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CHAPTER 26
MAINTAINING THE INTERNAL ENVIRONMENT
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.
CiliumFeedercanal
Contractilevacuole
Excretorypore
Endoplasmicreticulum
Anteriorcontractilevacuole
Posteriorcontractilevacuole
REGULATING THE BODY’S WATER CONTENT
• For example, protists, like Paramecium, employ contractile vacuoles.
REGULATING THE BODY’S WATER CONTENT
• Flatworms employ a system of excretory tubules called protonephridia to expel fluids and wastes from the body.
Flamecell
Excretorypores
Collectingtubule
Cilia
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
Bladder Capillarynetwork
Pore forurine excretion
Coelomic fluid Nephrostome
REGULATING THE BODY’S WATER CONTENT
• The excretory organs in insects are called Malpighian tubules, which are extensions of the digestive tract.
K+
Malpighiantubules
Waste molecules
Water
Mid gut
Waterand K+
AnusRectum
HindgutIntestine
RectumPoison sac
Mid gut
Air sac
Malpighiantubules
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
H2O
Glomerulus Neck
Distal arm
Proximal arm
H2O
NaCl
NaCl
H2O
Collectingduct
H2O
H2O
H2O
Divalentions
Amino acids
Glucose
Intermediatesegment
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• 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.
Active tubularreabsorptionof NaClNaCl
NaCl
Kidney tubule
Largeglomerulus
Food,fresh water(passes overgills)
Gills:Active absorption ofNaCl, water entersosmotically
Freshwater fish
Urine
Kidney: Excretionof dilute urine
Intestinalwastes
EVOLUTION OF THE VERTEBRATE KIDNEY
• Marine fish probably evolved from freshwater ancestors.• 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.
Marine fish
Food,seawater
Stomach:Passive reabsorptionof NaCl and water
Glomerulusreduced orabsent Active tubular
secretionof MgSO4
MgSO4
MgSO4
Gills:Active secretion ofNaCl, water loss
Intestinal wastes:MgSO4 voidedwith feces
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.
Glomerulus
UreaUrea
Kidney tubule
Kidney
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.
Salt glands
Salt secretion
EVOLUTION OF THE VERTEBRATE KIDNEY
• Birds have relatively few or no nephrons 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.
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 Ureter
Renalvein
Renalartery
Renalmedulla
Renalcortex
Nephron
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:
1. Pressure filtration2. Reabsorption of
water3. Selective
reabsorption4. Tubular secretion5. Further
reabsorption of water
http://youtu.be/TzwPmz5V6Xg
H2O
H2O
Na+
Cl–H2O
H2O
H2O
Na+
Cl–
2 3
4
5
To
tal
so
lute
co
nc
en
tra
tio
n (
mO
sm
)
300
600
Glomerulus Bowman'scapsule
Proximaltubule
Distal tubule
Nitrogenouswastes
Collectingduct
Urea
Inner medulla1200
Outer medulla
Cortex
Loop of Henle
1
ELIMINATING NITROGENOUS WASTES
• Amino acids and nucleic acids are nitrogen-containing molecules.• When animals metabolize these
substances, they produce nitrogen-containing by-products, 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
3 4
1
2
NH3HN
NH
O
O
HN
NH
O
O C
NH2
NH2
Amino acids and nucleic acids
Catabolism
Convertedto uric acid
Uric acid
Mammals, some others Reptiles and birds
Ammoniaby-product
Eliminateddirectly
Ammonia
Most fish
Convertedto urea
Urea
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