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Controlling the internal environment
Controlling the internal environment
Most organisms can survive fluctuations in the external environment that
are more extreme than any of their individual cells could tolerate.During the
day , a human may be exposed to substantial changes in outside
temperatures but will die if the internal body temperature goes more than a
few degrees above or below an average of about 37C.
The spring peeper, a North American tree frog often spends an entire
winter with much of its body frozen. Most of its blood and interstitial fluid
turns to ice, but its cells and a film of water surrounding them are kept from
freezing by specialized proteins and a high level of glucose throughout the
frog's body. Without these protectants, ice crystals would rupture the cell
membranes, and the animal would die.
Homeostatic mechanisms protect an animal's internal environment
from harmful fluctuations: an overview:
In most organism, the majority of cells are not in direct contact with the
external environment but are bathed by an internal body fluid. Insects and
other animals with an open circulatory system have an internal pond
composed of hemolymph, which bathes all body cells. In vertebrates and
other animals with a closed circulatory system, the internal pond is
interstitial fluid serviced by blood. Homeostatic mechanisms temper
changes in an animal's body fluids, cushioning them from the potentially
harmfiul impact of fluctuations in the extyernal environment (Fig. 40.1)
nP880.
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Cells require a balance between water uptake and loss:
Water enter our body's by three ways:
In our food
In our drink
Oxidative metabolism
Metabolic water is produced by cellular respiration when electrons and
hydrogen are added to oxygen.
Lose water by:
Urinating
defecating
And by evaporative loss due to
sweating and breathing.
Whether an organism inhabits land, fresh water, or salt water, or moves back
and forth between these environments, one general problem occurs:
The cells of the animal cannot survive a net water gain or loss. Water
continuously enters and leaves an animal cell across the plasma membrane,
however uptake and loss must balance.
An animal cells swell and burst if there is a net uptake of water or shrivel and
die if there is a net loss of water.
Osmosis, is the movement of water across a selectively permeable
membrane. It occurs whnever two solutions separated by the membrane differ
in total solute concentration, or osmolarity (total solute concentration
expressed as molarity, or moles of solute per liter of solution). Foe example,
the osmolarity of human blood is about 300 mosm/L, while sea water
commonly has an osmolarity of about 1000 mosm/L.
Osmoconformers and osmoregulators
There are two basic solutions to the problem of balancing water gain with
water loss. One solution for a marine organism is to be isotonic with its
saltwater environment. Such organisms, which do not aqctively adjust their
internal osmolarity, are known as osmoconformers.
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By contrast, organisms whose body fluids are not isotonic with the outside
environment, called osmoregulators, must either discharge excess water if
they live in ahyptonic environement or continuously take in water to offset
osmotic loss if they inhabit a hypertonic environment.
The ability to osmoregulate enables animals to live, for example, in fresh
water, where the osmolarity is too low to support cellular life, and on land,
where water is usually in short supply.
Most organisms, whether osmoconformers or osmoregulators, cannot tolerate
substantial changes in external osmolarity. Such organisms are said to be
stenohaline (stenos, "narrow": haline refers to salt). However, some
organisms called euryhaline animals, do survive radical fluctuations of
osmolarity in their surroundings. They either conform to the changes or
regulate their internal osmolarity within a narrow range even as the external
osmolarity changes. One example of euryhaline animal, a bony fish called
tilapia can adjust to any salt concentrations between fresh water and twice
that of seawater. All freshwater animals and many marine animals are
osmoregulators. Humans and other terrestrial animals, also osmoregulators,
must compensate for water loss.
Maintanance water balance in different environments
Marine animals. Animals Animals first evolved in the sea. Most marine
invertebrates are osmoconformers, even these animals differ from seawater
in their concentrations of specific salts.
Among the vertebrates, the hagfishes, (jawless members of the class
Agnatha) are isotonic with the surrounding seawater, but most marine
vertebrates osmoregulate. Sharks and most cartilaginous fishes maintain
internal salt concentrations lower than that of seawater. Their kidneys excrete
some salts, and a salt-excreting organ called the rectal gland excretes sodium
chloride out of the body through the anus.
A large amounts of urea dissolved in the body fluids accounts for a shark's
being slightly hypertonic to seawater. Sharks also produce and retain another
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organic compound, trimethylamine oxide (TMAO), which protects proteins
from being damaged by the urea.
Marine bony fishes constantly lose water by osmosis to their hypertonic
surroundings (Fig. 40.2a) P882. They compensate by drinking large amounts
of seawater, pumping out excess salts, and excreting urine in relatively small
amounts.
Freshwater animals
Freshwater animals are constantly taking in water by osmosis because the
osmolarity of their internal fluids is much higher than that of their surroundings.
Amoeba and paramecium have contractile vacuoles that pump out excess
water. Many freshwater animals, including fishes, bail out ( ‫ ) ينزح‬water by
excreting large amounts of very dilute urine and regaining lost salts in their
food. (Fig.40.2b) P882.
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Life in temporary waters
Dehydration kill most organisms, but some aquatic invertebrates living in
temporary ponds and films of water around soil particles can lose almost all
their body water and survive in a dormant state when their habitats dry up.
This remarkable adaptation is called anhydrobiosis (life without water) or
cryptobiosis (Hidden life). Example tardigrades, or water bears, tiny
invetebrates less than 1mm long (fig. 40.3 P882). In their active, hydrated
state, these organisms contain about 85% water by weight but can dehydrate
to less than 2% water and survive in an inactive state, dry as dust, for a
decade. Just add water, and within minutes the rehydrated tardigrades are
moving about and feeding.
Tardigrades survive drying out by containing a large amounts of sugars
disaccharides called trehalose, which protect the cells by replacing the water
associated with membranes and proteins. Many insects that survive freezing
in the winter also utilize trehalose as a membrane protectant.
Terrestrial animals
Few terrestrial organisms are capable of anhydrobiosis. Human's die if they
lose about 12% of their body water.
Adaptations that have made it possible for organisms, which consist
mostly of water, to survive on land is:
1. the waxy cuticle of plants on land.
2. Also insects have waxy exoskeletons.
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3. the shell of land snails.
4. and the multiple layers of dead, keratinized skin cells covering most
terrestrial vertebrates.
5. behavioral adaptations as nervous, hormonal mechanisms which control
thirst.
6. Desert animals are nocturnal, reduces dehydration.
7. The kidneys and other excretory organs of terrestrial organisms often
exhibit adaptations that help conserve water.
8. Some mammals are so well adapted to minimizing water loss that they
can survive in deserts without drinking. Example, Kangaroo rats lose so
little water that they can recover 90% of the loss by using metabolic
water . (Fig. 40.4 P883).
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Mosmoregulation depends on transport epithelia
Specialized epithelia, called transport epithelia, regulate the solute
movements.
A transport epithelium is usually a single sheet of cells facing the external
ebvironment or some channel that leads to the exterior through an opening on
the body surface. The cells of the epithelium are joined by impermeable tight
junctions (Fig. 7.34a), forming a continuous barrier at the tissue-environment
boundary. The solute passing between the extracellular fluid and the
environment must pass through the selectively permeable membranes of
cells. Transport epithelia vary in their passive permeabilities to water and
salts, and in number, type, and orientation of membrane proteins responsible
for active transport.
One of the most efficient transport epithilia is found in marine birds that spend
months or years at sea and obtain water from the ocean (Fig. 40.5 P884).
They have specialized salt-excreting glands whose transport epithelia are
dedicated exclusively to osmoregulation-maintaining salt and water balance.
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Tubular systems function in osmoregulation and excretion in many
invertebrates
Protonephridia:
The flame-Bulb system of flatworms
Flatworms have simple tubular excretory systems called protonephridia. A
protonephridium is a network of closed tubules lacking internal opening.
(Fig. 40.6) P885. The flame-bulb systems of freshwater flatworms function
mainly in osmoregulation, most metabolic wastes diffuse out from the body
surface or are excreted into the gastrovascular cavity and eliminated through
the mouth. Protonephridia are also found in rotifers, some annelids, and the
larvae of mollusks.
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Metanephridia of earthworms
Another type of tubular excretory system, the metanephridium, has internal
openings that collect body fluids. Metanephridia are found in most annelids,
including earthworms (Fig. 40.7) P885. Each segment of a worm has a pair of
metanephridia, which are tubules immersed in coelomic fluid and enveloped
by a network of capillaries. An earthworm's metanephridia have excretory and
osmoregulatory functions. As the fluid moves along the tubule, the transport
epithelium bordering the lumen pumps essential salts out of the tubule, and
the salts are reabsorbed into the blood circulating through the capillaries. The
urine that exists through the nephridiopore contains nitrogenous wastes and is
hypotonic to the body fluid.
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Malpighian tubules in insects
The excretory organs of insects and other terrestrial arthropods is called
Malpighian tubules, remove nitrogenous wastes from the hemolymph and also
function in osmoregulation (Fig. 40.8) P 886. The transport epithelium that
lines a tubule pumps certain solutes, including salts and nitrogenous wastes,
from the hemolymph into the lumen of the tubule.
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The kidneys of most vertebrates are compact organs with many
excretory tubules
The kidneys of vertebrates are compact organs containing numerous tubules
that are not segmentally arranged. A dense network of capillaries intimately
associated with the tubules is also part of the kidney. In vertebrates that
osmoregulate, the tubules function in both excretion and osmoregulation.
The kidneys, the blood vessels that serve them, and the structures that carry
urine formed in the kidneys out of the body are the components of the
vertebrate excretory system (Fig. 40.9) P887.
The mammalian excretory system
In humans, the kidneys are a pair of bean-shaped organs about 10 cm long
(Fig. 40.9a). Blood enters each kidney via the renal artery and leaves each
kidney via the renal vein. They receive about 20% of the blood pumped with
each heartbeat. Urine exists the kidney through a duct called the ureter. The
ureters of both kidneys drain into a common urinary bladder. During
urination, urine leaves the body from the urinary bladder through a tube called
the urethra.
The nephron and associated structures
The kidneys has two distinct regions, an outer renal cortex and an inner renal
medulla. The nephron, which is the functional unit of the vertebrate kidney,
consists of a single long tubule and a ball of capillaries called glomerulus.
(See Fig. 40.9b, 40.9c, 40.9 d.) P887.
Only mammals and birds have juxtamedullary nephrons; the nephrons of
other vertebrates lack loops of Henle.
The nephron and the collecting duct are lined by a transport epithelium that
processes the filtrate to form the urine. From about 1100 to 2000 L of blood
that flows through the human kidneys each day, the nephrons and collecting
ducts process about 180 L of filtrate, but the kidneys excrete only about 1.5 L
of urine. The rest of the filtrate, including about 99% of the water, is
reabsorbed into the blood.
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The kidney's transport epithelia regulate the composition of blood
Production of urine from a blood filtrate
The transport epithelia of nephrons and collecting ducts regulate the
composition of blood by a combination of three processes that transfer
material between the tubules and the capillaries that serves them: filtration,
secretion and reabsorption.
Filtration of the blood: Blood pressure forces fluid from the capillaries of
glomerulus across the epithelium of Bowman's capsule into the lumen of the
nephron tubule. The porous capillaries, along with specialized cells of the
capsule called podocytes, function as a filter, being permeable to water and
small solutes but not to bloos cells or large molecules such as plasma proteins
(Fig.40.10) P 889.
Filtration is nonselective with regard to small molecules; any substance small
enough to be forced through the capillary wall and between the podocytes by
blood pressure enters the lumen of the nephron tubule. The filtrate contains
solutes such as salts, glucose, and vitamins, nitrogenous wastes such as
urea.
Secretion as filtrate travels through the nephron tubule, it is joined by
substances that are transported across the tubule epithelium from the
surrounding interstitial fluid. The net effect of of renal secretion is the addition
of plasma solutes to the filtrate within the tubule. The proximal and distal
tubules are the most common sites of secretion.
Reabsorption because filtration is nonselective, it is important that small
molecules essential to the body be returned to the interstitial fluid and blood
plasma. This selective transport of substances across the epithelium of the
excretory tubule from the filtrate to the interstitial fluid is called reabsorption.
The proximal and distal tubules and the loop of Henle all contribute to
reabsportion as does the collecting duct.
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All sugars, vitamins and other organic nutrients present in the initial filtrate are
reabsorbed. Most of the water of the filtrate is also reabsorbed in the kidneys
of mammals and birds.
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The water-conserving ability of the mammalian kidney is a key terrestrial
adaptation
The osmolarity of human blood is about 300 mosm/L, but the kidney can
excrete urine up to four times as concentrated-about 1200 mosm/L.
The main two solutes responsible for this osmolarity gradient are NaCL, which
is deposited in the renal medulla by the loop of Henle, and urea, which leaks
across the epithelium of the collecting duct in the inner medulla.
Fig. 40.12 P893.
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Regulation of kidney function by feedback circuits
Fig. 40.13 P895
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Diverse adaptations of the vertebrate kidney have evolved in different
habitats
Mammals that excrete the most hypertonic urine, such as kangaroo rats and
other mammals adapted to the desert, have exceptionally long loops of Henle.
Long loops maintain steep osmotic gradients in the kindney, rsulting in urine
becoming very concentrated. In contrast, beavers, which spend much of their
time in fresh water and rarely face problems of dehydration, have nephrons
with very short loops, resulting in dilute urine.
Birds, like mammals, have kidneys with juxamedullary nephrons that
specialized in conserving water. The kidneys of reptiles, having only cortical
nephrons, produce urine that is, at best, isotonic to body fluids. Amphibians
kidneys function much like those of fresh water fishes. When in fresh water,
the skin of the frog accumulates certain salts from the water by active
transport, and the kidneys excrete dilute urine., On land, where dehydration is
the most pressing problem of osmoregulation, frogsconserve body fluid by
reabsorbing water across the epithelium of the urinary bladder.
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Animal's nitrogenous wastes are correlated with its phylogeny and
habitat
Metabolism produces toxic by-products. The most troublesome is the nitrogencontaining waste from the metabolism of proteins and nucleic acids. The
nitrogenous waste product is ammonia, a small and very toxic molecule.
Excreting ammonia directly is bioenergetically efficient way to dispose of
waste. Many organisms first convert ammonia to compounds such as urea or
uric acids.
Fig. 40.14 P897
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Ammonia
Most aquatic organisms excrete nitrogenous wastes as ammonia. Ammonia
molecules are very soluble in water. In fishes, most of the ammonia is lost as
ammonium ions (NH4+) across the epithelium of the gills.. In fresh water
fishes, the epithelium of the gills takes up Na+ from the water in exchange for
NH4+, which helps maintain concentration in the surrounding water.
Urea
Although it works in water, ammonia excretion is unsuitable for disposing of
nitrogenous wastes on land. Mammals and most adult amphibians excrete
urea. This substance can be tolerated in amuch more concentrated form
because it is about 100,000 times less tocix than ammonia. Urea is produced
in the liver by a metabolic cycle that combines ammonia and CO2.
The circulatory system carries urea to the kidneys, where some of it is
retained in kidneys, where it contributes to osmoregulation by helping maintain
the osmolarity gradient that functions in water reabsorption.
Amphibians that undergo metamorphosis generally switch from excreting
ammonia to excreting urea during the transformation from an aquatic larva,
the tadpole, to the terrestrial adult. This biochemical modification, however, is
not always coupled with metamorphosis.
Uric acid
Land snails, insects, birds, and many reptiles excrete uric acid as the major
nitrogenous waste. (Uric acid is less soluble in water than either ammonia and
urea. In birds and reptiles, urine is eliminated in pastelike form along with
faeces.
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Thermoregulation maintains body temperature within a range conductive
to metabolism
Thermoregulation is the maintenance of body temperature within a range that
enables cells to function efficiently. For example, the rate of cellular respiration
increases with temperature up to a certain point, and then declines when
temperatures are high enough to begin denaturing enzymes. The properties of
membranes also change with temperature.
Heat transfer between organisms and their surroundings
An organism exchanges heat with its external environment by four physical
processes: conduction, convection, radiation, and evaporation.
Conduction is the direct transfer of thermal motion (heat) between molecules
of the environment and those of the body surface. Example: animal sits in a
pool of cold water or on a hot rock. Heat will always be conducted from a body
of higher temperature to one of lower temperature. Water is 50 to 100 times
more effective than air in conducting heat.
Convection is the transfer of heat by the movement of air or liquid past the
surface of abody, as when a breeze contributes to heat loss from the surface
of an animal with dry skin.
Radiation is the emission of electromagnetic waves produced by all objects
warmer than absolute zero, including an animal's body and the sun. example
organism absorb heat from the sun.
Evaporation is the loss of heat from the surface of a liquid that is losing some
of its molecules as gas. Evaporation of water from an animal has a significant
cooling effect on the animal's surface.
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Ectotherms derive body heat mainly from their surroundings and
endotherms derive it mainly from metabolism
An ectotherm warms it s body mainly by absorbing heat from its
surroundings. Most invertebrates, fishes, amphibians, and reptiles are
ectotherms. In contrast, an endotherm derives most or all of its body heat
from its own metabolism. Mammals, birds some fishes, and numerous insects
are endotherms. Many endothermsmaintain a consistent internal temperature
even as the temperature of their surroundings fluctuates (Fig. 40.15) P899.
Also, the term cold-blooded and warm-blooded are misleading. Many
lizards, which are ectotherms, have higher body temperatures when active
than mammals. So, the terms ectotherm and endotherm are not based on
body temperature but rather on the main source of body heat.
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Thermoregulation involves physiological and behavioral adjustments
Both ectothermic and endothermic organisms thermoregulate using some
combination of up to four general categories of adaptations.
1. Adjusting the rate of heat exchange between the organism and its
surroundings. Body insulation, such as hair, feathers, and fat located
just beneath the skin, reduce an organism's heat loss. Other
mechanisms that regulate heat exchange usually involve adaptations of
the circulatory system. Example many endotherms and some
ectotherms can alter the amount of blood flowing to their skin
(vasodilation and vasoconstriction).
Another type of adaptation that alters heat exchange is a special
arrangement of arteries and veins called a countercurrent heat
exchanger (fig. 40.16)P 900.
2. Cooling by evaporative heat loss. Terrestrial endotherms and
ecotherms lose water in breathing and across their skin. If the humidity
of the air is low enough, the water will evaporate and the organism will
lose heat by evaporative cooling. Evaporation from the respiratory
system can be increased by panting. Evaporative cooling via the skin
can be increased by such means as sweating in mammals.
3. Behavioral responses. Many organisms can increase or decrease
body heat loss by relocating. They will bask in the sun or on warm rocks
in water; fine cool, damp areas or burrow in summer; or even migrate to
a more suitable climate (Fig. 40.17.) P901.
4. Changing the rate of metabolic heat production. This is applied to
endotherms, particularly mammals and birds. Many species of
mammals and birds can double or triple their metabolic heat production
when exposed to cold.
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Feedback Mechanisms in thermoregulation
The regulation of body temperature in humans is an example of a complex
homeostatic system facilitated by feedback mechanisms. Nerve cells that
control thermoregulation, are concentrated in the hypothalamus. The
hypothalamus serves as a thermostat, responding to changes in body
temperature above and below a set point by activating mechanisms that
promote heat loss or gain (Fig. 40.21) P905.
Nerve cells that sense body temperatures are located in the skin, the
hypothalamus itself, and some other parts of the nervous system. Some of
these are warm receptors that signal the thermostat in the hypothalamus
when the temperature of the skin or blood increases. Others are cold
receptors that signal the thermostat when the temperature decreases.
Responding to body temperature below the normal range, the thermostat
inhibits heat-loss mechanisms and activates heat-saving ones such as the
vasoconstriction of superficial vessels and the erection of fur, while
stimulating heat-generating mechanisms (shivering and nonshivering
thermogensesis).
In
response
to
elevated
body temperature,
the
thermostat shuts down heat-saving mechanisms and promotes body
cooling by vasodilation, sweating, or panting.
Torpoer: Conseving energy during environmental extremes
Torpor is an alternative physiological state in which metabolism decreases
and the heart and respiratory system slow down. Many endotherms enter a
state of torpor in which their body temperature declines.
Hibernation is long-term torpor during which the body temperature is
lowered as an adaptation to winter cold and food scaricity.
Aestivation, or summer torpor, is characterized by slow metabolism and
inactivity. It enables an animal to survive long periods of high temperatures
and scarce water supplies.
Many small mammals and birds exhibit a daily period of torpor that seems
to be adapted to their feeding patterns. Example, most bats and shrews
feed at night and go into torpor when they are inactive during daylight hours.
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