Download Chapter 44

Chapter 44
REGULATING INTERNAL ENVIRONMENT
OVERVIEW OF HOMEOSTASIS
The ability of animals to regulate their internal environment is called homeostasis.
A regulator is an animal that uses mechanisms of homeostasis to maintain an internal
environment when the external environment fluctuates.
A conformer is an animal that allows the internal environment to fluctuate in agreement with the
changes in the external environment. Conformers usually live in stable environments.
No organism is a perfect regulator or conformer. Organisms use a combination of mechanisms
when faced with environmental changes.
Homeostasis requires a careful balance of materials and energy: gains versus losses.
REGULATION OF BODY TEMPERATURE
The rate of most enzyme-mediated reactions increase by a factor of 2-3 for every 10°C
temperature increase, until the temperature is high enough to denature the enzyme.
Q10 effect is the multiple by which a particular enzymatic reaction or overall metabolic process
increases with a 10°C increase in body temperature.
Each animal has an optimal temperature range.
The properties of membranes change with temperature.
Four physical processes account for heat gain or loss
Heat flows from areas of higher temperature to areas of lower temperature.
All organisms exchange heat with the environment.
There are four ways to exchange heat with the environment:

Conduction: the direct transfer of heat between two objects that are in direct physical
contact. E.g. you sitting directly on a metal stand during a football game in winter.

Convection: when air or water moves over the body, heat is removed from the body. The
air or water in contact with the body is constantly replaced and not allowed to warm up.

Radiation: the emission of electromagnetic waves by all objects warmer than absolute zero;
the transfer of heat between two bodies that are not in direct contact.

Evaporation: when a liquid is converted into a gas, e.g. when you sweat, the evaporation of
the sweat removes heat from your skin.
Air is a poor conductor of heat and, therefore, it is a good insulator.
Animals have developed methods of trapping air to conserve heat, e. g. birds have feathers and
mammals have fur.
There are two general categories based on how animal obtain heat:

Endotherms produce heat in their own tissues and have higher basal metabolic rate.
Endotherms have a high metabolic rate and generate enough heat to maintain their body
temperature significantly higher than environmental temperature.
Mammals, birds, some fish, a few reptiles and many insects are endotherms.

Ectotherms obtain heat mostly from the environment and have low basal metabolic rate.
Ectotherms have a very low metabolic rate and generate little heat, which has little effect on
body temperature.
Most invertebrates, fishes, amphibians and reptiles are ectotherms.
Thermoregulation involves physiological and behavioral adjustments
1. Adjusting the rate of heat exchange between the animal and its environment.
Animals have developed methods of trapping air to conserve heat, e. g. birds have feathers and
mammals have fur; fat under the skin.
High blood flow in the skin normally results from vasodilation, an increase in the diameter of
superficial blood vessels. Vasodilation increases the heat transfer from the skin to the
environment by radiation, conduction and convection.
Vasoconstriction reduces blood flow and heat transfer by decreasing the diameter of
superficial vessels.
Countercurrent heat exchanger is found in several groups of aquatic animals and mammals
and birds that live in cold habitats.



The veins and arteries are next to each other in the limbs or tongue.
Arteries carry warm blood from inside the body to the extremities.
The heat flows from the arteries to the veins and is returned to the body instead of being lost
to the surroundings.
2. Cooling by evaporative heat loss.
Terrestrial animals lose water by evaporation off the skin and in breathing.
Water absorbs heat when it evaporates and this has a cooling effect in the body.
3. Behavioral responses
Changes in posture and location increase the cooling of the body, e. g. moving in the shade;
basking in the sun.
Hibernation and migration are behavioral adaptations to adjust to a drastic change in the
environment.
4. Changing the rate of metabolic heat production.
Endotherms are capable of changing their metabolic rate and increase or decrease heat
production.
Mechanisms of thermoregulation
1. Mammals and birds


Mammals usually maintain their body temperature between 36°C and 38°C.
Birds maintain a body temperature between 39°C and 42°C.
Heat production increases when moving or shivering.
In mammals, certain hormones can cause mitochondria to increase their metabolic activity and
produce heat instead of ATP. This is called nonshivering thermogenesis, NST.
Most endotherms have a specialized heat-producing tissue called brown adipose tissue.




This tissue has many mitochondria and large amount of stored fats.
When fats are oxidized no ATP is produced but much heat is released.
Brown adipose tissue releases about 10 times more heat than other body tissues.
It is an adaptation of small endotherms to achieve the required body temperature.
Insulation by hair, feathers, fat layers and counter current exchange is very effective.
Panting, fluttering of skin pouches below the mouth, use of saliva and urine over the body to
increase evaporation and cool the body, are mechanisms used when the environmental
temperature is high.
2. Amphibians and reptiles
The optimal temperature for amphibians varies substantially between the species, e. g.
salamanders may vary between 7°C and 25°C.
Amphibians lose heat rapidly when exposed to air. Behavioral adaptations help maintaining an
optimal temperature.
Reptiles also use behavioral adaptations.
Some reptiles have physiological adaptations used in thermoregulation, e. g. vasoconstriction in
marine iguanas found in the Galapagos Islands; female pythons incubating eggs produce heat
by shivering .
3. Fishes
Most fishes are conformers maintaining a body temperature within 1 or 2 degrees of the
surrounding water temperature.
Fishes lose most of their metabolic heat through the gills.
Fishes have countercurrent heat exchangers in the internal muscles.
In some fishes, specialized heat generating organs warm the eyes and brain.
4. Invertebrates
Aquatic invertebrates are thermoconformers.
Terrestrial invertebrates use behavior to maintain an optimal body temperature.
Some insects like bees and moths are endothermic. Their powerful flight muscles generate a
large amount of heat. They also have countercurrent heat exchangers.
In hot days, these endothermic insects can shut the countercurrent mechanism and allow heat
to be lost through the abdomen.
Honeybees transport water to the hive and use the fanning of their wings to increase
evaporation and cooling.
5. Feedback mechanisms in thermoregulation.
To regulate body temperature, the body has a sensor that monitors some aspect of the
environment.

Nerve cells in the skin sense temperature.
An integrator is part of the nervous system that evaluates the incoming sensory information
and decides if a response is necessary.

The hypothalamus in the brain acts as a thermostat that senses the body
temperature above and below certain points.
An effector is any structure that helps to restore the desired internal condition.

Vasoconstriction, vasodilation, shivering, panting are controlled by muscles and
nerve activity.
6. Acclimatization
Animals can adjust to a range of changing temperatures over a period of days or weeks. This is
called acclimatization.
A change in the amount of insulating fur and feathers is one way animals become acclimatized.
Acclimatization in ectotherms sometimes includes changes at the cellular level, e. g. production
of certain enzymes that have a different optimal temperature; changes in the amount of
membrane unsaturated lipids.
Antifreeze compounds called cryoprotectants prevent cells from freezing.
Stress-induced proteins protect against increase concentration of toxins, pH and temperature.
Heat-shock proteins are produced within minutes of a rapid increase of temperature and
prevent enzymes from denaturing.
Torpor
Torpor is a dormant state in which the activity of the animal is low and metabolism decreases.
Torpor occurs when conditions become unfavorable and/or food is not available.
Hibernation is long-term torpor that evolved as an adaptation to winter cold and food scarcity.
Body temperature may be reduced to as low as 1°C - 2°C.
Estivation is summer torpor that enables animals to survive high temperatures and low water
supply.
Some small mammals and birds exhibit daily torpor that seems to be adapted to their feeding
patterns, e. g. bats and shrews during the day, chickadees and hummingbirds at night.
WATER BALANCE AND WASTE DISPOSAL
Osmoregulation is the management of water content in the body and solute composition.
Water balance and waste disposal depend on transport epithelia.
Transport epithelia have the ability to move specific substances in controlled amounts in
particular directions.
In most animals, transport epithelia are arranged into complex tubular networks with extensive
surface areas.
The secretory cells of the transport epithelium actively secrete salts from the blood into the
tubules.
Nitrogenous wastes
Principal metabolic wastes are water, carbon dioxide and nitrogenous wastes (ammonia.
urea and uric acid).
Nitrogenous wastes are the products of deamination of amino acids.
Ammonia is highly toxic and it is usually converted to uric acid or urea.




Ammonia excretion is most common in aquatic species.
Ammonia is converted to ammonium, NH4+.
Ammonium ions are excreted through the gills.
In many invertebrates, ammonia is excreted across the whole body surface.
Urea is formed in the liver by combining ammonia and carbon dioxide.




Urea is soluble and less toxic than ammonia.
It requires less water than the same amount of ammonia.
Mammals, most adult amphibians and many marine fishes and turtles excrete urea.
The animal must spend energy to produce urea.
Uric acid is the product of nucleic acid and amino acid breakdown.




It is excreted in the form of a crystalline paste with little water loss.
It is relatively non-toxic.
Uric acid production requires more energy than urea production.
Birds, reptiles, land snails, and some insects secrete uric acid.
Osmolarity
Diffusion is the movement of solutes from the region of higher concentration to the region of
lower concentration.
Osmosis is the movement of water from the area of higher concentration to area of lower
concentration.
Osmolarity is the concentration of a substance expressed in moles per liter, mol/l.
The unit of osmolarity often used in physiology is milliosmoles per liter, mosm/L.



1 mosm/L = 10-3 M.
The osmolarity of the human blood is 300 mosm/L.
The osmolarity of seawater is 1000 mosm/L.
When two solutions separated by a selectively permeable membrane have the same osmolarity
are said to be isoosmotic. The terms hyperosmotic and hypoosmotic are also used.
Osmoconformers are isoosmotic to their surroundings.
Osmoregulators are animals that must control their internal osmolarity.


Stenohaline animals cannot tolerate substantial changes in external osmolarity.
Euryhaline animals can survive large fluctuations of external osmolarity.
Maintaining water balance
In saltwater ...

Most marine invertebrates and hagfishes are osmotic conformers. Their body fluids vary
with changes in the seawater.

The cells are hypotonic relative to the surrounding seawater.

Water tends to flow out of the gill cells.

The cells run the risk of plasmolysis, which is shriveling and dying.

Saltwater fish secrete large amounts of salt and drink lots of water.
Marine bony fish must replace lost fluid.




They lose water osmotically through their skin and gills.
Drink large amounts of water and take in salt.
Excrete excess salt through their gills
Excrete little urine in order to conserve water.
Chondricthyes accumulate and tolerate urea and their tissues are hypertonic to seawater.






Water diffuses into their body.
They maintain a high concentration of urea and trimethylamine oxide (TMAO), which
protects proteins from damage by urea.
Concentration of body salts, urea, TMAO and other compounds is greater than 1,000
mosm/L and therefore slightly hyperosmotic to seawater. This decreases the water
loss through the skin.
Water slowly enters the body of sharks and relatives.
Kidneys excrete large volume of urine.
Excess salt is excreted by the kidney and in some by the rectal gland.
In freshwater fish...





Freshwater is hypotonic to the cell.
Ions tend to move out of the cell into the surrounding water.
Electrolytes lost must be replace by eating and by active transport from the surrounding
water.
The gill cells are hypertonic relative to the surrounding water, therefore, the cells gain water
through osmosis.
Cells and tissues that are gaining water are under osmotic stress.




Osmotic stress means that the concentration of solutes in the cells and tissues is
abnormal.
The ability to achieve electrolyte balance is called osmoregulation.
Freshwater fish excrete large amounts of water in the urine and do not drink water.
Some protists have contractile vacuoles that pump out excess water.
Some animals that live in temporary ponds or films of water around soil particles can lose
almost all their body water and survive. This ability is called anhydrobiosis.

Tardigrades (water bears) contain 85% water in their body; in a dehydrated state
they have less than 2% water in their bodies.
These animals can live in this desiccated state for years. The mechanism is not understood.
Land animals...



Land animals constantly lose water to the environment through evaporation.
Gas exchange occurs through the wet surfaces of the lung epithelium.
Sweating and panting in order to keep their body cool also loses water.
EXCRETORY SYSTEMS
Most excretory systems produce urine by refining a filtrate derived from body fluids.
Blood, coelomic fluid or hemolymph is collected and then selective reabsorption of fluids takes
place with the secretion of unwanted solutes.




The initial fluid is filtrated by transport epithelia. Cells and large molecules are retained in the
fluid.
The filtrate contains small molecules like glucose, urea, electrolytes, amino acids and other
molecules.
Hydrostatic or blood pressures forces the water and solutes into the filtrate.
Selective reabsorption of valuable solutes takes place through active transport.
EXCRETORY SYSTEM OF INVERTEBRATES
1. Sponges and cnidarians use diffusion from cells to environment.
2. Protonephridia are found flatworms, nemerteans, rotifers, lancelets and some annelids.






Internal tubules with no openings.
Blind ends called flame bulbs have flame cells.
Fluid enters the lumen of the tubule through selectively permeable membranes of the
folding tubule cells.
The beating of the cilia of the flame cells keep the fluid moving towards the
nephridiopore.
Excrete through nephridiopores.
It functions mostly in osmoregulation; wastes diffuse through the skin or are excrete
through the lining of the gastrovascular cavity.

In some parasitic worms that are isoosmotic to their environment, the protonephridia are
used to expel wastes.
3. Metanephridia are found in most annelids, in mollusks.







Metanephridia are found in most annelids.
Each metanephridium is a tubule open at both ends.
The inner ends open into the coelum as a ciliated funnel called nephrostome, which
collects fluid from the coelum.
The outer end is a nephridiopore.
As coelomic fluids pass through the tubule, needed material is reabsorbed.
The urine is very diluted and balances the uptake of water through the skin.
Metanephridia have an osmoregulatory and excretory function.
4. Malpighian tubules are found insects and spiders.





Insects and other terrestrial arthropods have Malpighian tubules.
Blind end tubules of the digestive tract that stretch into the hemocoel.
Their cells transfer wastes and salts from the hemolymph to the lumen of the tubule by
diffusion and active transport. Water follows.
They empty into the intestine.
Water and some salts are reabsorbed in the rectum and almost dry nitrogenous wastes
are eliminated with the feces.
MAMMALIAN URINARY SYSTEM
Kidney produces urine.
Ureter brings urine to the urinary bladder.
Urinary bladder stores urine temporarily.
Urethra leads the urine to the outside.
The outer region of the kidney is called the renal cortex and the inner region the renal
medulla.
The renal medulla contains a number of cone-shaped structures called renal pyramids.
At the tip of each renal pyramid is a renal papilla into which the collecting ducts open.
The renal pelvis is a pyramidal chamber that collects and leads the urine to the ureter.
The nephron is the functional unit of the kidney.
1. The filtrate passes from capillaries  Bowman's capsule  proximal convoluted tubule
 loop of
Henle  distal convoluted tubule  collecting duct  renal pelvis.
2. Blood circulates through the kidney in the following sequence:
Renal artery  afferent arteriole  capillaries of glomerulus  efferent arteriole 
peritubular capillaries  small veins  renal veins.
3. Filtration, reabsorption and secretion produce urine.



Filtration is not selective with regard to ions and small molecules.
Reabsorption is highly selective.
Some substances are actively secreted from the blood.
KIDNEY STRUCTURE
The outer region of the kidney is called the renal cortex and the inner region the renal
medulla.
The renal medulla contains a number of cone-shaped structures called renal pyramids.
At the tip of each renal pyramid is a renal papilla into which the collecting ducts open.
The renal pelvis is a pyramidal chamber that collects and leads the urine to the ureter.
The nephron is the functional unit of the kidney. It consists of a single long tubule and a ball of
capillaries. The blind end of the tubule forms the Bowman's capsule, which surrounds the
glomerulus.
1. The filtrate passes from capillaries  Bowman's capsule  proximal convoluted tubule
 loop of
Henle  distal convoluted tubule  collecting duct  renal pelvis.
About 80% of the nephrons in human are cortical nephrons with a short loop, and 20% are
juxtamedullary nephrons with along loop of Henle.
Mammals and birds are the only animals with juxtamedullary nephrons. The nephrons of all
other animals lack the loop of Henle.
The tubules of the nephron are lined with a transport epithelium whose function is to reabsorb
water and solutes.



From 1,000 to 2,000 liters of blood flows through a pair of human kidneys each day.
Nearly all of the sugar, and other organic nutrients and most of the water is reabsorbed.
Only about 1.5 liter urine is discarded.
2. Blood circulates through the kidney in the following sequence:
Renal artery  afferent arteriole  capillaries of glomerulus  efferent arteriole 
peritubular capillaries  vasa recta  small veins  renal veins.
3. Filtration, reabsorption and secretion produce urine.
Bowman's capsule



Filtration is not selective with regard to ions and small molecules.
Reabsorption is highly selective.
Some substances are actively secreted from the blood.
Hydrostatic pressure in glomerular capillaries is higher than in other capillaries. Efferent arteriole
is smaller than the afferent arteriole.
The high pressure forces about 10% of the plasma out of the capillaries into Bowman's capsule.
Glomerular capillaries are highly permeable with numerous small pores (fenestration) present
between the endothelial cells.
There is a large permeable surface provided by the highly coiled capillaries.
Glucose, amino acids, ions and urea pass through and become part of the filtrate.
Reabsorption is highly selective.
Proximal tubule







H+ and NH3 are filtered into the lumen of the Bowman's capsule and the proximal tubule.
NH3 neutralizes the acid and maintains a constant pH.
HCO3- is a blood buffer and about 90% is reabsorbed here through active transport.
Toxins and foreign substances also pass into the filtrate.
K+, glucose and amino acids are reabsorbed.
NaCl and water are reabsorbed. Na+ is actively transported from the filtrate into the
interstitial fluid of the kidney; water follows by osmosis.
The osmolarity of the filtrate in the proximal tubule is about 300 mosm/L.
Descending limb of the loop of Henle





The transport epithelium in this section of the tubule is permeable to water but not to salts
and small solutes.
The interstitial fluid in this area, the outer medulla, is hyperosmotic to the filtrate.
The osmolarity of the interstitial fluid gradually increases from the outer cortex to the inner
medulla of the kidney.
As the filtrate descends, it loses water to the interstitial fluid and it becomes more
concentrated.
The osmolarity in the descending arm of the loop of Henle changes from 300 mosm/L at the
beginning to 1,200 mosm/L at the tip of the loop.
Ascending limb of the loop of Henle







The filtrate reaches the tip of the loop deep into the inner medulla in the case of the
juxtamedullary nephron, and then moves back to the cortex.
The transport epithelium of the ascending limb is permeable to salts but not water (in
contrast with the descending limb).
The filtrate passes first through a thin section of the ascending limb and NaCl, which had
become concentrated in the descending tube, now passes out by diffusion into the interstitial
fluid.
This addition of salt contributes to the high osmolarity of the inner medulla.
In the following thick section of the tubule, NaCl is excreted into the medulla by active
transport.
The filtrate passes out salts but no water, and becomes more diluted.
In the ascending arm, the osmolarity changes from 1,200 mosm/L to 100 mosm/L in the
distal tubule.
Distal tubule




K+ and H+ are actively secreted into the filtrate.
NaCl and HCO3- are actively reabsorbed, and water diffuses out by osmosis.
The control secretion of H+ and reabsorption of HCO3- contribute to the pH regulation of the
blood and interstitial fluids.
Osmolarity here is 100 mosm/L.
Collecting duct









The collecting duct brings the filtrate from the cortex to the inner medulla.
The filtrate is hypoosmotic to the interstitial fluid as it enters the collecting duct.
NaCl is actively reabsorbed here.
The transport epithelium here is permeable to water and urea (in the inner medulla) but not
to salt.
The filtrate becomes more concentrate as it loses more and more water to the hyperosmotic
interstitial fluid of the medulla.
NaCl and urea are the major contributors to the high osmolarity of the interstitial fluid in the
medulla.
By reabsorbing water, the urine becomes hyperosmotic to the general body fluids, but is
isoosmotic to the interstitial fluid of the medulla.
The high osmolarity of the interstitial fluid allows solutes to remain in the urine and be
eliminated with minimal water loss.
In the collecting tubule, the osmolarity changes from an initial 100 moms/L to 1,200 mosm/L.
ADAPTATION TO CONSERVE WATER
The ability of the kidney to conserve water is a an adaptation to terrestrial life.
The nephron especially in the area of the loop of Henle, uses energy in order to produce a
region of high osmolarity in the outer and inner medulla of the kidney, which can then be used to
extract water from the filtrate and urine in the collecting duct.
The principal solutes in this osmolarity gradient are NaCl, which is excreted by the loop of
Henle, and urea, which leaks across the epithelium of the collecting duct in the inner medulla.
Summary of osmolarity change in the filtrate:
Bowman's capsule: 300 mosm/L
Proximal tubule: 300 mosm/L
Descending loop of Henle: 300 to 1,200 mosm/L.
Ascending loop of Henle: 1,200 to 100 mosm/L
Distal tubule: 100 mosm/L.
Collecting duct: 100 to 1,200 mosm/L.
Urine: 1,200 mosm/L
The process depends on the salt concentration in the interstitial fluid in the kidney medulla.
The interstitial fluid has higher salt concentration around the loop of Henle.
There is a counterflow of fluid through the two limbs of the loop of Henle.
Water is drawn by osmosis from the filtrate as it passes through the collecting ducts and it
concentrates the filtrate.
Capillaries known as the vasa recta remove some of the water that diffuses from the filtrate into
the interstitial fluid.
The vasa recta are extensions of the efferent arteriole that extend deeply into the medulla and
then return fluid to the veins draining the kidney.
Urine is about 96% water, 2.5% urea, 1.5% salts and traces of other substances.
Urinalysis is the physical, chemical and microscopic examination of urine.
HORMONE REGULATION OF KIDNEY FUNCTIONS
Urine volume is regulated by the hormone ADH (antidiuretic hormone), which is produced by
the hypothalamus, and stored and released by the posterior lobe of the pituitary gland in
response to an increase in osmotic concentration of the blood, caused by dehydration.
An increase above the set point is 300 mosm/L releases ADH.


Low fluid intake decreases blood volume and increase osmotic pressure of blood.
ADH increases the permeability of collecting ducts to water, increasing reabsorption
of water and decreasing water excretion.


An increase in fluid uptake decreases the osmolarity of the blood below 300 mosm/L.
Little ADH is released and the permeability to water of the distal tubule and collecting
duct is reduced and more water is excrete.
A second regulatory mechanism involves the tissue called juxtaglomerular apparatus or
JGA.
Renin-angiotensin-aldosterone system, RAAS.
Decrease in blood volume → decrease in blood pressure → cells of juxtaglomerular apparatus
secrete renin → renin converts angiotensinogen in plasma to angiotensin → enzyme in lungs
converts angiotensin to angiotensin II → blood vessels constrict and aldosterone is secreted by
the adrenal gland→ aldosterone increases sodium and water reabsorption.
Angiotensin II causes an increase...




in blood pressure by constricting arterioles and decreasing blood flow to capillaries including
those of the kidneys;
in blood volume by increasing the reabsorption in the proximal tubules of NaCl and water;
this results in a decrease of urine volume, and an increase in blood volume and pressure.
In stimulating the adrenal gland to produce aldosterone.
Aldosterone increases sodium reabsorption by distal and collecting ducts.



Sodium is the most abundant extracellular ion.
It is produced by the adrenal gland as a reaction to a drop in blood pressure.
Decrease on blood pressure is caused by a decrease in blood volume due to
dehydration.
The function of ADH and RAAS counter different osmoregulatory problems, even if both
increase water reabsorption.
Injury and severe diarrhea will decrease blood volume and loss of electrolytes, but will not
change the osmolarity of the blood. The RAAS will detect the loss of blood volume and will react
but the ADH will not because the osmolarity remains the same.
Atrial natriuretic peptide (ANP) is produced by the heart and increases sodium excretion and
decreases blood pressure, and decreases the production of renin and ADH.
It works antagonistically to the renin-angiotensin system.
It is a response to an increase in blood volume and pressure.
Kidney adaptations
The vertebrate kidney has evolved in different habitats.

Mammals have long loop of Henle to produce concentrated urine.

Birds have short loop of Henle but the main water conservation is the production of uric acid;
birds will be too heavy to fly if they had a urinary bladder full of liquid.

Reptile have only cortical nephrons and produce urine that is isoosmotic to body fluids; the
cloaca reabsorbs water and reptiles secrete uric acid.

Fresh water fish excrete large amounts of diluted urine because they are hyperosmotic to
their environment; their kidneys reabsorb large amounts of salts.

Salt water bony fish are hypoosmotic to sea water; their kidneys secrete very little urine and
large amounts of divalent ions Ca2+, Mg2+ and SO42ˉ; the monovalent Na+ and Clˉ and
nitrogenous waste in the form of NH4+ through the gills.
Feedback mechanisms integrate the work of the nervous system and hormones in order to
maintain homeostasis.