Temperature and Body Fluid Regulation

Dr. Tamseela Mumtaz

Thermoregulation

• is a complex and important physiological process that maintains to varying degrees, an animal's body temperature, despite variations in environmental temperature
• Based on this regulation, animals can be categorized into
• endotherms
• or ectotherms,
• and homeotherms .
• or heterotherms

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osmoregulation

• In vertebrates, urinary system work as osmoregulation and excretion (necessary for internal homeostasis)

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• invertebrates have
• contractile vacuoles,
• flame-like systems,
• antennal (green) glands,
• maxillary glands, coxal glands,
• nephridia or
• Malpighian tubules

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Earth environments vary dramatically in temperature�

• In polar regions, the temperature remains of near 0°C (high mountain ranges and deep oceans)
• In equatorial deserts, temperature exceeds 40°C
• The temperate region has varying amount of water so in this region, temperature fluctuates

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Homeostasis and temperature regulation 

• The temperature of a living cell affects the rate of its metabolism
• A state of balance among all the body systems needed for the body to survive and function correctly is called homeostasis
• Zoologists believe that the ability of some higher animals to maintain a constant (homeostasis), relatively higher temperature is the major reason for their evolutionary success
• This ability to control the temperature of the body is called thermoregulation (heat control)  and involves the nervous, endocrine, respiratory, and circulatory systems in higher animals

The impact of temperature on Animal life

• Every animal's physiological functions are inexorably linked to temperature, because metabolism is sensitive to change in internal environment.
• Temperature has been a strong source of having a selective pressure on all animals
• The rate of respiration increases when temperature exceeds up to a point
• When the temperature exceeds then optimal temperature, the enzymes become denature
• The chemical interactions holding the enzyme  in 3-D shape
• These chemical reaction affects by animals habitat

The impact of temperature on Animal life

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• For example, a digestive enzyme in a trout might function optimally at 10°C, whereas another enzyme in the human body that catalyzes the same reaction in best at 37°C
• Higher temperature cause the nucleotides in nucleic acid to denature, and low temperature may cause membranes to change from a fluid to solid state, which can interfere with many environmental state (active transport pump)
• Animals can guard against these damaging effects of temperature fluctuations by balancing heat gain and heat lose with the environment

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Heat gain and loses 

• Animals produce heat as a by-product and by either heat gain or lose
• The total body temperature is a result of interaction of these factors and can be expressed as:

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• Animal use four physical process to exchange heat with the environment

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Body temperature = heat produced metabolically + heat gained from the environment + heat lost to the environment

Conduction

Convection

Evaporation

Radiation

Conduction

Conduction is the direct transfer of thermal motion (heat) between the molecules of the environment

This transfer is always from a high-temperature area to a low temperature For example, when you sit on cold ground, you lose heat, and when you sit on the warm sand, you gain heat

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Convection 

• Convection is the movement of air (or a liquid) over the surface of body
• It contributes to heat loss if the air is cooler than the body or the heat gain if air is warmer than body
• For example, on a cool day, your body loses heat by convection because your skin temperature is higher than surrounding air

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Evaporation�

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Example: humans and some other mammals (chimpanzees and horses) have seat glands that actively move watery solution through pores to the skin surface

• If the skin temperature is high, water at the surface absorbs enough thermal energy to break the hydrogen bonds that hold the water molecule in an individual and they depart from the surface after carrying heat
• If the environment humidity is low, sweating heap the individual by rid them of excess heat

Radiation �

• Radiation is emission of electromagnetic waves 
• Radiation can transfer heat between objects that are not in direct contact with each other, it happens when an animal suns itself

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Radiation warms an animal. After a cold night in its den on the kalahari desert, a meerkat (suricata suricatta) stands at attention allowing the large surface area of its body to absorb radiation from sun

• Conduction 

Conduction is the direct transfer of thermal motion (heat) between the molecule of the environment

This transfer s always from a high temperature area to the low temperature 

For example, when you sit on the cold ground, you lose heat, and when you sit on warm sand, you gain heat

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• Convection 

Convection is the movement of air (or a liquid) over the surface of body

It contributes to heat loss if the air is cooler than the body or the heat gain if air is warmer than body

For example, on a cool day, your body loses heat by convection because your skin temperature is higher than surrounding air

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• Evaporation

Evaporation is  a loss of heat from a surface as water molecules escape in the form of gas

It is useful only for terrestrial environment

For example, humans and some other mammals(chimpanzees and horses) have seat glands that actively move watery solution through pores to the skin surface

If the skin temperature is high, water at the surface absorbs enough thermal energy to break the hydrogen bonds that holding the water molecule in individual and they depart from surface after carrying heat

If the environment humidity is low, sweating heap the individual by rid them from excess heat

• Radiation 

Radiation is the emission of electromagnetic waves 

Radiation can transfer heat between objects that are not in direct contact with each other, it happens when an animal suns itself

Some solutions to Temperature Fluctuations

• Animal cope with temperature fluctuation in one of the basic three ways
• They can occupy a place in the environment where the temperature remains constant and compatible with their physio-logical processes
• Their physiological processes  may have adapted to the range of temperatures in which the animal are capable of living
• They can generate and trap heat internally to maintain a constant body temperature, despite fluctuations in the temperature of external environment

Ectotherms (Gr. Ectos, outside) or poikilotherms (Gr. Poikilos, variable + thermal)

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• It derive most of their body heat from the environment rather than from their own metabolism
• They have low rate of metabolism and are poorly isolated
• e.g., reptiles, amphibians, fishes, and invertebrates
• They tend to move about the environment and find places that minimize heat and cold stress to their bodies

Thermoregulation

• Animals fall into four general categories of thermal relations based on whether they display endothermy and whether they display thermoregulation 
• However, some animals do not fit easily into either classification scheme
• For example, heterotherms transcend these two classification schemes by using metabolic heat to regulate body temperature but also allowing body temperature to vary widely depending on the circumstances

Endotherms (Gr. Endos, within)

• They obtain their body heat from cellular processes like birds and mammals
• A constant source of internal heat allows them to maintain a nearly constant core temperature despite the fluctuating environmental temperature
• Core refers to the body's internal temperature as opposed to the temperature near its surface

Endotherms 

• Most endotherms have bodies insulated by fur and feathers and a relatively large amount of fat
• This isolation enables them to retain heat more efficiently and to maintain a high core temperature
• 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 to ectotherms

Homeotherms Vs Heterotherms

Homeotherms

• Most endotherms are homeotherms
• Homeotherms maintain a constant body temperature
• Some endotherms vary their body temperature seasonally (e.g., hibernation) other vary on other basis
• For example, some birds (hummingbirds) and mammals (shrews) can only maintain a high body temperature for a short period because they usually weigh less than 10g and have a body mass so small that they cannot generate enough heavy to compensate for the heat lost across their relatively large surface area

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Heterotherms

• most ectotherms are heterotherms
• Heterotherms have a variable body temperature
• 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

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Hummingbirds

• Hummingbirds must devote much of the day to locating and sipping nectar(a very high calorie food source) as a constant energy source metabolism
• When not feeding, hummingbirds rapidly run out of energy unless their metabolic rate decrease considerably
• At night, hummingbirds enter a sleep like state, called daily torpor and their body temperature approaches that of the cooler surroundings
• Some birds also undergo daily torpor to conserve energy

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Endotherm Vs Ectotherm

Ectotherm

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• In general, ectotherms are more common in tropics because they dont have to expend as much energy to maintain body temperature there, and they can devote more energy to food gathering and reproduction
• Indeed in the tropics, Amphibians are far more abundant than mammals

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Endotherm

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• In moderate to cool environments, endotherms have a selective advantage and are more abundant
• Their high metabolic rates and insulation allow them to occupy even the polar regions
• Infect, the efficient circulatory systems of birds and mammals can be thought of as adaptations to endothermy and a high metabolic rate 

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Temperature regulation in invertebrates

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• Many invertebrates
• have relatively low metabolic rates and
• have no thermoregulatory mechanisms
• thus they passively conform to the temperature of their external environment. These invertebrate are commonly known as thermoconformers

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Evidence indicates that some higher invertebrate can directly sense differences in environmental temperature however specific receptors are either absent or identified�

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Ticks of warm-blooded vertebrates can sense the earth of a nearby meal and drop on the vertebrate host

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Many arthropods have unique mechanisms for surviving temperature extremes

• For example,
• temperature zone insects avoid freezing by reducing the water content in their tissues as winter approaches
• Other insects can produce glycerol on other glycoproteins that act as a free antifreeze
• Some moths and humblebees warm up prior to flight by shivering contractions of their thoracic flight muscles

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Most large flying insects have evolved a mechanism to prevent overheating during flight�

• Hemolymph circulating through the flight muscles carried heat from the thorax to the abdomen, which get rid from heat (much as coolant circulating through an automobile engine passes through the radiator
• Certain cicadas (Diceroprocta apache) that live in the Sonoran desert have independently evolve the complete repertoire of evaporative cooling mechanisms that vertebrates use
• When threatened with overheating, these cicadas extract water from their hemeolymph and transport it thorough large ducts to the surface of their body where it passes through sweat pores and evaporate 
• In other words, these insects can sweat

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Body posture and orientation of the 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 adjustments

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Thermoregulation in Bumble bee

Many endotherms insects (such as bumblebees, honeybees and some moths) have a counter current heat exchanger that helps maintain a high temperature in the thorax where the insects flight muscles are located

This mechanism allows the insect to control heat gain and loss by regulating the amount of blood flowing through the heat exchanger

By allowing blood to flow through the heat exchanger or diverting it to other blood vessel in the body, the insect can alter the rate of heat loss as its physiological stage or environmental conditions change

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Thermoregulation in flying and ground dwelling insects

• If an insect is flying in very hot weather, it runs the risk of overheating due to the large amount of work done by the flight muscles
• Thus, the counter current heat  exchanger can be shutdown to allow the heat produced in the muscle to be lost from the thorax to abdomen and then to the environment
• To prevent overheating, many ground dwelling arthropods raise their bodies as high off the ground as possible to minimize heat gain from the ground
• Some caterpillars and locusts orient with reference to both the sun and wind to vary both radiation heat gain and convective heat lose

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Temperature regulation in invertebrates

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• Some desert dwelling beetle can exude waxes from thousands of tiny pores of their cuticle
• These wax blooms prevent dehydration and also are extra barrier against the desert sun

Colour has a significant effect on thermoregulation since 50% of the radiant energy from the sun is in the visible spectrum�

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• A black surface reflects less radiant energy than a white surface
• Thus many black beetle 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
• The endothermic temperature regulation of active insects apparently evolved because locomotion produces sufficient metabolic heat that thermoregulatory strategies could evolve

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Temperature regulation in fishes

• The temperature of the surrounding water determines the temperature of the body
• Fishes that live in extremely cold water have "antifreeze" material in their blood
• Polyalcohals (e.g., sorbitol, glycerol) or water soluble peptides and glycopeptides lower the freezing point of blood plasma and other body fluid 
• 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 supercooled state (i.e., at a temp below the normal freezing temperature of a solution; -2°C (28°F))

Temperature regulation in fishes

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• Some active fishes maintain a core temperature significantly above the temperature of water
• Bluefin tuna, swordfish and the great white shark have major blood vessels just under the skin
• Branches deliver food to the deeper, powerful, red swimming muscles, where smaller vessels are arrange in a counter current heat exchanger called the rete mirabile (miraculous net)
• The heat that these red muscles  generate is not lost because it is transferred in the rate mirabile from venous blood passing outward to cold atrial blood passing inward from the body surface
• This arrangement of blood vessels enhances vigorous activity by keeping the swimming muscle several degrees warmer than the tissue near the fish

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Temperature regulation in fishes

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• Their muscle contraction can have 4 times as much power as those of similar muscle in fishes with cooler bodies'
• Thus, they can swim  faster and a wide range through various depths in search of prey than can other predatory fishes more limited to given water depths and temperatures

Temperature regulation in Amphibians and Reptiles

• Animals that have air(amphibians and reptiles) rather than water as a surrounding medium face marked daily and seasonal temperature changes 
• These animals are mostly ecotherms
• They derive heat from the environment and their body temperatures vary with external temperatures
• Most amphibians have difficulty in controlling body heat because they produce most of it from their body surfaces
• However behavioural adaptations enable them to maintain their body temperature with in a homeostatic range 
• Amphibians have an additional thermoregulatory problem because they must exchange oxygen and carbon dioxide across their skin surface and this moisture layer acts as a natural evaporative cooling system

Temperature regulation in Amphibians and Reptiles

• This problem of heat loss through eveporation limits the habitate and activites of amphibians to warm the moist areas
• Some amphibians (bullfrogs) can vary the amount of mucus they secrete from their body surface (a physiological response that help to regulate cooling)
• Reptiles have dry rather than moist skin, which reduces the loss of body heat through evaporative cooling of the skin
• They also have an expendable rib cage, which allows for more powerful and efficient ventilation
• Reptiles are completely ectothermic

Temperature regulation in Amphibians and Reptiles

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• Reptiles have a low metabolic rate and warm themselves by behavioural adaptation
• In addition some of the most sophisticated regulatory mechanisms found in mammals are first found in reptiles
• For example, diving reptiles (sea turtles) conserve body heat by routing blood through circulatory shunts into the centre of the body
• These animals can also increase heat production in response to the hormones thyroxin and epinephrine
• Tortoise and land turtles can cool themselves by salivating and frothing at the mouth, urinating on the back legs, moistening the eyes and panting

Temperature regulation in birds and mammals

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• Birds and mammals are the most active and behaviourally complex vertebrates
• They can live in habitats all over the earth because they are homeoendotherm; they can maintain body can maintain body temperature between 35 and 42 with metabolic heat
• Various cooling mechanisms prevent excessive warming in birds (because they have no sweat gland, birds pant)
• Some species have a highly vascularized pouch (gular pouch) in their throat that they can flutter ( a process called gular flutter) to increase evaporation from respiratory system

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Temperature regulation in birds and mammals

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• Some birds posses mechanism for preventing heat loss 
• Feathers are excellent insulators for the body, especially, downy type feathers that trap a layer of air next to the body to reduce heat loss from the skin 
• Aquatic species (who loss heat from their legs and feet) have peripheral counter current heat exchange vessels in their legs to reduce heat loss

• A thick layer of down feathers keep chinstrap penguins warm

The counter current exchanger in bird foot

Temperature regulation in birds and mammals

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• Mammals that live in a cold region (arctic fox and barren ground caribou) also have exchange vessels in outer extremities (legs, tails, ears and nose)
• Aimals in hot climate (jackrabbits) have mechanisms (large ears) to rid the body of excess heat
• Thick pelts and a thick layer of insulating fat called blubber just under the skin help marine animals (seals and whales) to maintain a body temperature of around 36-38°C 
• In the tails and flippers (having no blubber) a counter current system of arteries and veins helps to minimize the heat loss

Temperature regulation in birds and mammals

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• Birds and mammals also use behavioural mechanisms to cope with external temperature changes 
• Like ectotherms, they sun themselves or seek shade a temperature fluctuates
• Many animals huddle to keep warm; other share burrows for protection from temperature extremes
• Migration to warm climates and hibernation enable many different birds and mammals to survive the harsh winter months
• Desert camel have a multitude of evolutionary adaptations for surviving in some of the hottest and driest climates on earth

Heat production in birds and mammals�

• In endotherms, heat generation can warm the body as it dissipates throughout tissues and organs
• Birds and mammals can generate heat (thermoregulation) by muscle contraction
• ATPase pump enzymes, oxidation of fatty acids in brown fat and other metabolic processes
• Every time a muscle contracts, the actin and myosin filaments sliding over each other and the hydrolysis of ATP molecules generate heat
• Heat generation by shivering is called shivering thermogenesis

Heat production in birds and mammals�

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• Birds and mammals have a unique capacity to generate heat by using specific enzymes of ancient evolutionary origin (the ATPase pump enzyme in the plasma membrane of most cells)
• When body cools, the thyroid gland releases the hormone thyroxine
• Thyroxine increases the permeability of many cells to sodium ion (which leaks into the cells)
• The ATPase pump quickly pumps these ions out
• In the process, ATP is hydrolysed and realizing heat energy
• The hormonal triggering of heat production is called Non shivering thermogenesis

Heat production in birds and mammals�

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• Brown fat is a specialized type of fat found in newborn mammals (live in cold climate and hibernate)
• The brown colour of this fat comes from the large number of mitochondria with their iron containing cytochromes
• Deposits of brown fat are beneath the ribs and in the shoulders
• A large amount of heat is produced when brown fat cells oxidize fatty acids because little ATP is made
• Blood flowing past brown fat is heated and contributes to warming the body
• The basal metabolic rate of birds and mammals are high and also produces heat as an inadvertent but useful byproduct

Heat production in birds and mammals�

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• In amphibians, reptiles birds and mammals, specialized cells in the hypothalamus of the brain control the thermoregulation
• The two hypothelamic thermoregulatory areas are the heating center and the cooling center
• The heating center controls the vasocontriction of superficial blood vessels, erection of hair and fur and shivering or nonshivering thermogenesis
• The cooling center controls the vasodilation of blood vessels, sweating and panting
• Overall, negative feedback mechanisms (with the hypothylamus acting as a thermostst) trigger either the heating or cooling of the body and thereby control body temperature
• Specialized neoronal 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

• Bats having the adipose tissue called brown fat between the shoulder blades

• The area of brown fat is much warmer than the rest of the body. Blood flowing through this is warmed

Heat production in birds and mammals�

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• During the winter various endotherms (bats, wooodchucks, chipmunks, ground squirrel) go into hibernation (L. Hiberna, winter)
• During hibernation, the metabolic rates slows as do the heart and breathing rates
• Mammals prepare for hibernation by building up fat reserves and growing  land winter pelts
• All hibernating animals have brown fat
• Decreasing day length stimulates both increase fat deposit and fur growth
• Some very small endotherms (bats, chickadees and humming birds) can also reduce both metabolic rate and body temperature to produce a stats of dormancy called TORPOR
• Torpor allows an animal to reduce the need for food by reducing metabolism 
• For example, hummingbirds allow their body temperature to drop as much as 25°C at night when food supplies are low
• This strategy is only found in small endotherms, as larger ones have too large of a body mass to allow rapid cooling

Heat production in birds and mammals�

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• Other vertebrates ( desert tortoise, pygmy mouse, ground squirrels) will enter a state of dormancy during the summer called estivation (L. Aestivus, summer) 
• In this state, both breathing rates and metabolism decrease when environmental temperatures are high, food is scarce and when dehydration is a problem
• Some animals( badgers, bears, opossums, raccoons, skunks) enter a state of prolonged sleep in the winter
• Because their body temperature remains near normal, this is not true hibernation

Control of water and solutes (osmoregulation and excretion)

• Excretion (L. Excretio, to eliminate) can be defined broadly as the elimination of metabolic waste products from an animal's body
• These products includes carbon dioxide and water (which cellular respiration primarily produces); excess nitrogen (which is produced from the deamination of amino acids); in the form of either ammonia, urea, or uric acid and solutes (various ions)
• The excretion of nitrogenous wastes is usually associated with the regulation of water and solute (ionic) balance by a physiological process called osmoregulation 
• Osmoregulation is necessary for all animals in all habitats
• If the osmotic concentration of a body fluid of an animal equals that of the medium (the animal's environment), the animal is an osmo-conformer
• When the osmotic concentration of the environmental changes ( so does that of the animal's body fluid)
• Obviously, this type of osmoregulation is not efficient and has limited the distribution of those animals using it

Control of water and solutes (osmoregulation and excretion)

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• In contrast, an animal that maintains its body fluids at a different osmotic concentration from that of its surrounding environment is an osmoregulator
• Animals living in seawater have body fluids with an osmotic concentration that is about a third less (hypoosmotic) than the surrounding seawater and water tends to leave their bodies continually
• To compensate with this problem, mechanisms evolved in these animals to conserve water and prevent dehydration
• Freshwater animals have body fluid that are hyperosmotic with respect to their environment and water tends to continually enter their bodies
• Mechanisms evolved in these animals that excrete water and prevent fluid accumulation

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Control of water and solutes (osmoregulation and excretion)

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• Land animals have a higher concentration of water in their fluids than in the surrounding air
• They tend to lose water to the air through evaporation and may use considerable amounts of water to dispose of wastes
• The form and function of organs or systems associated with excretion and osmoregulation are related both to environmental conditions (saltwater, freshwater and terrestrial)and to body size (especially the surface to volume ratio)

Invertebrate excretory system

• Aquatic invertebrates occur in a wide range of media, from freshwater to markedly hypersaline water (salt lakes)
• Generally, marine invertebrates have about the same osmotic concentration as sea water (osmoconformers)
• This eliminates any need to osmoregulator 
• Most water and ions are gained across the integument, via gills, by drinking and in food
• Ions and wastes are mostly lost by diffusion via integument, gills or urine
• freshwater invertebrates are strong osmoregulators because it is impossible to be iso-osmotic with dilute media
• Any water gain is usually eliminated as urine

Invertebrate excretory system

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• A number of invertebrate taxa have more or less successfully invaded terrestrial habitats
• The most successful terrestrial invertebrates are the arthropods (the insects, spiders, scorpions, ticks, mites, centipedes and millipedes)
• Overall, the water and ion balance of terrestrial invertebrates is quite different from that of aquatic animals because terrestrial invertebrates face limited water supplies and water loss by evaporation from their integument

Invertebrate excretory system

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• Although a few groups of metazoan possess no known excretory structures, most have nephridia (Gr. Nephros, kidney) (sing., nephridium) that serve for excretion, osmoregulation, or both
• Probably the earliest type of nephridium to appear in the evolution of animals was protonephridium

Contractile vacuole

• Many unicellular and simple multicellular 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 hyposonotic environments.

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

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Protonephridia 

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•  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.

Metanephridia

• 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
• are multicellular
•  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

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Earthworm Metanephridium. �The metanephridium opens by a ciliated nephrostome into the cavity of one segment, and the next segment contains the nephridiopore. The main tubular portion of the metanephridium is coiled and is surrounded by a capillary network. Waste can be stored in a bladder before being expelled to the outside. Most segments contain two metanephridia.

Metanephridia

• 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
• The excretory system of molluscs includes protonephridia in larval stages and metanephridia in adults.

Antennal (green) and Maxillary glands

• 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.

Antennal (Green) Gland of the Crayfish

The antennal gland, which lies in front of and to both sides of the esophagus is divided into an end sac, where fluid collects by filtration and a labyrinth. The labyrinth walls are greatly folded and glandular and appear to be an important site for reabsorption. The labyrinth leads via a nephridial canal into a bladder. From the bladder, a short duct leads to an excretory pore

Antennal (green) and Maxillary glands

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•  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.

Antennal (green) and Maxillary glands

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• 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 surrounding blood of the hemocoel, and this primary urine is modified substantially by selective reabsorption and secretion as it moves through the excretory system and rectum.

Maxillary Gland

Malpighian Tubules (named after Marcello Malpighi, Italian anatomist (1628-1694)

• Insects have open circulatory systems, with tissues bathed directly in haemolymph contained in chamber
• Their excretory organs, called Malpighian tubules, remove nitrogenous wastes from the haemolymph and also function in osmoregulation
• These organs open into the digestive tract at the point of the midgut and hindgut
• The tubules, with dead-end at the tips away from the digestive tract, are immersed in the haemolymph
• The transport epithelium that lines a tubule pumps certain solutes, including potassium ions and nitrogenous wastes, from the haemolymph 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 haemolymph, and water follows the salts by osmosis
• The nitrogenous wastes are eliminated as dry matter 

Malpighian Tubules

• Malpighian tubules remove nitrogenous wastes (uric acid) from the hemocoel. Various ions are actively transported across the outer membrane of the tubule. Water follows these ions into the tubule and carries amino acids, sugars, and some nitrogenous wastes along passively. Some water, ions, and organic compounds are reabsorbed in the basal portion of the Malpighian tubules and the hindgut; the rest are reabsorbed in the rectum. Uric acid moves into the hindgut and is excreted

Malpighian tubules

• Malpighian tubules are out folding of the digestive tract
• The tubules accumulate nitrogenous wastes and salts from the haemolymph, 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 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

Coxal glands

•  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 haemolymph of the hemocoel and discharged through pores on from one to several pairs of appendages near the proximal joint (coxa) of the leg
• Recent evidence suggests that the coxal 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 Malpighian tubules seem to function in silk production rather than in excretion.

Vertebrate excretory system

• 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
• by drinking 
•  by eating foods containing water
•  by the oxidation of nutrients (metabolic reactions yield water as end product)
• certain insects (e.g., desert roaches, certain ticks and mites, and the mealworm) are able to absorb water vapor directly from atmospheric air.

Comparison of water balance in human beings with that of kangaroo rats

• 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.

Vertebrate excretory system

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• 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).

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• 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.

 How vertebrates achieve osmoregulation?

• 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.
• Behavioural 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 behavioural 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.

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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

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Evolution of the vertebrate kidney

• 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
• Pronephros
• Mesonephros
• Metanephros

• Pronephros (L. pro, before + nephros, kidney)

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• 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 teleost's
• 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.

Types of Kidneys in Vertebrates and Their Association with the Male Reproductive System.

The primitive pronephric kidney is found in adult hagfishes and embryonic fishes and amphibians. It is anterior in the body and contains segmental renal tubules that lead from the body of the pronephros to the archinephric duct. Notice that the testes are separated from the kidneys.

• Mesonephros (Gr. Mesos, middle + L. nephros, kidney)

• 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.

Mesonephrose kidney

The mesonephros is the functional kidney in the amniote embryo, adult fishes, and amphibians. It is structurally similar to the nonsegmented opisthonephric (advanced mesonephric) kidney of most nonamniote vertebrates, such as sharks. The anterior portion of the opisthonephros functions in blood cell formation and secretion of sex hormones. Notice that the testes occupy the position of the anterior opisthonephros, and the archinephric duct carries both sperm and urine

Metanephros (Gr. Meta, beyond + L. nephros, kidney)

• 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.

Metanephric Kidney

The metanephric kidney of adult amniotes (reptiles, birds, and mammals) is the most advanced kidney. Notice the separate ureters (new ducts) for carrying urine. The archinephric duct becomes the ductus deferens for carrying sperm. The kidney is more compact and located more caudally in the body.

Physiological differences between types of kidneys

• 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.

Cartilaginous Fishes (Elasmobranchs) retain urea and pump out electrolytes

• Sharks and their relatives (skates and rays) have mesonephric kidneys and rectal glands that secrete a highly concentrated salt (NaCI) 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.

Cartilaginous Fishes (Elasmobranchs) retain urea and pump out electrolytes

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• 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.

Freshwater Teleosts (fishes) must keep water out and retain electrolytes

• Teleost fishes have mesonephric kidneys
• 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 same time, body ions tend to move outward into the water. To Solve this problem, freshwater fishes:
• usually do not drink much water,
• their bodies are coated with mucus, which helps stem inward water movement,
• water that inevitably enters by osmosis across the gills is pumped out by
• the kidney, which is capable of forming very dilute urine, special salt-absorbing cells located in the gills move salt ions, from the water to the blood.

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Osmoregulation. Osmoregulation by (a) freshwater

Marine teleosts (fishes) must keep water out and retain electrolytes

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• 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: 
• drink almost continuously to replace the water they are constantly losing. This seawater is absorbed from the intestine,
• they secrete Na^t, Cl, and K⁺ ions through specialized salt-secreting cells in their gills,
• 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.

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Osmoregulation by (b) marine fishes

Some fishes are both freshwater and marine teleosts

• Some fishes encounter both fresh-and salt water during their lives
• Newborn Atlantic salmon swim downstream from the freshwater stream after their birth and enter the sea. 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.

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Amphibians adapt to their environments

• 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:
• in their food and drink,
• 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
• 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.

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Salt glands

• 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

Reptiles, birds and mammals are able to retain water and excrete a concentrated urine

• 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.

Reptiles, birds and mammals are able to retain water and excrete a concentrated urine

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• Major sites of water loss in mammals are the lungs
• To^–reduce this evaporative loss, many mammals have nasal cavities that act as counter current 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 humidified
•  During exhalation, as the 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

(a) When this animal inhales, the cool, dry air passing through its nose is heated and humidified. At the same time, its nasal tissues are cooled. (b) When the animal exhales, it gives up heat to the previously cooled nasal tissue. The air carries less water vapor, and condensation occurs in the animal’s nose

Metanephric kidney functions

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• 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 filter
• Blood pressure forces fluid through these filters

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(a) Interior of a kidney, showing the positioning of the nephron and the blood supply to and from the kidney.

(b) Glomerular capsule. Red arrows show that high blood pressure forces water and ions through small perforations in the walls of the glomerular capillaries to form the glomerular filtrate

Metanephric kidney functions

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• 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.

Metanephric Nephron. The proximal convoluted tubule reabsorbs glucose and some ions. The distal convoluted tubule reabsorbs other ions and water. Final water reabsorption takes place in the collecting duct. Black arrows indicate the direction of movement of materials in the nephrons

Metanephric kidney functions

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• 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. Na^t, 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 (H+) and ammonium (NH4+) 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 concentration well above that of the blood 
• Thus the filtration, secretion and reabsorption activities of the nephron do not simply remove wastes, they also maintain water and ion balance and therein lies the importance of homeostasis function of the kidney

Metanephric kidney functions

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• Mammalian (sometimes avian and reptilian) kidneys can remove far more water from the glomerular filtrate than can the kidney of amphibians
• For example, human urine is four times as concentrated as blood plasma, a camel's urine is eight time concentrated, a gerbils is 14 times as concentrated and some desert rats and mice have 20 times as concentrated as blood plasma
• This concentrated waste enables them to live in dry and desert environment where little water is available from them to drink
• Most of their water is metabolically produced from the oxidation of carbohydrates, fats and proteins in the seeds that they eat
• Mammals and sometimes birds achieve this remarkable degree of water conservation by a unique evolutionary adaptation (the bending of nephron tube into loop
• By bending, the nephron can greatly increase the salt concentration in the tissue through which the loop passes and use this gradient to draw a large amount of waterout

Human urinary system

•  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 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

Counter current Exchange

•  The loop of the nephron increases the efficiency of reabsorption by a counter current 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
• The counter current 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%.

Counter current Exchange

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• However, the concentrations of salt and urea are still iso-osmotic 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 (Na+) ions are actively transported out of the filtrate into the extracellular fluid, with chloride (Cl-) 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. 

Counter current Exchange

• 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 counter current 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.

Countercurrent Exchange. Movement of materials in the nephron and collecting duct. Solid arrows indicate active transport; dashed arrows indicate passive transport. The shading at intervals along the tubules illustrates the relative concentration of the filtrate in milliosmoles.

Counter current Exchange

• 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 ureters (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.