Each day, an earthworm produces
urine equal to 60% of its body weight; it safely excretes ammonia.
D. Malpighian Tubules in Insects
1. Insects have a unique excretory system consisting of long, thin Malpighian
tubules attached to gut.
2. Malpighian tubules
take up metabolic wastes and water from hemolymph.
3. In gut, water and other useful
substances are reabsorbed.
4. Uric acid eventually passes out
of the gut.
5. Insects that live in water or
that eat large quantities of moist food reabsorb little water.
6. Insects in dry climates reabsorb
most water and excrete a dry, semisolid mass of uric acid.
40.4. Urinary
System in Humans
A. Human urinary system is an organ system with four
parts.
1. Human kidneys are two bean-shaped, reddish brown organs, each
about size of a fist.
a. They are
to sides of vertebral column below diaphragm and partly protected by lower rib
cage.
b. Kidneys
are sites of urine formation.
2. Each kidney is connected to a ureter;
each conducts urine from a kidney to urinary bladder.
3. Urinary bladder
stores urine from kidneys until it is voided from body through urethra.
4. A single urethra
conducts urine from the urinary bladder to exterior of the body.
5. Male urethra runs
through penis and conducts semen; in females, it opens ventral to
vaginal opening.
B. Kidneys
1. If sectioned longitudinally, three major regions can be distinguished.
a. Renal
cortex is thin, outer region of a kidney and appears granular. (Fig.
45.6)
b. Renal
medulla consists of striped, pyramid-shaped regions that lie on inner
side of the cortex.
c. Renal
pelvis is innermost hollow chamber of the kidney.
2. Each human kidney is composed of
about one million tiny tubules called nephrons.
3. Some nephrons are located
primarily in cortex but others dip down into medulla.
C. Nephrons
1. Each nephron is comprised of several parts.
2. End of a nephron pushes in to
form cuplike structure called glomerular capsule (Bowman's
capsule).
a. Its outer
layer is composed of simple squamous epithelium.
b. Inner
layer is made of specialized cells that allow easy passage of molecules.
3. Nearest the glomerular capsule is
proximal convoluted tubule lined by cells with many mitochondria
and tightly
packed microvilli.
4. Simple squamous epithelium is loop
of the nephron (loop of Henle), the middle portion of
nephron tubule
with an
descending and ascending limb.
5. Distal convoluted tubule
is distal portion nephron tubule; several deliver urine into collecting
ducts.
6. Loop of nephron and collecting
duct give pyramids of the medulla their striped appearance.
7. Each nephron has its own blood
supply.
a. Renal
artery branches into small arteries, which branch into afferent
arterioles, one for each nephron.
b. Each afferent
arteriole divides to form a capillary bed or glomerulus.
c.
Glomerular capillaries drain into efferent arteriole which
branches into a peritubular network.
d.
Peritubular capillaries drain into a venule; venules from many nephrons drain
into a small vein; small
veins join to form the renal vein, a vessel that enters inferior
vena cava.
D. Urine Formation
1. Urine production requires three distinct processes.
2. Glomerular filtration
occurs at glomerular capsule.
3. Tubular reabsorption,
occurs at proximal convoluted tubule.
4. Tubular secretion
occurs at distal convoluted tubule.
E. Glomerular Filtration
1. When blood enters glomerulus, blood pressure moves small molecules from
glomerulus across inner
membrane of
glomerular capsule into lumen of glomerular capsule; this is pressure filtration.
2. Glomerular walls are 100 times
more permeable than walls of most capillaries.
3. Molecules that leave blood and
enter glomerular capsules are glomerular filtrate.
4. Plasma proteins and blood cells
are too large to be part of glomerular filtrate.
5. Failure to restore fluids would
soon cause death from loss of water, nutrients, and low blood pressure.
F. Tubular Reabsorption
1. Tubular reabsorption from nephron to blood occurs through
walls of proximal convoluted tubule.
2. Reabsorption recovers much of the
contents of glomerular filtrate.
a.
Osmolarity of blood equals the filtrate so osmosis of water does not occur.
b. Sodium
ions are actively reabsorbed, pulling along chlorine.
c. This
changes the osmolarity of the blood so that water moves passively from tubule
to blood.
d. About
60-70% of salt and water are reabsorbed at the proximal convoluted tubule.
3. Cells of the proximal convoluted
tubule have numerous microvilli increasing surface area for absorption,
and numerous
mitochondria, which supply energy needed for active transport.
4. Only molecules by carrier
molecules are transported through the tubule into interstitial spaces.
5. Diabetes Mellitus
a. Glucose
is usually reabsorbed completely; there are plentiful carriers for the glucose
molecules.
b. If all
carriers are in use, excess glucose can appear in the urine.
c. In
diabetes mellitus, there is a too much glucose because liver fails to store
glucose as glycogen.
G. Tubular Secretion
1. Tubular secretion moves substances in blood back to tubular
lumen by other than glomerular filtration.
2. Secretion back into filtrate is
primarily associated with distal convoluted tubule.
3. This helps rid body of
potentially harmful compounds that were not filtered into glomerular capsule.
4. Uric acid, hydrogen ions,
ammonia, and penicillin are eliminated this way.
H. Reabsorption of Water
1. Loop of nephron is comprised of a descending limb
(going down) and an ascending limb (going up).
2. This countercurrent flow enables
us to excrete a hypertonic urine.
3. Salt (NaCl) passively diffuses
out of lower portion of ascending limb, but the upper, thick portion of limb
actively
transports salt out in to tissue of outer renal medulla.
4. Less salt is available for
transport from tubule as fluid moves up thick portion of ascending limb.
5. Urea leaks from lower portion of
collecting ducts causing concentrations in lower medulla to be highest.
6. Because of the solute
concentration gradient within renal medulla, water leaves descending limb of
loop
of nephron
along its length.
7. Decreasing water concentration in
descending limb encounters an increasing solute concentration.
8. Fluid received by a collecting
duct from distal convoluted tubule is isotonic to cells of cortex; as this
fluid passes
through renal medulla, water diffuses out of collecting duct into renal
medulla.
9. Urine finally delivered to renal
pelvis is usually hypertonic to blood plasma.
10. Antidiuretic hormone (ADH)
is released from posterior lobe of pituitary.
a. It acts
on collecting ducts by increasing permeability to H2O, thereby increasing H2O
retention.
b. When ADH
is released, more water is reabsorbed and there is less urine.
c. When ADH
is not released, more water is excreted and more urine forms.
d. If an
individual does not drink, the pituitary releases ADH; if hydrated, ADH is not
released.
e. Diuresis
means increased urine; antidiuresis is decreased amount of urine.
11. Reabsorption of Salt
a. More than
99% of sodium filtered at glomerulus is returned to blood.
b. Most is
reabsorbed at proximal tubule, 25% is extruded by ascending limb of loop of
nephron and
rest from distal convoluted tubule and collecting duct.
12. Aldosterone is
secreted by adrenal cortex.
a. It acts
on distal convoluted tubules to increase reabsorption of Na+ and excretion of
K+.
b. Increased
Na+ in blood causes water to be reabsorbed, increasing blood volume and
pressure.
c.
Aldosterone production is triggered by renin-angiotensin-aldosterone
system.
d. Blood
pressure is constantly monitored within juxtaglomerular apparatus.
e. If blood
pressure is insufficient to promote glomerular filtration, afferent arteriole
cells secrete renin.
f. Renin
catalyzes conversion of angiotensinogen (a protein produced by
liver) into angiotensin I.
g. Later,
angiotensin I is converted to angiotensin II by angiotensin-converting
enzyme.
h.
Angiotensin II stimulates cells in adrenal cortex to produce aldosterone.
i.
Angiotensin II increases blood pressure as a vasoconstrictor.
13. Atrial natriuretic hormone
(ANH) is produced by the atria of heart when cardiac cell
stretch.
a. When
blood pressure rises, heart produces ANH to inhibit secretion of renin and
release of ADH.
b.
Therefore, this hormone decreases blood volume and pressure and is a balance to
the other two.
I. Adjustment of Blood pH
1. The bicarbonate buffer system and breathing work together to maintain blood
pH.
2. Excretion of H+ ions and NH3, and
reabsorption of bicarbonate ions (HCO3 -) is adjusted.
a. If blood
is basic, fewer hydrogen ions are excreted and fewer sodium and bicarbonate
ions are reabsorbed.
H+ + HCO3 - <==> H2CO3 <==> H2O +
CO2
b. If blood is acidic, H+ ions are excreted with ammonia, while Na+ and HCO3 -
ions are reabsorbed;
Na+ ions promote formation of hydroxide ions and bicarbonate takes up H+ ions
when carbonic acid is formed.
NH3 + H+ NH4+
c. Ammonia is produced in tubule cells by deamination of amino acids.
3. Reabsorption or excretion of ions
by kidneys is a homeostatic function; maintains pH of blood and osmolarity.
J. Artificial Kidney Dialysis
1. Hemodialysis is a therapy for persons who have suffered renal
failure.
2. Blood of a patient is processed
by an artificial kidney machine.
3. Blood passes through a
semipermeable membranous tube in contact with balanced salt (dialysis)
solution.
4. Waste molecules diffuse out of
blood while others can be added to blood, such as bicarbonate ions.
5. The system is efficient enough to
permit the treatment to be spaced at twice a week