Miller/Harley: Zoology 9e
Instructor’s Manual
28
Lecture Outline
Homeostasis involves all body systems; thermoregulation is a homeostatic mechanism to minimize internal temperature changes in the face of external temperature variation. The evolution of endothermy may be responsible for the success of several animal groups. Thermoregulation involves a number of organ systems, including the nervous and circulatory systems. The temperature range for an organism has shaped its enzyme evolution so that the optimum temperature reflects that temperature at which its enzymes are fully functional (i.e., its metabolism operates most efficiently at this temperature range). At temperatures that are too high, enzymes will be denatured and reactions will slow or cease. If temperatures fall too low, enzymes will also be adversely affected. The body temperature of an organism is the net result of heat produced as a metabolic by-product and heat gained and lost from the environment. There are 4 mechanisms of heat exchange:
1. Conduction is direct transfer of heat across a gradient between the environment and the animal.
2. Convection is transfer of heat due to movement of the air or water over the body of the animal.
3. Evaporation is a loss of heat from the surface of a wet animal as water molecules escape as a gas.
4. Radiation is the emission of electromagnetic waves; typically between the sun and the animal.
How do animals cope with temperature fluctuations?
Ectotherms (sometimes referred to as poikilotherms) derive their body heat from the environment—they have a low metabolic rate and poor insulation, so they change sites to increase or decrease heat (seek basking areas or shaded areas). These animals tend to be more common in the tropics, as life there requires less energy expenditure. Some examples of these types of animals include invertebrates, most fishes, amphibians, and reptiles.
Endotherms derive their heat from metabolic processes. They may have an insulating layer of fat in the hypodermis of the skin. Fur and feathers also provide insulation. Endothermic animals can colonize a wide range of habitats. Examples include birds and mammals.
Homeotherms maintain a relatively constant body temperature, but they may be ectotherms and move between habitats as necessary.
Heterotherms experience significant changes in body temperature daily or seasonally. Hummingbirds are a good example as they are very active and warm during the day, but go into torpor at night.
Ectotherms are more common in the tropical regions, whereas endothermic animals have a definite advantage in temperate regions.
Most invertebrates are ectotherms and tend to be thermoconformers (meaning that they match the environmental temperature. Invertebrates may withstand environmental fluctuations in temperature by:
reducing the water content of their tissues,
circulating the blood past the muscles,
producing antifreeze chemicals (glycerol) within their hemolymph or blood during the winter,
producing heat by shivering.
orienting their bodies in relation to the sun.
Other invertebrates can avoid overheating by literally sweating! Behavioral adaptations allow some invertebrates to gain heat from the sun by basking, or to avoid heat gain from the sun by raising the body off the ground. Body color affects heat absorption.
Fish, amphibians, and reptiles have classically been thought of as ectotherms and heterotherms, but exceptions exist. Air has a greater heat exchange rate than does water. Some large fish, such as bluefin tunas and great white sharks, have a countercurrent heat exchanger (the rete mirabile) that allows them to maintain a higher core temperature, allowing greater activity. In these fishes, heat is gained from muscle contraction and the internal body temperature remains fairly constant despite swimming in deeper or colder waters. Some make an anti-freeze such as sorbitol or glycerol.
Amphibians are limited to warm moist places by the evaporative cooling of the skin. However, the moist skin is necessary for respiratory exchange.
Reptiles rely on behavioral adaptations for temperature regulation, but they do have some unique physiological adaptations such as circulation shunts (marine reptiles, such as sea turtles), salivation, and panting (lizards).
Birds and mammals have complex mechanisms for producing, retaining or dissipating heat as necessary—they are homeothermic endotherms. Birds employ gular flutter to prevent overheating. The gular pouch is highly vascularized and the passage of air through the pouch allows for convective cooling. Feathers act to retain heat—birds cannot sweat. Aquatic birds also may have countercurrent heat exchange vessels in their legs to retain heat and use feathers as insulation.
Many marine mammals have a countercurrent exchange system similar to birds in their tail and flippers (it is also present in caribou legs and in some other arctic mammals). Blubber acts as insulation in aquatic species, but thick fur works better in polar terrestrial organisms.
Migration may also allow an animal to avoid temperature extremes. Birds and mammals may also bask in the sun or seek shaded areas in order to regulate body temperatures.
There are some behavioral mechanisms employed to change body temperature:
Heat production in endotherms is accomplished by muscle contraction and other metabolic processes. Voluntary or involuntary contraction of muscle fibers generates heat, so shivering produces heat. This is shivering thermogenesis.
Nonshivering thermogenesis produces heat via an ATPase pump. Thyroxine hormones increase the permeability of cells to sodium ions. The ATPase pumps remove the sodium ions and hydrolyze ATP as an energy source. The hydrolysis of ATP releases heat energy.
Brown fat has an enormous number of mitochondria with an iron-containing cytochrome (hence the brown color) and is important in for heat generation in newborn mammals (even human newborns have some), and those that hibernate (bats, for instance).
The hypothalamus of the brain contains both a heating center (for vasoconstriction and shivering) and a cooling center (for vasodilation and panting). The hypothalamus triggers both hibernation and aestivation, and the metabolic changes associated with these processes. These processes include growth of winter pelage and accumulation of fat reserves and brown fat. Metabolism slows and the body temperature of the animal will fall dramatically (ice will sometimes form on hibernating bats!).
Excretion is the elimination of metabolic waste products such as carbon dioxide, water, nitrogen and ions. However, some ions are required. Osmoregulation is the excretion of nitrogenous wastes, water, and ions to maintain homeostasis, but regulation is not required in all cases:
Osmoconformers are animals whose body fluids are equal in isosmotic concentration to their environment (obviously referring to aquatic organisms). Ions enter the body from food, drink and across the gills; ions leave the body by diffusion. Marine invertebrates are typically osmoconformers.
Osmoregulators maintain their body fluids at different concentrations than the environment; freshwater invertebrates are osmoregulators. Freshwater protozoans and sponges often pump ions in and have contractile vacuoles to rid the cells of excess water. Protonephridia, used for excretion and osmoregulation, are the simplest nephridia, including flame cells, and are found in rotifers, some annelids and flatworms. Metanephridia open internally to the body fluids and are multicellular; they occur in annelids and adult molluscs. Some crustaceans may excrete ammonia (nitrogen) via their gills, but others have antennal (green glands), or maxillary glands.
Insects have Malpighian tubules that transport uric acid into the gut. Coxal glands are typical in arachnids (for collecting waste and perhaps releasing pheromones), although some arachnids have Malpighian tubes as well. The requirement for osmoregulation depends on the difference between internal and external osmotic concentrations. All vertebrates have an internal osmotic concentration about one-third that of seawater (hypoosmotic). In seawater, they gain ions by diffusion and lose water. In freshwater, the internal concentration is greater than the external concentration, so they lose ions by diffusion and gain water.
In vertebrates osmoregulation occurs by filtration of blood, reabsorption of ions from blood, secretion of ions into the filtrate, and excretion. Water is gained through absorption of liquids and metabolism of solid foods. Water may be lost through processes of sweating, evaporation from respiratory surfaces, elimination in fecal matter, and excretion. Solutes (minerals and ions) are acquired through absorption in the intestines and/or through the gill epithelium, and from metabolism of foods. Solutes are lost in ways related to water loss. The vertebrate kidneys are responsible for maintaining proper water and solute balances. Kidneys are paired, but their structure varies widely—there are 3 types of kidney that vary in the number of blood-filtering units:
1. The pronephros is a primitive kidney, and is typically replaced during development. Larval amphibians and fishes have pronephros kidneys. Lampreys retain a pronephros kidney as adults. The pronephros kidney forms in the anterior portion of the body cavity and is sometimes referred to as a "head kidney".
2. The mesonephros is the functional kidney of adult fishes and amphibians.
3. The metanephros is the functional kidney of adult reptiles, birds, and mammals.
Elasmobranchs (sharks, skates, rays) have high circulating levels of trimethylamine oxide (TMO) and urea that raise the internal concentration so the organism is hypertonic to the ocean water. This conserves water. They also have a rectal gland that secretes excess sodium chloride.
Teleost fish face different problems with osmoregulation depending on their environment. Marine fish are hypotonic, and excrete a small volume of isosmotic urine, and freshwater fish are hypertonic, and excrete a large volume of hypoosmotic urine.
A pumping mechanism involving specialized cells of the gills allow fish to maintain salt balance by adding ions in freshwater and releasing them in seawater. Freshwater fish do not actively drink water, whereas saltwater fishes drink large quantities of water. Additionally, freshwater fishes have a mucus coating over their bodies to help prevent water loss.
Amphibians have a kidney similar to that of freshwater fish, but when they are in water, the bladder acts as a water reservoir, increasing or decreasing concentration as needed. Also, amphibians may acquire water through absorption from foods, through the skin, and by drinking. Reptiles, birds, and mammals all have complex metanephric kidneys that are much more efficient than amphibian kidneys. Some desert and marine reptiles (and marine birds) have salt glands to excrete excess sodium chloride (accumulated from salt water or salty foods) from the body.
The functional unit of the metanephric kidney is the nephron. The glomerular capsule (Bowman’s capsule) is the first filtering device in the outermost kidney. It branches into the glomerulus. Water, ions, sugar, and salts are forced out of the glomerular capillaries into the lumen of the glomerular capsule by blood pressure. In the proximal convoluted tubule, the loop of the nephron, and the distal convoluted tubule, this filtrate is modified by selective reabsorption of water and by tubular secretion (both passively and by active transport). Secretion and reabsorption are fine-tuned so that the osmotic concentration of the urine maintains the correct water and salt balance in the animal. A countercurrent mechanism occurs in the loop of the nephron, and the longer the loop, the more concentrated the resultant urine (the more water is reabsorbed); the urine can be up to 20 times as concentrated as the blood. Urine flows from the tubules of the nephron to collecting ducts which empty into the renal pelvis. The collecting ducts of the kidney are the final sites where water may be reabsorbed and the urine more greatly concentrated. Urine then flows, without further modification, to the ureters, the urinary bladder, and the urethra, where it exits the body.
Teaching Suggestions
Students often have a hard time keeping track of the osmoregulatory mechanisms. However, if you set up the idea that there is an internal concentration of ions that the animal must maintain, and then describe the external concentrations, students often recall that ions and water move in opposite directions to equal the concentration gradient. If the initial internal concentration is anything other than identical to the external concentration, the animal will have problems if the concentration gradient is equalized.
So, if the internal concentration is about 295 milliOsmoles (in a vertebrate) and the external concentration is near 1000, as it is in seawater, the animal will gain ions and lose water. The problem is too many ions and dessication—the solution is to get rid of ions via gills, wastes, etc., and to take in water by drinking. Drinking seawater adds more ions, but this is the only water source present. Clearly, only a small amount of urine is released.
In freshwater, a vertebrate with an internal concentration of 295 milliOsmoles is surrounded by an external environment with an osmotic concentration near zero. So, it will gain water (bloating) and lose ions—these are problems. The solution is to take in ions via food and the gills, and excrete water by making copious amounts of dilute urine.
Marine invertebrates typically have an internal concentration near seawater. Contrast a marine mammal that eats invertebrates with a marine mammal that eats other vertebrates. Which would have the most problem with salt balance?
Lecture Enrichment
Water balance
It is important to emphasize that input must equal output. The typical human inputs and outputs of water are:
Input:
(liters per day) 2.75
Fluid intake 1.2–1.8
Metabolic water 0.75
Intake via food 0.5–1
Output:
(liters per day) 2.75
Urine 1.5–2 (and salts and urea)
Sweat 0.5 (and salts)
Gastrointestinal 0.1 (and salts)
Lung <0.5 (and CO2)
Human body temperature
Although it is commonly stated that the normal human body temperature is 98.6° F, it is actually 98.2° F (36.8° C). According to an article in Science, the original value of 98.6° came from averaging earlier measurements that were then rounded to the nearest Celsius degree (37°), then it was converted to Fahrenheit. A good lesson in rounding and significant figures!
Death due to temperature extremes
Death due to high temperatures is due to a variety of factors, such as thermal denaturation of proteins or inactivation of enzymes, inadequate oxygen supply (in aquatic organisms) or temperature effects on membrane structure. The most heat tolerant vertebrate is Cyprinodon diabolis, the Devil’s Hole Pupfish, a small minnow-like fish living in the desert waters of California. It lives at 33.9° C and survives at temperatures up to 43° C. The least heat tolerant fish is Trematomus, the Antarctic ice fish, which dies at a chilly
6° C.