MAINTAINING THE
INTERNAL ENVIRONMENT
CHAPTER 26
HOW THE ANIMAL BODY
MAINTAINS HOMEOSTASIS
• Homeostasis may be defined as the
dynamic constancy of the internal
environment.
• Conditions fluctuate continuously within narrow
limits.
HOW THE ANIMAL BODY
MAINTAINS HOMEOSTASIS
• To maintain internal constancy, the
vertebrate body uses:
• Sensors that measure each condition of the
internal environment.
• An integrating center that contains the set point,
or proper value for a particular internal condition.
• Effectors, which are muscles or glands that can
change the value of the condition back toward
the set point.
• The activity of the effectors is influenced by the
effects they produce in a negative feedback
loop.
HOW THE ANIMAL BODY
MAINTAINS HOMEOSTASIS
• Regulating body temperature
• Humans, as well as other mammals and birds, are
endothermic.
• This means that they can maintain relatively
constant body temperature.
• Other vertebrates are ectothermic, meaning their
body temperatures depend more or less on the
environmental temperature.
• But they can modify their behavior to affect
body temperature.
HOW THE ANIMAL BODY
MAINTAINS HOMEOSTASIS
• Regulating blood glucose
• Excess glucose is stored in the liver as glycogen
under the influence of the hormone insulin, which
is released from the pancreas.
• When glucose levels are low in the blood, the
pancreas releases the hormone glucagon, which
stimulates the liver to convert glycogen back to
glucose.
CONTROL OF BLOOD GLUCOSE
LEVELS
REGULATING THE BODY’S WATER
CONTENT
• Animals use various mechanisms for
osmoregulation, the regulation of the body’s
osmotic composition.
• This refers to how much water and salt the body
contains.
• The proper operation of many vertebrate organ
systems requires that the osmotic concentration
of the blood be kept within narrow bounds.
REGULATING THE BODY’S WATER
CONTENT
• In many animals and single-celled
organisms, the removal of water and salts
from the body is coupled with the removal
of metabolic wastes through the excretory
system.
REGULATING THE BODY’S WATER
CONTENT
• For example,
protists, like
Paramecium,
employ contractile
vacuoles.
Anterior
contractile
vacuole
Cilium
Feeder
canal
Contractile
vacuole
Excretory
pore
Endoplasmic
reticulum
Posterior
contractile
vacuole
REGULATING THE BODY’S WATER
CONTENT
• Flatworms employ a
system of excretory
tubules called
protonephridia to
expel fluids and
wastes from the
body.
Excretory
pores
Flame
cell
Cilia
Collecting
tubule
REGULATING THE BODY’S WATER
CONTENT
• Other invertebrates
have a system of
tubules that open
both to the inside
and to the outside
of the body.
• In annelids, these
tubules are called
nephridia.
Nephridium
Coelomic fluid
Bladder Capillary
network
Pore for
Nephrostome
urine excretion
REGULATING THE BODY’S
WATER CONTENT
• The excretory organs in insects are called
Malpighian tubules, which are extensions of
the digestive tract.
Malpighian
tubules
Waste molecules
K+ Water
Air sac
Malpighian
tubules
Mid gut
Mid gut
Intestine
Hindgut
Rectum
Poison sac
Rectum
Anus
Water
and K+
REGULATING THE BODY’S
WATER CONTENT
• Kidneys are the excretory organs in
vertebrates.
• Kidneys create a tubular fluid by filtration.
• The filtrate contains many valuable nutrients in
addition to waste products.
• Selective reabsorption ensures that these
nutrients and water are reabsorbed into the
blood, while wastes remain in the filtrate.
EVOLUTION OF THE VERTEBRATE
KIDNEY
• The kidney is a complex organ made up of
many repeating units called nephrons.
• Blood pressure forces the fluid in the blood
through a capillary bed at the top of each
nephron, called a glomerulus.
• The glomerulus excludes blood cells, proteins,
and other large molecules from the filtrate.
• The remainder of the nephron tube reabsorbs
anything else useful from the filtrate
BASIC ORGANIZATION OF THE
VERTEBRATE NEPHRON
Proximal arm
Distal arm
Glomerulus
Neck
Glucose
Amino acids
Divalent
ions
Intermediate
segment
H2O
NaCl
H2O
H2O
H2O
NaCl
H2O
H2O
Collecting
duct
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Only birds and mammals can reabsorb
water from the glomerular filtrate to
produce a urine that is hypertonic to (more
concentrated than) blood.
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Kidneys are thought to have evolved first
among the freshwater fish.
• The body fluids of a freshwater fish have a greater
osmotic concentration than the surrounding
water. So,
• Water tends to enter the body from the
environment.
• Solutes tend to leave the body and enter the
environment.
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Freshwater fish
address these
problems by
NaCl
Large
glomerulus
NaCl
Active tubular
reabsorption
of NaCl
Kidney tubule
• Not drinking water.
• Excreting a large
volume of dilute urine.
• Reabsorbing ions
(mainly NaCl) from the
nephron.
• Actively transporting
NaCl across the gills
from the surrounding
water into the blood.
Food,
fresh water
(passes over
gills)
Kidney: Excretion
of dilute urine
Gills:
Active absorption of
NaCl, water enters
osmotically
Intestinal
wastes
Urine
Freshwater fish
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Marine fish probably
evolved from
freshwater ancestors.
Glomerulus
reduced or
absent
Stomach:
Passive reabsorption
of NaCl and water
• Their bodies are
hypotonic to the
surrounding seawater.
So,
• Water tends to leave
their bodies through
osmosis across the gills.
• They lose water in their
urine.
• To compensate, marine
fish drink lots of seawater
• They excrete isotonic
urine.
MgSO4
MgSO4
Active tubular
secretion
of MgSO4
Food,
seawater
Gills:
Active secretion of
NaCl, water loss
Intestinal wastes:
MgSO4 voided
with feces
Marine fish
Kidney:
Excretion of MgSO4,
urea, little water
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Elasmobranchs solve the osmotic problem
posed by their seawater environment by
reabsorbing urea from the nephron tubules.
• The blood is approximately isotonic to the
surrounding sea.
Kidney
Glomerulus
Urea
Urea
Kidney tubule
Cartilaginous fish
EVOLUTION OF THE VERTEBRATE
KIDNEY
• The amphibian kidney is like that of
freshwater fish.
• Amphibians produce a very dilute urine and
actively transport Na+ across their skin.
• The kidneys of terrestrial reptiles reabsorb
much of the salt and water in the nephron
tubules.
• Their urine is still hypotonic but they can absorb
additional water in the cloaca
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Because mammals and
birds can produce
hypertonic urine, they can
excrete their waste
products in a small volume
of water.
• The kidneys of some mammals
are even more extremely
efficient at conserving water.
• The kidneys of the kangaroo
rat are so efficient it never
has to drink water; it can
obtain all the water it needs
from its food and aerobic
cell respiration.
EVOLUTION OF THE VERTEBRATE
KIDNEY
• Birds have relatively
few or no nephrons
Salt glands
with long loops.
• At most, they can only
reabsorb enough water
to produce a urine that
is about twice the
concentration of their
blood
• Marine birds solve the
problem of water loss by
drinking sea water and
excreting excess salt
through salt glands near
the eyes.
Salt secretion
THE MAMMALIAN KIDNEY
• In mammals, each
kidney receives blood
from a renal artery,
and it is from this blood
that urine is produced.
• Urine drains from each
kidney through a ureter.
• The ureters carry urine to
a urinary bladder.
• Urine passes out of the
body through the
urethra.
THE MAMMALIAN KIDNEY
• Within the kidney,
the mouth of the
ureter flares open to
form a funnel-like
renal pelvis.
• The renal tissue is
divided into:
• An outer renal
cortex
• An inner renal
medulla
Nephron
Renal
cortex
Renal
medulla
Renal
artery
Renal
vein
Ureter
THE MAMMALIAN KIDNEY
• The mammalian kidney is comprised of
roughly 1 million nephrons, each of which is
composed of three regions:
• Filter
• The filtration device at the top of each nephron is called
the Bowman’s capsule which receives filtrate from the
glomerular capillaries.
• Tube
• The Bowman’s capsule is connected to a long renal
tubule, which includes the Loop of Henle, that acts as a
reabsorption device.
• Duct
• The renal tubule empties into a collecting duct that
operates as a water conservation device.
THE MAMMALIAN KIDNEY
There are five steps
involved in the
formation of urine in
the kidney:
Total solute concentration (mOsm)
•
1. Pressure filtration
2. Reabsorption of
water
3. Selective
reabsorption
4. Tubular secretion
5. Further reabsorption
of water
Glomerulus
Proximal Distal tubule
tubule
1
Na+ –
Cl
H2O
300
4
Nitrogenous
wastes
Collecting
duct
Cortex
2
H2O
600
3
Na+ H2O
Cl–
Outer medulla
5
H2O
Loop of Henle
1200
http://youtu.be/TzwPmz5V6Xg
Bowman's
capsule
Inner medulla
Urea
H2O
ELIMINATING NITROGENOUS
WASTES
• Amino acids and nucleic acids are nitrogencontaining molecules.
• When animals metabolize these substances,
they produce nitrogen-containing byproducts, called nitrogenous wastes, that
must be eliminated by the body.
ELIMINATING NITROGENOUS
WASTES
• The first step in the metabolism of amino
acids and nucleic acids is the removal of
the amino (—NH2) group.
• This group is then combined with H+ to form
ammonia (NH3).
• This takes place in the liver.
ELIMINATING NITROGENOUS
WASTES
• Ammonia is quite toxic and is safe only in
very dilute concentrations.
• Fish and tadpoles, ammonia can be directly
eliminated across the gills or excreted in dilute
urine.
• In sharks, adult amphibians, and mammals, the
nitrogenous waste is eliminated as urea, which is
less toxic.
• Reptiles, birds, and insects excrete nitrogenous
wastes in the form of uric acid, which can be
excreted with very little water.
NITROGENOUS WASTES
Amino acids and nucleic acids
1
Catabolism
Ammonia
by-product
2
3
Converted
to urea
Eliminated
directly
Urea
NH2
Ammonia
O
NH3
4
Converted
to uric acid
Uric acid
O
H
N
C
HN
NH2
O
O
N
H
N
H
Most fish
Mammals, some others
Reptiles and birds