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
- 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.
- 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.
- 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.
- 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.