Answer of Question of Temperature & Body Fluid Regulation
Thermoregulation maintains body temperature within a range conducive to metabolism. The maintenance of body temperature within a range that enables cells to function efficiently involves heat transfer between the organism and the external environment. Heat exchange involves the physical processes of conduction, convection, evaporation and radiation. Ectotherms derive body heat mainly from their surroundings and endotherms derive it mainly from metabolism. Homeotherms generally have a relatively constant core body temperature, while heterotherms have a variable body temperature. Comparative physiology reveals diverse mechanisms of Thermoregulation among animals. Many large flying insects generate metabolic heat by muscle contractions, and many have countercurrent heat exchangers that retain it. Although the body temperature of most fishes matches the environment, some large, active species maintain a higher current heat exchanger. Reptiles and amphibians maintain internal temperatures within tolerable ranges mainly by various behavioral adaptations. Birds may thermoregulate by panting, increasing evaporation from a vascularized pouch in the mouth, and by passing blood going to legs through a rete mirabile system. Mammals and birds can adjust their rate of metabolic heat production by shivering and non-shivering thermogenesis. The marine mammals maintain their high body temperatures in cold waters by a thick layer of insulating blubber and countercurrent heat exchange between arterial and venous blood. Thermogenesis involves mainly shivering, enzymatic activity, brown fat, and high cellular metabolism. Thermoregulatory areas of the hypothalamus serve as the body’s thermostat, receiving nerve signals from warm and cold receptors and responding by initiating either cooling or warming processes. Torpor, including hibernation and aestivation, is a physiological state characterized by a decrease in metabolic, heart, and respiratory rates. This state enables the animal to temporarily withstand varying periods of unfavorable temperatures or the absence of food and water. Some Invertebrates have contractile vacuoles, flame-cell systems, antenna! (green) glands, maxillary glands, coxal glands, nephridia, or Malpighian tubules for osmoregulation. The osmoregulatory system of vertebrates governs the concentration of water and ions; the excretory system eliminates metabolic wastes, water, and ions from the body. Freshwater animals tend to lose ions and take in water. To avoid hydration, freshwater fishes rarely drink much water, have impermeable body surfaces covered with mucus, excrete a dilute urine, and take up ions through their gills. Marine animals tend to take in ions from the seawater and to lose water. To avoid dehydration, they frequently drink water, have relatively permeable body surfaces, excrete a small volume of concentrated urine, and secrete ions from their gills. Amphibians can absorb water across the skin and urinary bladder wall. Desert and marine reptiles and birds have salt glands to remove and secrete excess salt. In terrestrial animals, such as reptiles, birds and mammals, the kidneys are important osmoregulatory structures. The functional unit of the kidney is the nephron, composed of the glomerular capsule, proximal convoluted loop of the nephron, distal convoluted tubule, and collecting duct. The loop of the nephron and the collecting duct are in the kidney’s medulla; the other nephron parts lie in the kidney’s cortex. Urine passes from the pelvis of the kidney to the urinary bladder. To make urine, kidneys produce a filtrate of the blood and reabsorb most of the water, glucose, and needed ions, while allowing wastes to pass from the body. Three physiological mechanisms are involved: filtration of the blood through the glomerulus, reabsorption of the useful substances, and secretion of toxic substances. In those animals with a loop of nephron, salt (Nace) and urea are concentrated in the extra cellular fluid around the loop, allowing water to move by osmosis out of the loop and into the peritubular capillaries.
Answers to the Questions
Q.1. What is Thermoregulation?
Ans. Thermoregulation is the maintenance of body temperature within a range that enables cells to function efficiently. It involves the nervous, endocrine, respiratory and circulatory systems in higher animals. Metabolism is very sensitive to changes in the temperature of an animal’s internal environment. For example, the rate of cellular respiration increases when temperatures are high enough to begin denaturing enzymes. The properties of membranes also change with temperature. Although different species of animals are adapted to different temperature range, within that range many animals can maintain a constant internal temperature as the external temperature fluctuates.
Q.2. How do temperature extremes affect metabolic reactions?
Ans. Every animal’s physiological functions are inexorably linked to temperature, because metabolism is sensitive to changes in internal temperature. Biochemical reactions are also extremely sensitive to temperature. All enzymes have an optimum temperature range beyond which (above or below) enzyme function is impaired. Temperature therefore, is a severe constraint for animals, all of which must maintain biochemical stability. When body temperature drops too low, metabolic processes slow down, reducing the amount of energy the animal can muster for activity and reproduction. If body temperature rises too high, metabolic reactions become unbalanced and enzymatic activity is hampered or even destroyed. Thus animals can succeed only in restricted range of temperature, usually between 0°C to 40°C. For example, a digestive enzyme in a trout might function optimally at 10C, whereas another enzyme in the human body that catalyzes the same reaction functions best at 37°C. Higher temperatures cause the proteins in nucleic acids to denature, and lower temperatures may cause membranes to change from a fluid to a solid state, which can interfere with many cellular processes, such as active-transport pumps.
Animals can guard against these damaging effects of temperature fluctuations by: either finding a habitat where they do have to contend with temperature extremes, or they must develop means of stabilizing their metabolism independent of temperature extremes.
Q.3. How can you express the animal’s body temperature?
Ans. Animals produce heat as a by-product of metabolism and either gain heat from, or lose it to, the environment. The total body temperature is a result of an interaction of these factors and can be expressed as:
Body temperature = heat produced metabolically
+ Heat gained from the environment
– Heat lost to the environment
Q.4. Describe the processes by which animals exchange heat with the environment.
Ans. An organism like all objects, exchanges heat with its external environment by four physical processes: conduction, convection, evaporation, and radiation.
Conduction is the direct transfer of thermal motion (heat) between molecules of the environment and those of the body surface, as when an animal sits in a pool of cold water or in a hot rock. Heat will always he conducted from a body of higher temperature to one of lower temperature. For example, when we sit on the cold ground, we lose heat, and when we sit on warm sand, we gain heat.
Water is 50 to 100 times more effective than air in conducting heat. This is one reason we can rapidly cool our body on a hot day just by standing in cold water.
Convection is the transfer of heat by the movement of air or liquid past the surface of a body, as when a breeze contributes to heat loss from the surface of an animal with dry skin. Convection also contributes to the comfort a fan brings on a hot, still day, but most of this effect is due to evaporative cooling. On the other hand, a wind-chill factor compounds the harshness of cold winter temperature. On a cool day, our body loses heat by convection because our skin temperature is higher than the surrounding air temperature.
Evaporation is loss of heat from a surface as water molecules escape in the form of a gas. It is useful only to terrestrial animals. For example, humans and some other mammals (chimpanzees and horses) have sweat glands that actively move watery solutions through pores to the skin surface. When skin temperature is high, water at the surface absorbs enough thermal energy to break the hydrogen ‘bonds holding the individual water molecules together, and they depart from the surface, carrying heat with them. As long as the
environment humidity is low enough to permit complete evaporation, sweating can rid the mammalian body of excess heat; however, the water must evaporate. Sweat dripping from a mammal has no cooling effect at all as we experience in humid atmosphere.
Radiation is the emission of electromagnetic waves produced by all objects warmer than absolute zero, including an animal’s body and sun. Radiation can transfer heat between objects that are not in direct contact with each other, as an animal absorbs heat radiating from the sun. Researchers have recently discovered a specific adaptation for exploiting solar radiation in polar bears. The fur of these animals is actually clear, not while. Each hair functions somewhat
like an optical fiber that transmits ultraviolet radiation to the black skin, where the energy is absorbed and converted to body heat.
If you were to sit at rest in still air at a comfortable temperature cooler than your body (for example, an air temperature of 23.-C), conduction could account for only about 1% of your heat loss, convection for about 40%, radiation for another 50%, and evaporation for about 9%• Convection and evaporation are the most variable causes of heat loss. A breeze of just 15km/hr will increase total heat loss substantially by increasing convection fivefold. Fig. 6.1, 6.2.
Q.5. What are three basic ways animals cope with temperature fluctuations?
Ans. Animals cope with temperature fluctuations in one of three basic ways.
- They can occupy a place in the environment where the temperature remains constant and compatible with their physiological processes.
- Their physiological processes may have adapted to the range of temperatures in which the animals are capable of living; or
- They can generate and trap heat internally to maintain a constant body temperature, despite fluctuations in the temperature of the external environment.
Q.6. How can animals be categorized on the basis of their source of body heat?
Ant Animals can be categorized as ectotherms or endotherms, based on whether their source of body heat is from internal processes or derived from the environment.
Ectotherms (Gr.etos, outside) warm their body mainly by absorbing heat from its surroundings. The amount of heat it derives from its own metabolism is usually negligible. They have low rates of metabolism and are poorly insulated. Most invertebrates, fishes, amphibians, and reptiles are ectotherms, although a few reptiles, insects, and fishes can raise their internal temperature. Ectotherms tend to move about in their environment and find places that minimize heat or cold stress to their bodies. For example, many ectothermic marine fishes and invertebrates inhabit water with such stable temperatures that their body temperature varies less than that of humans and other endotherms.
An endotherm (Gr.endos, within) derives most or all of its body heat from its own metabolism. Mammals, birds, some fishes, and numerous insects are endotherms. Many endotherms maintain a consistent internal temperature even as the temperature of their surroundings fluctuates. Most endotherms have bodies insulated by fur or feathers and a relatively large amount of fat. This insulation enables them to retain heat more efficiently and to maintain a high core temperature (“Core” refers to the body’s internal temperature as opposed to the temperature near its surface). Endothermy allows animals to stabilize their core temperature so that biochemical processes and nervous system functions can proceed at steady, high levels. Endothermy allows some animals to colonize habitats denied lo ectotherms.
Q.7. What are homeotherms and heterotherms?
Ans. The term heterotherm (variable body tempenzture) and homeotherms (constant body temperature) are frequently used by zoologists as alternatives to “cold-blooded” and “warm blooded” respectively. Most endotherms are homeotherms, and most ectotherms are heterotherms. These terms, which refer to variability of body temperature, are more precise and more informative, but still offer difficulties. Some endotherms vary their body temperature seasonally (e.g., hibernation); other vary it on a daily basis. For example, deep-sea fishes live in an environment having no perceptible temperature change. Even though their body temperature is absolutely stable, day in and day out, to call such fishes homeotherms would distort the intended application of the term. Further more, among the homeothermic birds and mammals there are many that allow their body temperature to change between day and night, or, as with hibernators, between seasons. Some ectotherms can maintain fairly constant body temperatures. Among these are a number of reptiles that can maintain fairly constant body temperatures by changing position and location during the day to equalize heat gain and loss.
Q.8. Define (a) daily torpor (b) hibernation (c) aestivation.
Ans. (a) Daily Torpor: Many small mammals and birds, such as bats and humming birds etc, maintain high body temperatures when active but allow their body temperature to drop profoundly when inactive and asleep. This is called daily torpor, an adaptive hypothermia that provides enormous saving of energy to small endotherms that are never more than a few hours away from starvation at normal body temperatures.
(b) Hibernation: Many small and medium-sized mammals in northern temperate regions solve. the problem of winter scarcity of food and low tomperature by entering a prolonged and controlled state of dormancy called hibernation. True hibernators, such as ground squirrels, jumping mice, marmots, and wood chucks prepare for hibernation by storing body fat.
Some mammals, such as bears, badgers, raccoons, and opossums, enter a state of prolonged sleep in winter (winter sleep) with little or no decrease in body temperature. Prolonged sleep is not true hibernation. Bears of the northern forest, sleep for several months.
(c) Aestivation: Aestivation, or summer torpor, is characterized by slow metabolism and inactivity. It enables an animal to survive long periods of the temperatures and scarce water supplies. Hibernation and aestivation are often triggered by seasonal changes in the length o; daylight. As the day shorten, some animals will eat huge quantities of food before hibernating. Ground squirrels, for instance, will more than double their weight in a month of gorging.
Q.9. Describe the geographic distribution of ectotherms and endotherms?
Ans. In general, ectotherms are more common in the tropics because they do not have to expend as much energy to mairktain body temperature there, and they can devote more energy to food gatharing and reproduction. Indeed in the tropics, amphibians are far more abundant than mammals. Conversely, in moderate to cool environments, endbthermS have d.eelective advantage and are
more abundant. Their high metabolic rates and insulation allow them to occupy even the polar regions (e.g.; polar bears). In fact, the efficient circulatory systems of birds and mammals can be thought of as adaptations to endothermy and a high metabolic rate.
Q.10. What are thermo conformers?
Ans. Many invertebrates have relatively low metabolic rates and have no thermoregulatory mechanisms; thus, they passively conform to the temperature of their external environment. These invertebrates are termed thermoconformers. Some higher invertebrates can directly sense differences in environmental temperatures; however, specific receptors are either absent or unidentified. Many arthropods, such as insects, crustaceans, and the horseshoe crab (Limu/us), can sense thermal variation. For example, ticks of warm-blooded vertebrates can sense the ‘warmth of a nearby meal” and drop on the vertebrate host.
Q.11. How temperature is regulated in invertebrates?
- Most invertebrates have very little control over their body temperature, but some
do adjust temperature by behavioral or physiological mechanisms. For example;
1. Temperate-zone insects avoid freezing by reducing the water content in their tissues as winter approaches.
2. Some insects of temperate zone can produce glycerol or
other glycoproteins that act as an antifreeze
4. The desert locust, must reach a certain temperature to become active. It orients in a direction that minimizes the absorption of sunlight.
5. Some species of large flying insects, such as bees and large moths, can generate internal heat and are endothermic. They are able to “warm up” before taking off by contracting all of the flight muscles in synchrony, so that only slight movements of the wings occur but large amounts of heat are produced. This higher temperature of the flight muscles enables the insects to sustain the intense activity required for flight on cold days and nights.
6. Endotherms such as bumblebees, honeybees, and certain moths called noctuids that survive and fly during cold winter moths have a countercurrent heat exchanger that helps maintain a high temperature in the thorax.
7. Most large, flying insects have evolved a mechanism to prevent overheating during flight; blood circulating through the flight muscles carries heat from the thorax to the abdomen, which gets rid of the heat.
8. Certain cicadas (Diceroprocta apache) that live in the sonorant desert have cooling by evaporation like vertebrates. When threatened with overheating, these cicadas extract water from their blood and transport it through large ducts to the surface of their body, where it passes through sweat pores and svaporates. In other words, these insects can sweat.
- Body posture and orientation of wings to the sun can markedly affect the body temperature of basking insects. For example, perching dragonflies and butterflies can regulate their radiation heat gain by postural adjustment. Fig.6.3.
- To prevent overheating, many ground-dwelling arthropods (Tenebrio beetles, locusts, scorpions) raise their bodies as high off the ground as possible to minimize heat gain from the ground.
10 Some caterpillars and locusts orient with reference to both the sun and wind to vary both radiation heat gain and convective heat loss.
11 Some desert-dwelling beetles can exude waxes from thousands of tiny pores on their cuticle. These “wax blooms” prevent dehydration and also are an extra barrier against the desert sun.
12. Color has a significant effect on thermoregulation. Many black beetles may be more active earlier in the day because they absorb more radiation and heat faster. Conversely, white beetles are more active in the hotter parts of the day because they absorb less heat.
13. Honeybees use an additional mechanism that depends on social organization to increase body temperature. In cold weather, they increase their movements and huddle together, thereby retaining heat. They maintain a relatively constant temperature by changing the density of the huddling. Individuals move from the cooler outer edges of the cluster to the warmer center and back again, thus circulating and distributing the heat.
14. Honeybees also control the temperature of their hive by transporting water to it in hot weather and fanning with their wings, which promotes evaporation and convection.
Q.12. How do fishes maintain their body temperature? Ans. Temperature Regulation in Fishes
Ans. The body temperature of most fishes is usually within 1° — 2° of the surrounding water. temperature. Fishes that live in extremely cold water have “antifreeze” materials in their blood. i.e.
- Polyalcohols (e.g. sorbitol, glycerol) or water soluble peptides and glycopeptides lower the freezing point of blood plasma and other body fluids.
- These fishes also have proteins or protein-sugar compounds that stunt the growth of ice crystals that begin to form.
These adaptations enable these fishes to stay flexible and swim freely in a super cooled state i.e. at a temperature below the normal freezing temperature of a solution. Some active fishes maintain a core temperature significantly above the temperature of the water. Endothermic fishes include several large, active species such as blue fin tuna, swordfish, and the great white shark. Their swimming muscles produce enough metabolic heat to elevate temperatures at the body core, and adaptations of the circulatory system retain the heat. Large arteries convey most of the cold blood from the gills to tissues just under the skin. Branches deliver blood to the deep muscles, where the small vessels are arranged into a countercurrent heat exchanger. This arrangement of blood vessels enhances vigorous activity by keeping the swimming muscles several degrees warmer than the tissue near the surface of the fish. Their muscular contractions can have four times as much power as those of similar muscles in fishes with cooler bodies. Thus, they can faster and range more widely through various depths other predatory fishes more limited to given water depths and temperatures. Fig. 6.4.
Q.13. What is the function of rete mirabile in fishes?
Ans. Rete Mirabile (L. wonder net) is a network of small blood vessels so arranged that the incoming blood runs countercurrent to the outgoing blood and thus makes possible efficient exchange between the two blood streams. Such a mechanism serves to maintain the high concentration of gases in the fish swim bladder. The amazing effectiveness of this device is exemplified by a fish living a depth of 2400 m (8000 feet). To keep the bladder inflated at the depth, the gas inside (mostly oxygen, but also variable amounts of nitrogen, carbon dioxide, argon, and even some carbon monoxide) must have a pressure exceeding 240 atmospheres, which is much greater than the pressure in a fully charged steel gas cylinder. Yet the oxygen pressure in the fish’s blood cannot exceed 0.2 atmosphere – equal to the oxygen pressure at the sea surface. In brief, the gas gland secretes lactic acid, which enters the blood, causing a localized high acidity in the rete mirabile that forces hemoglobin to release its load of oxygen. The capillaries in the rete are arranged so that the released oxygen accumulates in the rete, eventually reaching such a high pressure that the Oxygen diffuses into the swim bladder. The final gas pressure attained in the swim bladder depends on the length of the rete capillaries; they are relatively short in fishes living near the surface, but are extremely long in deep-sea fishes.
the oxygen pressure at the sea surface. In brief, the gas gland secretes lactic acid, which enters the blood, causing a localized high acidity in the rete mirabile that forces hemoglobin to release its load of oxygen. The capillaries in the rete are arranged so that the released oxygen accumulates in the rete, eventually reaching such a high pressure that the Oxygen diffuses into the swim bladder. The final gas pressure attained in the swim bladder depends on the length of the rete capillaries; they are relatively short in fishes living near the surface, but are extremely long in deep-sea fishes.
Q.14. Describe the temperature regulation in amphibians and reptiles? Ans. Temperature Regulation in Amphibians and Reptiles
Ans. Animals, such as amphibians and reptiles, that have air rather than water as a surrounding medium face marked daily and seasonal temperature changes. Most of these animals are ectotherms. They divert heat from their environment, and their body temperatures vary with external temperatures. Amphibians The optimal temperature range for amphibians varies substantially with the species. Amphibians produce very little heat, and most lose heat rapidly by evaporation from their body surfaces, making it difficult to control body temperature. However, behavioral adaptations enable them to maintain body temperature within a sa:isfactory range, most of the time, by moving to a location, where solar heat is available or into water. When the surroundings are too warm. the animals seek cooler microenvironments, such as shaded areas. Some amphibians, including bullfrogs, can vary the amount of mucus they secrete from their surface, a physiological response that regulates evaporative cooling. Reptiles Reptiles have dry rather than moist skin, which reduces the loss of body heat through evaporative cooling of the skin. They also have an expandable rib
cage, which allows for more powerful and efficient ventilation. Reptiles are generally ectotherms with
relatively low metabolic rates that contribute little to normal body temperatures. Reptiles warm themselves mainly by behavioral adaptations. They seek warm places, orienting themselves toward heat sources to increase heat uptake and expanding the body surface exposed to a heat source. Reptiles do not simply maximize heat uptake, however; they may behave in such a way as to truly regulate their temperature within a range If a sunny spot is too warm, for instance, a lizard may sit alternately in the sun and in the shade, or turn in another direction, thereby reducing the surface area exposed to the sun. By seeking favorable microclimates within the environment, many reptiles maintain body temperatures that are quite stable. Some reptiles also have physiological adaptations that regulate heat loss. For example, diving reptiles (e.g., sea turtles, sea snakes) conserve body heat by routing blood through circulatory shunts into the center of the body. These animals can also increase heat production in response to the hormones thyroxin and epinephrine. In addition, tortoises and land turtles can cool themselves through salivating and frothing at the mouth, urinating on the back legs, moistening the eyes, and panting. A few reptiles are endothermic for brief periods of time. For instance when incubating eggs, female phythons increase their metabolic rate by shivering, generating enough heat to maintain their body temperature 5° to 7°C above the surrounding air.
0.15. How do birds and mammals regulate their body temperature? Ans. Temperature Regulation in Birds and Mammals?
Ans. Birds and mammals are the most active and behaviorally complex vertebrates. They can live in habitats all over the earth because they are homeothermic endotherms; they can maintain body temperatures between 35 and 42°C with metabolic heat. Birds Various cooling mechanisms prevent excessive warming in birds.
- As they have no sweat glands, birds pant to lose heat through evaporative cooling.
- Some birds have a vascularized pouch in the floor of the mouth that they can flutter to increase evaporation from the respiratory system.
- Feathers are excellent
insulators for the body, especially down type feathers (gular flutter), that trap a layer of air next to the body to reduce heat loss from the skin.
4. Aquatic species, which lose heat from their legs and feet, have peripheral
countercurrent heat exchange vessels called a rete mirabile in their legs to reduce heat loss. The arteries carry warm blood down the legs to warm the cooler blood in the veins, so that the heat is carried back to the body rather than lost through the feet that are in contact with a cold surface. Fig. 6.6.
- Mammals that live in cold regions, such as the arctic fox and barren-ground caribou, have rete mirabile in their extremities (e.g., tails, ears, and nose).
- Animals in hot climates, such as jack rabbits, have mechanisms, (e.g., large ears) to rid the body of excess heat. Fig. 6.2.
- Humans rely more on a layer of fat just beneath the skin as insulation against heat loss.
4 Thick pelts and a thick layer of insulating fat called blubber just under the skin help marine animals, such as seals and whales, to maintain a body temperature of around 36 to 38°C.
- The flippers or tail of a whale or seal lack insulating blubber, but countercurrent heat exchangers effectively reduce heat loss in these extremities, as they do in the legs of many birds.
- Many terrestrial mammals have sweat glands, which are controlled by the nervous system
- Other mechanics that promote evaporative cooling include spreading saliva on body surfaces, an adaptation of some kangaroos and rodents for combating severe heat stress.
- Some bats use both saliva and urine to enhance evaporative cooling.
Birds and mammals also use behavioral mechanisms to cope with external temperature changes. Like ectotherms, they sun themselves or seek shade as the temperature fluctuates. Many animals huddle to keep warm: others share burrows for protection from temperature extremes. Migration to warm climates and hibenation enable many different birds and mammals to survive the harsh winter months. The desert camel, have a multitude of evolutionary adaptations for surviving in some of the hottest and driest climates on earth.
Q.16. What is blubber?
Ans. Blubber is a thick pelt and a thick layer of fat found between the skin and muscle of whales and other cetaceans, from which oil is made. The function of blubber is to insulate the body of animal and to maintain a body temperature of around 36° to 38°C.
Q.17 What is gular flutter?
Ans. Gular flutter is a type of breathing in some birds. Some species of birds have a highly vascularized pouch (gular pouch) in their throat that they can flutter to increase evaporation from the respiratory system, as in pelicans.
Q.18. How do birds and mammals generate heat? Ans. Heat Production in Birds and Mammals
Ans. In endotherms, heat generation can warm the body as it dissipates throughout tissues and organs. Birds and mammals can generate heat (thermogenesis) by muscle contraction, ATPase pump enzymes, oxidation of fatty acids in brown fat, and other metabolic process.
In severely cold conditions all mammals can produce more heat by augmented muscular activity through exercise or shivering. Every time a muscle cell contracts, the actin myosin filaments sliding cver each other, and the hydrolysis of ATP molecules generate heat. Both voluntary muscular work (e.g., running flying, jumping) and involuntary muscular work (e.g., shivering) generate heat. Heat generation by shivering is called shivering thermogenesis.
The hormonal triggering of heat production is called nonshivering thermogenesis. Birds and mammals have a unique capacity to generate heat by using specific enzymes of ancient evolutionary origin—the ATPase pump enzymes in the plasma membranes of most cells. When the body cools, the thyroid gland releases the hormone thyroxin. Thyroxin increases the permeability of many cells to sodium (Na”) ions, which leak into the cells. The ATPase pump quickly pumps these ions out. In the process, ATP is hydrolyzed, releasing heat
Brown fat is a specialized type of fat found in newborn mammals, in mammals that live in cold climates, and in mammals that hibernate. Deposits of brown fat are beneath the ribs and in the shoulders. A large amount of heat is produced when brown fat cell oxidize fatty acids, because little ATP is made. Blood flowing past brown fat is heated and contributes to warming the body. Fig. 6.7.
Hypothalamic Control to Thermogenesis
In amphibians, reptiles, birds, and mammals, specialized cells in the hypothalamus of the brain control The two hypothalamic thermoregulatory areas are the heating center and the cooling center. The heating center controls vasoconstriction of
superficial blood vessels, erection of hair and fur, and shivering or nonshivering thermogenesis. The cooling center controls vasodilatation of blood vessels, sweating and panting. Overall, feedback mechanism with the hypothalamus acting as a thermostat) trigger either the heating or cooling of the body and thereby control body temperature. Specialized neuronal receptors in the skin and other parts of the body sense temperature changes. Warm neuronal receptors excite the cooling center and inhibit the heating center. Cold neuronal receptors have the opposite effects. Fig. 6.8
Torpor is an alternative physiological state in which metabolism decreases and the heart and respiratory system slow down. Many endotherms e.g. hummingbirds, bats etc. at night enter a state of daily torpor in which their, body temperature declines. In effect, their body’s thermostat is turned down, thereby conserving energy when food supplies are low and environmental temperatures are extreme. Humming birds can only maintain a high body temperature for a short period, because they usuey weigh less than 10 gm, and have almost no reserve energy providing source. They devote much of the day to locating and sipping nectar. When not feeding (during night) they run out of energy, and as such their metabolic rate decreases.
During the winter, various endotherms (e.g., bats, wood chucks, chipmunks, ground squirrels) go into hibernation. During hibernation, the metabolic rate slows, as do the heart and breathing rates. Mammals prepare for hibernation by building up fat reserves and growing long winter pelts. All hibernating animals have brown fat. Decreasing day length stimulates both increased fat deposition and fur growth.
It is characterized by slow metabolism and inactivity. It enables some animals to survive long periods of high temperatures and scarce water supplies. Aestivation is an adaptation in desert environments. Thus most of the animals including predators are nocturnal (active during night), when temperature is relatively low.
Some animals, such as badgers, bears, opossums, raccoons, and skunks, enter a state of prolonged sleep in winter. Since their body temperature remains near normal, this is not true hibernation. The basal metabolic rate of birds and mammals is high and also produces as an inadvertent but useful by product.
Q.19. What is brown fat?
- Placental mammals are unique in having a dark adipose tissue called brown fat,
specialized for generation of heat. Newborn mammals, including human infants, have much more brown fat than adults. In human infants brown fat is located in the chest, upper back, and near the kidneys. In adults it is mostly found in the neck and between the shoulders. The abundant mitochondria in brown fat contain a membrane protein called thermogenin that acs to uncouple production of ATP during oxidative phosphorylation.
Q.20. How do excretion and osmoregulation differ?
Ans. Excretion (. excretion, to eliminate) can be defined broadly as “The disposal of nitrogen-containing waste products of metabolism from an animal’s body”. These products include carbon dioxide and water (which cellular respiration primarily produces), excess nitrogen (which is produced as ammonia, urea or uric acid from metabolism of proteins and nucleoproteins), and solutes (various ions). Osmoregulation is the maintenance of proper internal salt and water concentrations in a cell or in the body of a living organism. Animal cells require more critical balance of water and solutes in body as they cannot survive a net water gain or loss. Water continuously leaves and enters the cells; however, the quantity of the water and the solutes is kept in balance.
Q.21. How do osmoconformers differ from osmoregulators?
Ans. The animals, which do not actively adjust their internal osmolarity are know as
osmoconformers. By control, animals whose body fluids are not isotonic with the outside environment, called osmoregulators, must either discharge excess water if they live in a hypotonic environment or continuously take in water to offset osmotic loss if they inhabit a hypertonic environment. A net movement of water occurs only in an osmotic gradient (from a region of lower osmolarity to a region of higher osmolarity), and osmoregulators must expend energy to maintain osmotic gradients, to move water lither in or out. They do so by manipulating solute concentrations in their body fluids. Most marine invertebrates are osmoconformers. Among the vertebrates, the hagfishes are isotonic with the surrounding seawater. All freshwater, terrestrial. and many marine animals are osmoregulators.
Q.22. What is the function of the contractile vacuole, and where would you find one?
Ans. Contractile Vacuoles Many unicellular and simple multicellulair animals have no special excretory structures. Nitrogenous wastes are simply excreted across the general cell membranes into the surrounding water. Many freshwater species (protozoa, sponges), do, however, have a special excretory organelle, the contractile vacuole that pump out excess water. Though there is now evidence that contractile vacuoles excrete some nitrogenous wastes, it seems clear that their primary function is elimination of excess water. In most protozoa the vacuole is surrounded by a layer of tiny vesicles and these, in turn, are surrounded by a layer of mitochondria. The vesicles initially contain a fluid isotonic with the cytosol, but later actively pump out ions, using energy from ATP manufactured in the mitochondria. Thus contractile vacuoles are energy requiring devices that expel excess water from individual cells exposed to hypoisonotic environments.
Q.23. How do protonephridia and metanephridia function?
Ans. Protonephridia A protonephriaium (Gr. porotos, first + nephridium) is a network of closed tubules lacking internal openings. The tubules branch throughout the body, and the smallest branches are capped by a cellular unit called a flame bulb. Interstitial fluid bathing the tissues of the animal passes through the flame bulb and enters the tubule system. The flame bulb has a tuft of cilia projecting into
the tubule, and the beating of these cilia propels fluid along the tubule, away from the flame bulb. In planaria, tributaries of the tubular system drain into excretory ducts that empty into the external environment through numerous openings called nephridiopores. Fig. 6.9. The flame-bulbs systems of fresh water flatworms function mainly in osmoregulation; most metabolic wastes diffuse out from the body surface or are exerted into the gastrovascular cavity and eliminated through the mouth. However in some parasitic flatworms, which are isotonic to the surrounding fluids of their host organisms, protonephridia function mainly in excretion, disposing of nitrogenous wastes. Protonephridia are also found in rotifers, some annelids, the larvae of molluscs, and lancelets, which are invertebrate chordates.
A more advanced type of excretory structure among invertebrates is the metanephridium (Gr. meta, beyond + nephridium). Protonephridia and metanephridia have critical structural differences. Both open to the outside, but metanephridia:
- Also open internally to the body fluids, and
2.are multicullular. Metanephridia are found in most annelids (including earthworms) and a variety of other invertebrates. Each segment of earthworm has a pair of metanephridia, which are tubules immersed in coelomic fluid and enveloped by a network of capillaries. The internal opening of a metanephridium is surrounded by a ciliated funnel, the nephrostome, that collect coelomic fluid. 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 exits through the nephridiopore contains nitrogenous wastes and is hypotonic to the body fluids. By excreting this dilute urine in amounts up to 60% of the body weight of the worm per day. the metanephridia offset the continuous osmosis taking place across the skin of the animal from the damp soil. Fig. 6.10. The excretory system of molluscs includes protonephridia in larval stages and metanephridia in adults.
Q.24. How do antenna! (green) glands and maxillary glands function? or How do excretion occurs in Crustacea? Ans. Antenna! (Green) and Maxillary Glands
Ans. In crustaceans that have gills, nitrogenous wastes are removed by simple diffusion across the gills. Most crustaceans release ammonia, although they also produce some urea and uric acid as waste products. Thus, the excretory organs of fresh water species may be more involved with the reabsorption of ions and elimination of water than with the discharge of nitrogenous wastes. The excretory organs in some crustaceans (crabs, crayfish) are antennal glands or green glands because of their location near the antenna and their green color. The glands remove the water and nitrogenous waste substances from the surrounding blood into the end sac by the process of ultra filtration. The filtrate called the primary urine, passes to the labyrinth. The useful and necessary substances are reabsorbed and then passed to the blood. The remaining fluid is now called the final urine. The urine passes into the bladder and then expelled out of the body through the renal aperture.
In other crustaceans (some malacostracans [crabs, shrimp, pillbugs]), the excretory organs are near the mixillary segments and are termed maxillary glands. In maxillary glands fluid collects within the tubules from the urrounding blood of the hemocoel, and this primary urine is modified substantiallythy selective reabsorption and secretion as it moves through the excretory system and rectum. Fig. 6.11..
Q.25. Write an account on Malpighian tubules of insects,
Ans. Malpighian tubules Insects have open circulatory systems, with tissues bathed directly in hemolymph contained in sinuses. Their excretory organs, called Malpighian tubules, remove nitrogenous wastes from the hemolymph and also function in osmoregulation. These organs open into the digestive tract at the juncture of the midgut and hindgut. The tubules, with dead-end at the tips away from the digestive tract, are immersed in the hemolymph. The transport epithelium that lines a tubule pumps certain solutes, including potassium ions and nitrogenous wastes, from the hemolymph into the lumen of the tubule. The fluid within the tubule then passes through the hindgut into the rectum. The epithelium of the rectum pumps most of the salt back into the hemolymph, and water follows the salts by osmosis. The nitrogenous wastes are eliminated as almost dry matter along with the
Malpighian tubules of insects. Malpighian tubules are ouffoldings of the digestive tract. The tubules accumulate nitrogenous wastes and salts from the hemolymph, and water follows these solutes by osmosis. Most of the salts and water are reabsorbed across the epithelium of the rectum, and the dry nitrogenous wastes are eliminated with the faces. feces. The insect excretory system is one adaptation that has contributed to the tremendous success of these animals on land, where conserving water is essential. Fig. 6.12. 6.12a
Q.26. Describe the excretory organs in arachnids. OR What are coxal–glands?
Ans. Coxal (L coxa, hip) glands are common among arachnids (spiders, scorpions, .ticks mites) These spherical sacs resemble. annelid nephridia Wastes an collected from the surrounding hemolymph of the hemocoel and discharged through pores on from one to several pairs of appendages near the proximal joint (coxa) of the leg Recen’ evidence suggests that the coxai glands may also function in the release of pheromones. Other arachnid species have Malpighian tubules instead of. or in addition to, the coxal glands. In some of these species, however Malpighian tubules seem to function in silk production rather than in excretion. Fig. 6.13.
Q.27. How does a vertebrate losewater from its body? How does it gain water?
Ans. On land the greatest threat to life is desiccation. Water is lost by (1) evaporation from the respiratory surfaces- lungs. trachea, etc.) (2) by evaporation from the general body surface, (3) by sweating or panting (4) by elimination in the feces, and (5) by excretion in the urine. The lost water must obviously be replaced if life is to continue. It is replaced (1) by drinking (2) by eating foods containing water (3) by the oxidation of nutrients (metabolic reactions yield water as end product) (4) certain insects (e.g., desert roaches, certain ticks and mites, and the mealworm) are able to absorb water vapor directly from atmospheric air.
Of particular interest is a comparison of water balance in human beings (non-desert mammals that drink water) with that of kangaroo rats (desert rodents that may drink no water at all). Kangaroo rats acquire all their water from their food: 90% is metabolic water derived from oxidation of foods, and 10% as free moisture in food. Even though we eat foods with a much higher water content than ‘ the dry seeds that make up much of a kangaroo rat’s diet, we still must drink half our total water requirement.
Q.28. How solute losses and gains are done in vertebrates?
Ans. Solute losses must be balanced by solute gains. Vertebrates take in solutes:
1. by absorption of minerals from the small and large intestines.
2. through the integument or gills,
3. from secretions of various glands or gills, and
4. by metabolism (e.g., the waste products of degradative reactions).
Vertebrate lose solutes in sweat, feces, urine, and gill secretions, and as metabolic wastes. The major metabolic wastes that must be eliminate are ammonia, urea or uric acid.
Q.29. How various vertebrates maintain water and salt balance?
Q.30. How do vertebrates achieve osmoregulation?
Ans. A variety of mechanisms have evolved in vertebrates to cope with their osmorgulatory problems. These are:
- Most terrestrial animals are covered by relatively impervious surfaces that help prevent dehydration.
- The multiple layers of dead, keratinized skin cells covering most terrestrial vertebrates prevents loss of water.
- Behavioral adaptations, such as nervous and hormonal mechanisms that control thirst, are important osmoregulatory mechanism in land-dwelling animals.
- Many terrestrial animals, especially in deserts, are nocturnal, the important behavioral adaptation that reduces dehydration.
- The kidneys and other excretory organs of terrestrial animals often exhibit adaptations that help conserve water.
- Some mammals are so well adapted to minimizing water loss that they can survive in deserts without drinking.
Q.31. What are three functions of the kidneys?
- Following three key functions take place in kidneys:
- Filtration: During filtration blood passes through a filter that retains blood cells, proteins, and other large solutes but lets small molecules, ions, and urea to pass through.
- Reabsorption: During reabsorption selective ions and molecules (such as vital nutrients and water) are reabsorbed from the filtrate into the blood stream.
- Secretion: During secretion, drugs, selected ions, and end products of metabolism (e.g., K+,. H+, NH3) that are in the blood are selectively secreted into the filtrate for removal from the body. The overall effect of filtration, secretion, and reabsorption is analogous to cleaning out a drawer (blood) by first removing all the small articles (filtration), returning useful items to the blood (reabsorption), adding additional useless items to the refuse pile (secretion), and then is carding all the unwanted objects (excretion). These main functions of the kidneys are central to homeostasis, for they enable the kidney to clear the blood of metabolic wastes and respond to imbalances in body fluids by excreting more or less of a particular ion.
Q.32. Describe the types (or variations) of kidneys in vertebrates. Ans. Vertebrate Kidney Variations
Ans. Vertebrates have two kidneys that are in the back of the abdominal cavity, on either side of the aorta. Each kidney has a coat of connective tissue called the renal capsule (L. renes, kidney). The inner portion of the kidney is called the medulla; the region between the capsule and the medulla is the cortex. There are three kinds of vertebrate kidneys.
The most primitive type; of kidney functional in adult vertebrates is the Pronephros. The Pronephros is believed to represent the most anterior part of the ancestral archinephros. Distribution of Pronephros: The pronephric tubules continue to function in the adult hagfish and in some teleosts. The pronephros is also a functional structure in many immature fishes as in the larvae of some amphibians and appears transitorily in the embryos of all the higher vertebrates.
The mesonephros is the kidney tissue that develops posterior to the pronephros. These kidneys form discrete organs that readily look like kidneys. Distribution of Mesonephros: The mesonephros is the functional kidney of the adult lamprey, cartilaginous fishes, bony fishes, and amphibians. The mesonephros also functions in the embryos of reptiles, birds and mammals.
The metanephric kidney develop from the most posterior portion of the mesonephros and it’s the most compact of any of the vertebrate renal structures. The body of metanephros has a two fold origin. Part of it develops from the posterior end of the mesonephros, while part forms as a new and unique metanephric structure.Distribution of the Metanephros The metanephros becomes functional in most reptilian, avian and mammalian embryos and is the functional kidney of all adult amniotes. Fig. 6.14.
Q.33. What are the physiological differences between three types of vertebrate kidneys?
Ans. The physiological differences between three kidney types are primarily related to the number of blood-filtering units they contain. The pronephric kidney forms in the anterior portion of the body cavity and contains fewer blood-filtering units than either the mesonephric or metanephric kidneys. The large number of filtering units in the latter has allowed vertebrates to face the rigorous osmoregulatory and excretory demands of freshwater and terrestrial environments.
Q.34.How sharks have solved their osmotic problems?
Ans. Sharks and their relatives (skates and rays) have mesonephric kidneys and rectal glands that secrete a highly concentrated salt (fnlaCI) solution. Despite its relatively low salt concentration, a marine shark is slightly hypertonic to seawater. It does not drink water, and the water that enters its body by osmosis is disposed of in urine, the waste fluid formed by the excretory organs, the kidneys. To reduce water loss, sharks use two organic molecules—urea and trimethylamine oxide (TMO) in their body fluids to raise the osmotic pressure to a level equal to or higher than that of the seawater. Urea denatures proteins and inhibits enzymes, whereas TMO stabilizes proteins and activates enzymes. Together in the proper ratio, they counteract each other, raise the osmotic pressure, and do not interfere with enzymes or proteins. This reciprocity is termed the counteracting osmolyte strategy.
Q.35. How do teleost fishes osmoregulate?
Ans. Teleost FishesMost teleost fishes have mesonephric kidneys.
1. Freshwater Fishes: Because the body fluids of freshwater fishes are hyperosmotic relative to freshwater, water tends to enter the body of fishes, causing excessive hydration or bloating. At the some time, body ions tend to move outward into the water. To Solve this problem, freshwater fishes:
(i) usually do not drink much water,
(ii) their bodies are coated with mucus, which helps stem inward water movement,
(iii)water that inevitably enters by osmosis across the gills is pumped out by
the kidney, which is capable of forming very dilute urine,
(iv) special salt-absorbing cells located in the gills move salt ions, from the water to the blood. Fig. 6.15a.
2. Marine Fishes: Marine bony fishes are hypotonic relative to the surrounding water, and have the problem of excessive water loss and excessive salt intake. To compensate dehydration, marine fishes: Fig. 6.15b.
(i) drink almost continuously to replace the water they are constantly losing. This seawater is absorbed from the intestine,
(ii) they secrete Nat, C[, and K+ ions through specialized salt-secreting cells in their gills,
(iii)channels in plasma membranes of their kidneys activity transport the multivalent ions that are abundant in seawater (e.g., Ca2+, Mg2+, S024–, and PO°4 ) out of the extracellular fluid and into the nephron tubes. The ions are then excreted in a concentrated urine. 3 Some fishes encounter both fresh-and saltwater during their liVes. Newborn Atlantic salmon swim downstream from the freshwater stream after their birth and enter the Instead of continuing to pump ions in, as they have done in freshwater, the salmon must now rid their bodies of salt. Years later, these same salmon migrate from the sea to their freshwater home to spawn. As they do so, the pumping mechanism reverse themselves.
Osmoregulation, Osmoregulation by (a) freshwater and (b) marine fishes. Large black arrows indicate passive uptake or loss of water or ions. Small black and white arrows indicate active transport processes at gill membranes and kidney tubules. Insets of kidney nephrons depict adaptations within the kidney. Water, ons, and small organic molecules are filtered from the blood at the glomerulus of the nephron. Essential components of the filtrate can be reabsorbed within the tubule system of the nephron. Marine fishes conserve water by reducing the size of the glomerulus of the nephron, and thus reducing the quantity of water and ions filtered from the blood. Ions can be secreted from the blood into the kidney tubules. Marine fishes can produce urine that is isoomotic with the blood. Freshwater fishes have enlarged glomeruli and short tubule systems. They filter large quantities of water from the blood, and tubules reabsorb some ions from the filtrate. Freshwater fishes produce a hypoosmotic urine.
Q.36. How do amphibians conserve water?
Ans. The amphibian kidney is identical to that of freshwater fishes, because amphibians spend a large portion of their time in freshwater, and when on land, they tend to seek out moist place. Amphibian take up water and ions:
(i) in their food and drink,
(i) through their skin that is in contact with moist substrate, and through the urinary bladder, This uptake counteracts what is lost through evaporation and prevents osmotic imbalance, Figure 6.16.The urinary bladder of frog, toad or salamander is an important water and ion reservoir. For example, when the environment becomes dry, the bladder enlarges for storing more urine. If an amphibian becomes dehydrated, a brain hormone causes water to leave the bladder and enter the body fluid.
0.37. What are salt glands? In which type of animals these are found?
Ans. Some desert and marine birds and reptiles have evolved an effective solution for execrating large loads of salt eaten with their food. In these animals, salt glands are
present, located above each eye. These are capable of excreting a highly concentrated
solution of sodium chloride (Nacl), up to twice the concentration of seawater. In marine birds, the salt solution runs out the nares. Marine lizards and turtles, like Alice in wonderland’s Mock turtle, shed their salt gland secretion as salty tears. Salt glands are important accessory organs of salt exertion in these animals
because their kidney cannot produce concentrated urine, as can mammalian kidney. Fig. 6.17.
Q.38. What are the primary regulatory organ for osmotic balance in amniotes?
Ans. Reptiles, birds, and mammals all possess metanephric kidneys. Their kidneys are by far the most complex animal kidneys, well suited for these animal’s high rates of metabolism In most reptiles, birds, and mammals, the kidneys can remove far more water than can those in amphibians, and the kidneys are the primary regulatory organs for controlling the osmotic balance of the body fluids.
Q.38. How do nasal cavities help conserve water? •
Ans. Major sites of water loss in mammals are the lungs. To–reduce this evaporative loss, many mammals have nasal cavities that act as countercurrent exchange systems. When the animal inhales, air passes through the nasal cavities and is warmed by the surrounding tissues. In the process, the temperature, of this tissue drops. When the air gets deep into the lungs, it is further warmed and humiuiried. During exhalation, as me warm moist air passes up the respiratory tree, it gives up its heat to the nasal cavity. As the air cools, much of the water condenses on the nasal surfaces and does not leave the body. This mechanism explains why a dog’s nose is usually cold and moist. Fig. 6.18.
Q. 40. How does metanephric kidney function?
Ans. Metanephric Kidney The filtration device of the metanephric kidney consists of over one million individual filtration, secretion, and absorption structures called nephrons (Gr. nephros, kidney + on, neuter). At the beginning of the nephron is the filtration apparatus called the glomerular capsule (formerly Bowman’s capsule), which looks rather like a tennis ball that has been punched in on one side. The capsules are in the cortical (outermost) region of the kidney. In each capsule, an afferent (“going to”) arteriole enters and branches into a fine network of capillaries called the glomerulus. The walls of these glomerular capillaries contain small perforations called filtration slits that act as filters. Blood pressure forces fluid through these filters. The fluid is now known as glomerular filtrate Because the filtration slits are so small, large proteins and blood remain in the blood and leave the glomerulus via the efferent (“outgoing”) arteriole. The efferent arteriole then divides into a set of capillaries called the peritubular capillaries that wind profusely around the tubular portion of the nephron. Eventually they merge to form veins that carry blood out of the kidney. and contains small molecules, such as glucose, ions (Ca2+, PO4 ), and the primary nitrogenous waste products of metabolism — urea and uric acid.
Figure 6-19 Urinary system of humans, with enlargements showing detail of the kidney and a single nephron,
Beyond the glomerular capsule are the proximal convoluted tubule, the loop of the nephron (formerly the loop of Henle), and the distal convoluted tubule. At various places along these structures, the glomerular filtrate is selectively reabsorbed, returning certain ions (e.g. Nat, K+, CI ) to the bloodstream. Both active (ATP requiring) and passive procedures are involved in the recovery of these substances. Potentially harmful compounds, such as hydrogen (he) and ammonium (NH;) ions, drugs, and various other foreign materials are secreted into the nephron lumen. In the last portion of the nephron, called the collecting duct, final water reabsorption takes place so that the urine contains an ion
Q.41. Describe the human urinary system.
Ans. In humans, the kidneys are a pair of bean-shaped organs about 10 cm long. Blood enters each kidney via the renal artery and leaves each kidney via the renal vein. Although the kidneys account for less than 1% of the weight of the human body, they receive about 20% of the blood pumped with each heart-beat. Urine exits the kidney through a duct called the ureter. The ureters of botll kidneys drain into a common urinary bladder. During urination, urine leaves the body from the urinary bladder through a tube called the urethra, which empties near the vagina in females or through the penis in males. Sphincter muscles near the junction of the urethra and the bladder control urination. Fig. 6.20.
Q.42. How does the countercurrent flow mechanism in the kidney functions?
Ans. Countercurrent Exchange The loop of the nephron increases the efficiency of reabsorption by a countercurrent flow. Generally, the longer the loop of the nephron, the more water and ions that can be reabsorbed. It is why that desert rodents (e.g., the kangaroo rat) that form highly concentrated urine have long nephron loops. Similarly, amphibians that are closely associated with aquatic habitats have nephrons that lack a loop. Figure 6.21 shows the countercurrent flow mechanism for concentrating urine. The process of reabsorption in the proximal convoluted tubule removes some salt (NaCI) and water from the glomerular filtrate and reduces its volume by approximately 25%. However, the concentrations of salt and urea are still isoosmotic with the extracellular fluid. As the filtrate moves to the descending limb of the loop of the nephron, it becomes further reduced in volume and more concentrated. Water moves out of the tubule by osmosis due to the high salt concentration (the “brine-bath”) in the extracellular fluid. As the filtrate passes into the ascending limb, sodium (Nat) ions are actively transported out of the filtrate into the extracellular fluid, with chloride (Cr) ions following passively. Water cannot flow out of the ascending limb because the cells of the ascending limb are impermeable to water. Thus, the salt concentration of the extracellular fluid becomes very high. The salt flows passively into the descending loop, only to move out again in the ascending loop, creating a recycling of salt through the loop and the extracellular fluid. Because the flows in the descending and ascending limbs are in opposite directions, a countercurrent gradient in salt is set up. The osmotic pressure of the extracellular brine bath is made even higher because of the abundance of urea that moves out of the collecting ducts. Finally, the distal convoluted tubule empties into the collecting duct, which is permeable to urea, and the concentrated urea in the filtrate diffuses out into the surrounding extracellular fluid. The high urea concentration in the extracellular fluid, coupled with the high concentration of salt, forms the urea-brine bath that causes water to move out of the filtrate by osmosis as it moves down the descending limb. Finally, the many peritubular capillaries surrounding each nephron collect the water and return it to the systemic circulation. The renal pelvis of the mammalian kidney is continuous with a tube, the ureter that carries
urine to a storage organ called the urinary bladder. Urine from two ui eters (one from each kidney) accumulates in the urinary bladder. The urine leaves the body through a single tube, the urethra, which opens at the body surface at the end of the penis On human males) or just in front of the vaginal entrance (in human females). As the urinary bladder fills with urine, tension increases in its smooth muscle walls. In response to this tension, a reflex response relaxes sphincter muscles at the entrance to the urethra. This response is called urination. The two kidneys, two ureters, urinary bladder, and urethra constitute the urinary system of mammals.