How do animals lose heat?

How do animals lose heat?

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I remember being told as a child that dogs stick their tongue out to stay cool, because they can't sweat like us. This made sense to me.

But only a couple of days ago I suddenly thought: What about all the other animals?! I mean, I don't see a cat or a horse putting their tongues out, and yet I don't see them sweating either. How do they lose heat? How do different kinds of animals lose heat?

There are a number of ways, which are briefly summarized here. I also recommend reading this nice Scitable article on thermoregulation. It is not too technical. Felines do pant when they get hot. Horses do sweat. Jackrabbits can enlarge the blood vessels in their large ears to eliminate excess heat. Bird use a process called gular flapping, which is similar to panting of dogs and cats. Animals can move into shaded areas. The possible mechanisms are many.

Koalas hug trees to lose heat

In a study published in the Royal Society journal Biology Letters, scientists used thermal cameras to reveal that, in hotter weather, the animals moved to the lower, cooler parts of the trees.

They also pressed their bodies even closer to the trunks.

The team, led by researchers from the University of Melbourne, was studying how koalas regulated their temperature.

This is part of a wider research project investigating the effect of climate on land-dwelling animals in Australia, a country which experienced an extreme heat wave earlier this year.

While PhD student Natalie Briscoe was studying the koalas' behaviour, she noticed that in the winter the animals would stay high in the trees - up near the leaves feeding.

In the hotter summer weather though, they would move down.

Dr Michael Kearney from the University of Melbourne explained: "Theyɽ just flop over the [lower] tree trunks.

"It looked like they were spread-eagled and uncomfortable it seemed like the wrong thing to do."

But measurements of the temperatures of the tree trunks showed that, on days as hot as 39C, they were up to seven degrees cooler than the air.

"That's what made us wonder if the koalas were using the trees as a heat sink," said Dr Kearney.

The team used a thermal camera to take pictures of koalas on a particularly hot day.

"When we got the images, back it was so obvious what the koala was doing," explained Dr Kearney. "You could see the koala sitting on the coolest part of the tree trunk with its bottom wedged right into the coolest spot.

"If we had thermal vision, it would have been an obvious thing."

8 Ways Wild Animals Beat the Heat

A hippo scans the river. Photo © Malcolm Macgregor / Flickr

During blizzards and extreme cold, many people worry about the impacts on wildlife. In fact, blogs on this topic are among the most popular on Cool Green Science. By summer, most of us don’t worry about the birds and other neighborhood critters. In the Northern Hemisphere, warm weather means a season of plenty. There’s ample vegetation, plentiful insects and no shortage of lush cover.

Of course, extreme heat waves can be deadly to wildlife, and recent ones in Europe proved deadly for everything from butterflies to hedgehogs.

And as is the case with snow and cold, some wild animals are supremely adapted to thrive in conditions of heat and drought. These species live in some of the most seemingly inhospitable environments on earth. To do so means adaptations that allow them to beat the heat. Here are eight examples.

Hippo Sunscreen

A hippopotamus at Lake Naivasha, Kenya. Photo © Robert Granzow

While a hippo’s hide may look tough, its skin is actually prone to drying and sunburn. Like beach-going tourists, hippos use plenty of sunscreen – but they supply their own.

The hippo secretes a mucus-like substance that coats the skin, providing effective protection. This multi-purpose substance also moisturizes the skin while it at the same time repels water, allowing the hippo to stay submerged for long periods of time. And if that wasn’t enough, the secretion also serves as an antibiotic, protecting the skin from infection.

When exposed to the sun, this substance turns red, which led European explorers to declare that hippos sweat blood. We now know it is neither sweat nor blood. Common and pygmy hippos are the only species to have this substance. Researchers have taken some dramatic steps to understand it (one intrepid scientist dressed in a hippo suit and rested in mud and dung to collect samples as hippos secreted it). But much about this red substance remains poorly understood.

Dried But Alive Lungfish

The Marbled African Lungfish (Protopterus aethiopicus) breathes air, “walks” underwater with its fins, and can grow nearly six feet long! © Solomon David

A dry riverbed is generally bad news for fish, but not the African lungfish. This species can breathe air. It also has an astonishing ability to weather drought.

As aquatic ecologist and blogger Solomon David writes in a previous Cool Green Science story:

“During periods of drought, African lungfishes are unique in that they can survive near-complete desiccation (drying out) by estivating: undergoing a period of slowed metabolism, almost like hibernation. Scientists have tested African lungfish estivation in the lab and found they can be revived even after seven years of ‘sleep!’”

The Long Sleep of Ground Squirrels

A Columbia ground squirrel. Photo © Matt Miller/TNC

Lungfish aren’t the only species that estivate. Many species undergo torpor during hot weather as a means of conserving energy. Several species of ground squirrels that live near my Idaho home are extreme examples. I start to see them in March or April, but by early July they are back underground. They will not re-emerge until the following spring.

Think about this. For eight to nine months of the year, they are dormant. I live on the edge of the high desert, with hot, dry summer conditions. Staying cool and finding food requires a lot of energy, so estivation makes sense.

Interestingly, if I travel to north into the higher-elevation mountains, I often find the same species of ground squirrels throughout the summer. With cooler temperatures and greener vegetation, they do not need to spend as much time underground.

A Mud Spa You Should Avoid

Bull elk in Yellowstone. Photo © Phil Parsons /Flickr

Elk are one of the few creatures in the forest that I often smell before I see. They stink. I know some wilderness lovers who wax rhapsodic about the musky scent of elk, but it actually smells quite terrible.

One reason for this stench is the elk wallow. Bull elk, in particular, create wallows where springs seep through the earth, or near natural mineral licks. These become like giant mud baths. Bull elk soak in them, while also urinating and occasionally ejaculating. Then they soak some more, coating themselves in the mud (and other fluids), giving them a somewhat startling appearance and an even more startling odor.

It’s well known among biologists and hunters that these wallows serve an obvious territorial purpose. But research has shown that elk, including cows, also use wallows as a way to cool off and for protection against summer insects. But if you come across a mud bath in the wilderness and think it would make a nice spa: Don’t.

The Shady Future of Lizards

Eastern Collared Lizard on post in southern Utah's Cedar Mesa. Photo © Tana Kappel/TNC

For many animals, the secret to beating the heat is pretty simple: find some shade. For lizards, cold-blooded animals found in often-warm environments, shade is particularly important. Researchers have found that lizards survive best where there are lots of small pockets of shade, rather than a few large shaded areas.

This, unfortunately, presents a dire future for many lizard species. With climate change, many of the tiny pockets of shade in arid environments will heat up considerably, rendering them as ineffective refuges for lizards.

Some predict that 20 percent of lizards will go extinct by 2080 due, in no small part, to a decrease in shade.

The Big Ears of Jackrabbits

A black-tailed jackrabbit. Photo © Aaron Fellmeth Photography / Flickr

I often kick up black-tailed jackrabbits on my desert hikes, and often all you can see are those giant ears. Those large appendages aren’t just for hearing they also serve as radiators. The ears are packed with blood vessels. As blood flows to through the ear, the heat transfers to the air.

According to National Geographic’s Education Blog, “the blood vessels can widen (a process called vasodilation), allowing more warm blood to circulate to the ears for even greater heat loss.”

If the outside air temperature is below 86 degrees, the jackrabbit can shed all excess body heat through its ears. It does not have to sweat or pant, both of which shed water – important in the arid environments where this animal resides.

Kangaroo Rats and Extreme Water Conservation

An Ord's kangaroo rat in Alberta, Canada. Photo photographed in Texas. Photo © Andy Teucher / Flickr

If you run in hot weather, you know how difficult staying hydrated can be. Kangaroo rats hop around the hottest, driest parts of North America, and they don’t shrivel up. They can do this because they are like little water conservation machines.

Nearly every aspect of their physiology, physical shape and habit help conserve water. Their nose has large passages that allow them to reabsorb water from their own breath. Their kidneys can extract moisture from seemingly dessicated seeds, and they pass urine that is five times more concentrated than humans. Basically they can pee with very little liquid.

They stay cool via oily coats and by digging burrows in the ground, so again, they lose very little moisture during the heat of the day.


A captive great horned owl in Florida. Photo © Mark Conlin, courtesy of Tallahassee Natural History Museum

Any backyard naturalist knows that birds regularly takes baths, and while this can help cool off our feathered friends, it is not the primary function. Many birds also seek out shade. If you observe them closely, you’ll see that some species (including my backyard chickens) panting in a way that resembles dogs.

For some birds, this “panting” appears rapid and dramatic, what biologists call “gular fluttering.” According to the U.S. Fish and Wildlife Service: “Fluttering is a combination of rapid, open-mouth breathing and quick vibration of the moist throat membranes that causes evaporation. As excess heat leaves the bird’s body with each exhalation, the bird cools.”

You can see this behavior with whip-poor-wills, common nighthawks, double-crested cormorants, owls and doves.

Matthew L. Miller is director of science communications for The Nature Conservancy and editor of the Cool Green Science blog. More from Matthew

How Do Dogs Regulate Their Body Temperature?

Dogs don’t use their skin to perspire, like humans, because of their insulating coat. Their coat keeps them both cool in hot weather and warm in cool weather. Dogs do have sweat glands, located in the pads of their feet and in their ear canals, but sweating plays a minor role in regulating body temperature.

When the temperature is very hot and especially when it is humid, everything heats up…including a dog’s body. His body responds by trying to cool off and it basically attempts to use conduction, convection, radiation, and evaporation. He will seek a cool place in the shade to lie down to absorb the coolness (conduction). His blood vessels will dilate in his skin and tongue bringing hot blood close to the surface radiating his internal heat. He will seek out fans or breezes to blow air to transfer the heat from body to air (convection). He will pant to bring air into his upper respiratory system to evaporate water from his mucous membranes. He will drink a lot of water to compensate for the evaporation.

Stationary cars or other enclosed areas in that are in direct sunlight heat up very rapidly and stay heated even though there may be some slight ventilation. This is sometimes called the “hot house” effect. Basically, the windows allow the sun’s rays to enter but preclude the heat waves to exit. The whole interior of the car heats up quite quickly (seats, steering wheel, dash board) and hold the heat. Putting an animal into this situation is like putting an animal into an oven and turning on the heat.

How do dogs cool themselves down?

Once their body temperature rises, dogs can’t sweat through their skin like we do to cool off. Dogs do sweat through their paw pads, but it’s by panting that dogs circulate the necessary air through their bodies to cool down. Note: Dogs with short faces, because of the structure of their upper airways, do not effectively cool by panting and do not tolerate high temperatures.

How does a dog maintain homeostasis?

Sweating is your body’s way of cooling down, thus maintaining homeostasis. As the liquid dries on your skin, it cools your skin and lowers your temperature. Because dogs do not have sweat glands, they pant. … The major blood vessel in a dog’s head runs very close to the surface of its nose.

How do I cool down a hot dog?

Let your dog stand in a cool pool. Aside from panting, dogs cool down through the sweat glands in their paws. Having them stand in a cool pool of water or giving them a quick foot soak can help lower their body temperature. It can also be helpful to put some cold water on your dog’s chest.

Should I shave my dog in the summer?

[excerpt] “The “no shave” rule applies not only to super-furry northern breeds like Samoyeds, Huskies or Malamutes but to other double-coated breeds as well. Herding breeds like Aussie Shepherds, Border Collies, and Shelties are double-coated. So are Golden Retrievers, Newfoundlands, Bernese Mountain Dogs and many more.

Double-coated breeds have two layers to protect against arctic weather. The long guard hairs form the outer layer and protect against snow or ice and even shed water. The soft undercoat lies close to the skin and keeps your dog warm and dry. In winter this undercoat can be so thick you may have trouble finding your dog’s skin.

In summer, your dog should shed his soft undercoat, leaving just the guard hairs. The job of the guard hairs in warm weather is to protect your dog from sunburn and insulate him against the heat. Without the undercoat, air can circulate through the guard hairs, cooling the skin.

Unlike single coated breeds, who have hair that just keeps growing, double coats grow to a certain length and don’t get any longer. So you can shave a single-coated breed down and the coat will grow back again without really changing it. But that’s not true for double coats. Shaving a double-coated breed can really ruin the coat.” Learn more >>

How do you know if it’s too hot to walk your dog?

Use the five-second rule to make sure it’s safe to walk your dog. This tip comes via Moon Valley Canine Training, and it’s pretty simple. Whenever you take your dog out, place the back of your hand on the pavement. If you can’t hold it there for five seconds, it’s too hot to walk your dog.

The Transport of Heat

Many animals (including humans) have another way to conserve heat. The arteries of our arms and legs run parallel to a set of deep veins. As warm blood passes down the arteries, the blood gives up some of its heat to the colder blood returning from the extremities in these veins.

Such a mechanism is called a countercurrent heat exchanger. When heat loss is no problem, most of the venous blood from the extremities returns through veins located near the surface.

Countercurrent heat exchangers can operate with remarkable efficiency. A sea gull can maintain a normal temperature in its torso while standing with its unprotected feet in freezing water.

When you consider that the blood of fishes passes over the gills which are bathed in the surrounding water, it is easy to see why fishes are "cold-blooded". Nonetheless, some marine fishes (e.g., the tuna) are mesotherms &mdash able to keep their most active swimming muscles warmer than the sea by using a countercurrent heat exchanger.

This photograph on the right (courtesy of E. D. Stevens, Dept. of Zoology, University of Guelph, Ontario) shows a cross section through a skipjack tuna. The dark muscle on either side of the vertebral column is maintained at a higher temperature than the rest of the fish thanks to its countercurrent heat exchanger.

The cold, oxygen-rich arterial blood passes into a series of fine arteries that take the blood into the active muscles. These fine arteries lie side by side with veins draining those muscles. So as the cold blood passes into the muscles, it picks up the heat that had been generated by these muscles and keeps it from being lost to the surroundings.

Thanks to this countercurrent heat exchanger, a tuna swimming in the winter can maintain its active swimming muscles 14°C warmer than the surrounding water.

The photomicrograph on the left (also courtesy of Dr. Stevens) is of a cross section through the heat exchanger. Note the close, parallel packing of the arteries (thick walls) and veins (thin walls).

Countercurrent exchangers also operate in the kidney and are built into the design of artificial kidneys.

The circulatory system is also responsible for cooling an animal. If the animal's "core" body temperature gets too high, the blood supply to the surface and extremities is increased enabling heat to be released to the surroundings. If this is insufficient, the animal can evaporate water from the blood &mdash in the form of sweat for those animals with sweat glands. The evaporation of 1 gram of water absorbs some 540 calories of heat.

Most endotherms cannot tolerate a rise in body temperature of more than 5°C or so. The brain is the organ most susceptible to damage by a high temperature. Some mammals, dogs for example, have a countercurrent heat exchanger located between the carotid arteries and the vessels that distribute blood to the brain. This heat exchanger transfers some of the heat of the arterial blood to the relatively cool venous blood returning from the nose and mouth. This cools their arterial blood before it reaches the brain.

A second region of the hypothalamus triggers warming responses:

It is the hypothalamus that executes the fever response. In effect, the hypothalamus is the body's thermostat. The release of prostaglandins during inflammation increases the setting that is, turns the thermostat "up". If the body temperature is not yet there, the body begins shivering violently &mdash causing "chills" &mdash to generate the heat needed. The result is fever when the new set point is reached.

Homeostasis: Thermoregulation

Animals use different modes of thermoregulation processes to maintain homeostatic internal body temperatures.

Learning Objectives

Outline the various types of processes utilized by animals to ensure thermoregulation.

Key Takeaways

Key Points

  • In response to varying body temperatures, processes such as enzyme production can be modified to acclimate to changes in the temperature.
  • Endotherms regulate their own internal body temperature, regardless of fluctuating external temperatures, while ectotherms rely on the external environment to regulate their internal body temperature.
  • Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.
  • Heat can be exchanged between environment and animals via radiation, evaporation, convection, or conduction processes.

Key Terms

  • ectotherm: An animal that relies on external environment to regulate its internal body temperature.
  • endotherm: An animal that regulates its own internal body temperature through metabolic processes.
  • homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
  • poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

Thermoregulation to Maintain Homeostasis

Internal thermoregulation contributes to animal’s ability to maintain homeostasis within a certain range of temperatures. As internal body temperature rises, physiological processes are affected, such as enzyme activity. Although enzyme activity initially increases with temperature, enzymes begin to denature and lose their function at higher temperatures (around 40-50 C for mammals). As internal body temperature decreases below normal levels, hypothermia occurs and other physiological process are affected. There are various thermoregulation mechanisms that animals use to regulate their internal body temperature.

Types of Thermoregulation (Ectothermy vs. Endothermy)

Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, and are colloquially referred to as “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. Ectotherms are colloquially referred to as “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.


Ectotherm: The Common frog is an ecotherm and regulates its body based on the temperature of the external environment.

An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments in which temperatures are constant, such as the tropics or ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.


In contrast to ectotherms, endotherms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is usually generated from the animal’s normal metabolism, but under conditions of excessive cold or low activity, an endotherm generate additional heat by shivering. Many endotherms have a larger number of mitochondria per cell than ectotherms. These mitochondria enables them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food more often. For example, a mouse (endotherm) must consume food every day to sustain high its metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.

Homeothermy vs. Poikilothermy

Homeotherm vs. Poikilotherm: Sustained energy output of an endothermic animal (mammal) and an ectothermic animal (reptile) as a function of core temperature. In this scenario, the mammal is also a homeotherm because it maintains its internal body temperature in a very narrow range. The reptile is also a poikilotherm because it can withstand a large range of temperatures.

A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Poikilotherm’s internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs that have such stable ambient temperatures that their internal temperature remains constant.

Means of Heat Transfer

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and from dry skin the same manner. When a mammal sweats, evaporation removes heat from a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

Mechanisms for heat exchange: Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction.

Keeping Cool

When you get hot, what’s the first thing that happens? You start to sweat. The average adult has 3 million sweat glands. It’s not the sweating that cools you, but rather the evaporation of this sweat. Evaporation is an endothermic phase change, meaning it must absorb energy to occur. This energy is drawn from your body, making you cooler.

Anytime you lose energy, your body will feel cool. Evaporation requires energy because forces of attraction between water molecules—called intermolecular forces—need to be broken when water goes from a liquid to a gas. In liquid water, the molecules are close together and are attracted to one another. Evaporation requires energy because the intermolecular forces of attraction between water molecules in the liquid phase must be overcome when water goes from a liquid to a gas. The energy that goes into overcoming these attractive forces comes from your body.

Do animals sweat? Most don’t, but some do. Dogs sweat mainly between the pads on the bottom of their paws. One notable exception is the American hairless terrier, which has sweat glands all over its body, illustrating the fact that fur tends to inhibit sweating because if the sweat can’t evaporate it doesn’t help in the cooling process.

Cats not only have sweat glands on the pads of their feet, but also on their tongues! When a cat licks itself, it may not be just to keep clean, but it could also be to cool itself as the saliva on their fur evaporates. Kangaroos will lick their forearms for the same reason.

The key to surviving in hot climates is not only to keep your body from overheating but also to prevent water loss. Animals that are adapted to desert life are not heavy sweaters—because water is scarce, they cannot afford to lose it by sweating. Also, a great deal of water is lost through breathing out, so desert animals expel dry air, reabsorbing the water in their breath before it has a chance to be expelled.

The ability of animals to adapt to extreme environments is quite remarkable. Whether it is in the freezing corners of Siberia or the sizzling hot desert of the Sahara, animals always find ways to survive, and how they do it will never cease to amaze us!

  • Denny, M. McFadzean, A. Engineering Animals: How Life Works Harvard University Press: Cambridge, MA, 2011.
  • Mone, G. 20 Things You Didn’t Know About… Hibernation. Discover, March 2013, p 74.
  • Streever, B. Cold: Adventures in the World’s Coldest Places Little, Brown and Company: New York, 2009.
  • Streever, B. Heat: Adventures in the World’s Fiery Places Little, Brown and Company: New York, 2013.

Brian Rohrig teaches chemistry at Metro Early College High School in Columbus, Ohio. His most recent ChemMatters article, “Not Milk? Living with Lactose Intolerance,” appeared in the April 2013 issue.

Excretion in Animals, Humans and Plants (with diagram)

Chemical reactions occur in the cells of living organisms all the time to carry out the life processes.

The sum of these reactions is called metabolism. Metabolism produces useful products as well as toxic (poisonous) by-products.

These toxic substances have to be removed as they are harmful if allowed to accumulate. The removal of metabolic waste products from the body of an organism is known as excretion.

The major excretory products are carbon dioxide, excess water, and nitrogenous compounds like ammonia, urea, uric acid, etc. Carbon dioxide and water are produced in the process of tissue respiration. Nitrogenous compounds are formed from the breakdown of proteins and amino acids. Water and salts in excess of the body’s needs are also excreted.

We acquire most of the water with our food and drink and some by metabolism, e.g., the water produced during cellular respiration. Other excretory products include chemicals from medicines, toxic substances, and circulating hormones that have already served their purpose. We will learn how metabolic wastes get eliminated.

Excretion in animals:

Many unicellular organisms like Amoeba throw out their wastes by diffusion from their body surface. Protozoan’s have no organs for excretion. As they live in an aquatic habitat, their wastes are eliminated by diffusion through the plasma membrane.

Simple multicellular organisms like Hydra throw out solid waste matter through their mouth. Higher multicellular organisms have well-defined specialized excretory organs. These organs could be simple tubular structures as in flatworms and leech.

The excretory organs of insects (e.g., grasshopper, cockroach and housefly) are also tubular. They remove nitrogenous wastes from the body fluid and help in maintaining the water balance in the body.

In vertebrates, the main organs of excretion and maintenance of water balance are the kidneys.

Excretion in human beings:

Although the kidneys are the main organs of excretion, the skin, lungs and liver also help in excretion.

Our skin has sweat glands, through which we excrete small amounts of water, urea and salts.

The liver excretes bile, which contains bile pigments. These are produced by the breakdown of old RBCs in the liver. As hemoglobin breaks down, its iron is retained, while the pigment (haem) is excreted with the bile. The liver also excretes cholesterol.

The lungs help in getting rid of carbon dioxide, formed as a result of cellular respiration, through exhalation.

Excretory System in Man:

Our excretory system consists of kidneys, blood vessels that join them, ureters, urinary bladder and urethra. They help produce and excrete urine.

There are two bean-shaped kidneys that lie in the abdominal cavity, one on either side of the vertebral column. The kidneys are reddish brown. Each of them is about 10 cm long and weighs about 150 g. Although they weigh less, they receive a lot of blood for filtration.

A volume of blood nearly equivalent to that in the whole body passes through the kidneys every four or five minutes. The kidneys produce urine to filter out the waste products, like urea and uric acid, from the blood.

Urine leaves each kidney through a tube called ureters. The ureters from both the kidneys are corrected to the urinary bladder that collects and stores urine. Ureters carry urine from the kidneys into the urinary bladder. The urethra is a canal that carries urine from the bladder and expels it outside the body.

Internal Structure of a Kidney:

Each kidney is enclosed in a thin, fibrous covering called the capsule. A renal artery brings blood into the kidney, along with nitrogenous waste materials. After filtration in the kidney, the purified blood leaves the kidney through a renal vein.

Two distinct regions can be seen in the section of a kidney:

(1) An outer, dark, granular cortex and (2) an inner, lighter medulla. The hollow space from where the ureter leaves the kidney is called the pelvis. Each kidney is made up of numerous (about one million) coiled excretory tubules, known as nephrons, and collecting ducts associated with tiny blood vessels. A nephron is the structural and functional unit of a kidney, having three functions— filtration, reabsorption and secretion.

A cluster of thin-walled blood capillaries remains associated with the cup-shaped end of each nephron tubule. These capillaries bring blood from the body to the nephron for filtration. The network of capillaries spreads over the nephron tubules also. These capillaries finally carry purified blood to the body.

Structure and Function of a Nephron:

A nephron consists of a long coiled tubule and the Malpighian corpuscle. The tubule of the nephron is differentiated into the proximal convoluted tubule, Henle’s loop and the distal convoluted tubule. The distal tubule opens into the collecting duct.

At the proximal end of the nephron is the Malpighian corpuscle, which consists of Bowman’s capsule and the glomerulus. Bowman’s capsule is a double-walled cuplike structure which surrounds the dense network of blood capillaries called the glomerulus.

The process of excretion in nephron:

The process of excretion may be divided into three stages- tubular secretion.

Filtration of blood occurs under high pressure in the nephrons of the kidney. Blood enters the glomerulus through the afferent arteriole (with a wider lumen) and leaves through the efferent arteriole (with a narrow lumen). Therefore, blood passes through the glomerulus under pressure. This results in filtration of blood.

Water and small molecules are forced out of the walls of the capillaries of the glomerulus and Bowman’s capsule and enter the tubule of the nephron. Large molecules remain in the blood of the glomerulus. The filtrate contains water, glucose, salts, urea, vitamins, etc. It is called the glomerular filtrate.

Selective reabsorption:

Some molecules of the glomerular filtrate are selectively reabsorbed into the blood. The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped Henle’s loop and the distal convoluted tubule. It contains many useful substances such as glucose, amino acids and salts.

These are reabsorbed by a process, which requires energy. Without reabsorption, these nutrients could have been lost with the urine. The filtrate now contains urea, some salts and water. Reabsorption of solutes into the blood increases the water concentration of the filtrate.

Then water is reabsorbed into the blood by the process of osmosis, and the osmotic balance is restored. The amount of water reabsorbed depends on the amount of excess water in the body and that of the dissolved waste to be excreted.

This reabsorption of water from the filtrate to maintain the water balance of the body fluid is known as osmoregulation. In this way the kidneys serve as water-conserving organs. After reabsorption from 180 L of filtrate in the kidney, only 1-2 L of urine is produced.

Some nitrogenous waste products like creatinin and some other substances like potassium ions are removed from the blood by the distal convoluted tubule, and are then added to the urine. This is called tubular secretion.

The urine that is formed continually collects in the urinary bladder. As the bladder expands, its pressure creates an urge to pass urine through the urethra. As the bladder is muscular, the urge to urinate is under voluntary nervous control.

Kidney Failure and the Survival Kit—Haemodialysis:

The kidneys may be damaged due to infection, injury, diabetes, and extremes of blood pressure. A damaged kidney cannot function efficiently to remove urea, ions, water, etc., from the blood. This malfunctioning results in the accumulation of toxic wastes like urea (uremia), which can lead to death.

One of the ways to treat kidney failure is to use a ‘dialysis machine’ that acts as an artificial kidney. It has a long tube like structure made of Cellophane suspended in a tank (dialyser) of a fresh dialysis fluid (dialysis). The Cellophane tube is partially permeable and therefore allows solutes to diffuse through. The dialysis fluid has the same concentration as normal tissue fluid, but nitrogenous wastes and excess salts are absent.

During dialysis, the blood of the patient is withdrawn from an artery and cooled at 0°C. It is maintained in a liquid state by adding an anticoagulant and by other special treatments. It is pumped through the dialysis machine. Here, the nitrogenous waste products from the blood diffuse into the dialysis fluid. The purified blood is then warmed to the body temperature and pumped back into the patient’s body through a vein.

The dialyser is specific for each patient to avoid infections. Dialysis through an artificial kidney has to be carried out at frequent intervals. This process of purification of blood is called haemodialysis.

A dialysis machine works like a kidney except that no selective reabsorption takes place in the former.

(1) Helps remove harmful wastes, extra salts and water

(2) Controls blood pressure and

(3) Maintains the balance of sodium and potassium salts in a patient whose kidneys have failed.

Excretion in plants:

Compared to animals, plants do not have a well-developed excretory system to throw out nitrogenous waste materials. This is because of the differences in their physiology. Therefore, plants use different strategies for excretion.

The gaseous waste materials produced during respiration (carbon dioxide) and photosynthesis (oxygen) diffuse out through stomata in the leaves and through lenticels in other parts of the plant. Excess water evaporates mostly from stomata and also from the outer surface of the stem, fruits, etc., throughout the day. This process of getting rid of excess water is called transpiration.

The waste products, like oxygen, carbon dioxide and water, are the raw materials for other cellular reactions. The excess of carbon dioxide and water are used up in this way. The only major gaseous excretory product of plants is oxygen!

Many plants store organic waste products in their permanent tissues that have dead cells, e.g., in heartwood. Plants also store waste within their leaves or barks. These wastes are periodically removed as the leaves and barks fall off.

Some of the waste products are stored in special cells or cellular vacuoles. Various waste products such as tannins, essential oils, gums, resins, etc., are produced during catabolic processes. Tea leaves, amla and betel nuts (supari) contain tannin. Tannins are found also in the barks of trees.

The leaves of many plants, like Eucalyptus, lemon, sacred basil (tulsi), etc., contain essential oils. The rind of oranges and lemons and the petals of flowers like rose and jasmine also contain oils. Some plant wastes are stored as a thick, white fluid. You may have seen white fluid ooze out when you pluck a papaya or a fig or the leaves of yellow oleander (pila kaner). This white fluid is called latex.

Gums are a group of sticky, water- soluble wastes found in the common gum tree (babul). Resins are another group of wastes found commonly in the stems of conifers (e.g., pine, fir).

Alkaloids are a group of toxic waste products. But some of these are useful to us. Quinine and morphine are medicines derived from alkaloids stored in Cinchona bark and opium poppy flowers respectively. Caffeine found in coffee seeds and nicotine in tobacco leaves is also alkaloids.

Organic acids, which might prove harmful to plants, often combine with excess cations and precipitate out as insoluble crystals that can be safely stored in plant cells. Calcium oxalate crystals accumulate in some tubers like yam (zamikand).

Aquatic plants lose most of their metabolic wastes by direct diffusion into the water surrounding them. Terrestrial plants excrete some waste into the soil around them.

1911 Encyclopædia Britannica/Animal Heat

ANIMAL HEAT. Under this heading is discussed the physiology of the temperature of the animal body.

The higher animals have within their bodies certain sources of heat, and also some mechanism by means of which both the production and loss of heat can be regulated. This is conclusively shown by the fact that both in summer and winter their mean temperature remains the same. But it was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained. It was then found that local differences were present, since heat production and heat loss vary considerably in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to determine the temperature of those parts which most nearly approaches to that of the internal organs. Also for such results to be comparable they must be made in the same situation. The rectum gives most accurately the temperature of internal parts, or in women and some animals the vagina, uterus or bladder. ​ Occasionally that of the urine as it leaves the urethra may be of use. More usually the temperature is taken in the mouth, axilla or groin.

Warm and Cold Blooded Animals.—By numerous observations upon men and animals, John Hunter showed that the essential difference between the so-called warm-blooded and cold-blooded animals lies in the constancy of the temperature of the former, and the variability of the temperature of the latter. Those animals high in the scale of evolution, as birds and mammals, have a high temperature almost constant and independent of that of the surrounding air, whereas among the lower animals there is much variation of body temperature, dependent entirely on their surroundings. There are, however, certain mammals which are exceptions, being warm-blooded during the summer, but cold-blooded during the winter when they hibernate such are the hedgehog, bat and dormouse. John Hunter suggested that two groups should be known as “animals of permanent heat at all atmospheres” and “animals of a heat variable with every atmosphere,” but later Bergmann suggested that they should be known as “homoiothermic” and “poikilothermic” animals. But it must be remembered there is no hard and fast line between the two groups. Also, from work recently done by J. O. Wakelin Barratt, it has been shown that under certain pathological conditions a warm-blooded (homoiothermic) animal may become for a time cold-blooded (poikilothermic). He has shown conclusively that this condition exists in rabbits suffering from rabies during the last period of their life, the rectal temperature, being then within a few degrees of the room temperature and varying with it. He explains this condition by the assumption that the nervous mechanism of heat regulation has become paralysed. The respiration and heart-rate being also retarded during this period, the resemblance to the condition of hibernation is considerable. Again, Sutherland Simpson has shown that during deep anaesthesia a warm-blooded animal tends to take the same temperature as that of its environment. He demonstrated that when a monkey is kept deeply anesthetized with ether and is placed in a cold chamber, its temperature gradually falls, and that when it has reached a sufficiently low point (about 25° C. in the monkey), the employment of an anaesthetic is no longer necessary, the animal then being insensible to pain and incapable of being roused by any form of stimulus it is, in fact, narcotized by cold, and is in a state of what may be called “artificial hibernation.” Once again this is explained by the fact that the heat-regulating mechanism has been interfered with. Similar results have been obtained from experiments on cats. These facts—with many others—tend to show that the power of maintaining a constant temperature has been a gradual development, as Darwin's theory of evolution suggests, and that anything that interferes with the due working of the higher nerve-centres puts the animal back again, for the time being, on to a lower plane of evolution.

Chart showing diurnal variation in body temperature, ranging from about 37.5° C. from 10 A.M. to 6 P.M. , and falling to about 36.3° C. from 2 A.M. to 6 A.M.

Variations in the Temperature of Man and some other Animals.—As stated above, the temperature of warm-blooded animals is maintained with but slight variation. In health under normal conditions the temperature of man varies between 36° C. and 38° C., or if the thermometer be placed in the axilla, between 36·25° C. and 37·5° C. In the mouth the reading would be from ·25° C. to 1·5° C. higher than this and in the rectum some ·9° C. higher still. The temperature of infants and young children has a much greater range than this, and is susceptible of wide divergencies from comparatively slight causes.

Of the lower warm-blooded animals, there are some that appear to be cold-blooded at birth. Kittens, rabbits and puppies, if removed from their surroundings shortly after birth, lose their body heat until their temperature has fallen to within a few degrees of that of the surrounding air. But such animals are at birth blind, helpless and in some cases naked. Animals who are born when in a condition of greater development can maintain their temperature fairly constant. In strong, healthy infants a day or two old the temperature rises slightly, but in that of weakly, ill-developed children it either remains stationary or falls. The cause of the variable temperature in infants and young immature animals is the imperfect development of the nervous regulating mechanism.

The average temperature falls slightly from infancy to puberty and again from puberty to middle age, but after that stage is passed the temperature begins to rise again, and by about the eightieth year is as high as in infancy. A diurnal variation has been observed dependent on the periods of rest and activity, the maximum ranging from 10 A.M. to 6 P.M. , the minimum from 11 P.M. to 3 A.M. Sutherland Simpson and J. J. Galbraith have recently done much work on this subject. In their first experiments they showed that in a monkey there is a well-marked and regular diurnal variation of the body temperature, and that by reversing the daily routine this diurnal variation is also reversed. The diurnal temperature curve follows the periods of rest and activity, and is not dependent on the incidence of day and night in monkeys which are active during the night and resting during the day, the body temperature is highest at night and lowest through the day. They then made observations on the temperature of animals and birds of nocturnal habit, where the periods of rest and activity are naturally the reverse of the ordinary through habit and not from outside interference. They found that in nocturnal birds the temperature is highest during the natural period of activity (night) and lowest during the period of rest (day), but that the mean temperature is lower and the range less than in diurnal birds of the same size. That the temperature curve of diurnal birds is essentially similar to that of man and other homoiothermal animals, except that the maximum occurs earlier in the afternoon and the minimum earlier in the morning. Also that the curves obtained from rabbit, guinea-pig and dog were quite similar to those from man. The mean temperature of the female was higher than that of the male in all the species examined whose sex had been determined.

Meals sometimes cause a slight elevation, sometimes a slight depression—alcohol seems always to produce a fall. Exercise ​ and variations of external temperature within ordinary limits cause very slight change, as there are many compensating influences at work, which are discussed later. Even from very active exercise the temperature does not rise more than one degree, and if carried to exhaustion a fall is observed. In travelling from very cold to very hot regions a variation of less than one degree occurs, and the temperature of those living in the tropics is practically identical with those dwelling in the Arctic regions.

Limits compatible with Life.—There are limits both of heat and cold that a warm-blooded animal can bear, and other far wider limits that a cold-blooded animal may endure and yet live. The effect of too extreme a cold is to lessen metabolism, and hence to lessen the production of heat. Both katabolic and anabolic changes share in the depression, and though less energy is used up, still less energy is generated. This diminished metabolism tells first on the central nervous system, especially the brain and those parts concerned in consciousness. Both heart-beat and respiration-number become diminished, drowsiness supervenes, becoming steadily deeper until it passes into the sleep of death. Occasionally, however, convulsions may set in towards the end, and a death somewhat similar to that of asphyxia takes place. In some recent experiments on cats performed by Sutherland Simpson and Percy T. Herring, they found them unable to survive when the rectal temperature was reduced below 16° C. At this low temperature respiration became increasingly feeble, the heart-impulse usually continued after respiration had ceased, the beats becoming very irregular, apparently ceasing, then beginning again. Death appeared to be mainly due to asphyxia, and the only certain sign that it had taken place was the loss of knee jerks. On the other hand, too high a temperature hurries on the metabolism of the various tissues at such a rate that their capital is soon exhausted. Blood that is too warm produces dyspnoea and soon exhausts the metabolic capital of the respiratory centre. The rate of the heart is quickened, the beats then become irregular and finally cease. The central nervous system is also profoundly affected, consciousness may be lost, and the patient falls into comatose condition, or delirium and convulsions may set in. All these changes can be watched in any patient suffering from an acute fever. The lower limit of temperature that man can endure depends on many things, but no one can survive a temperature of 45° C. (113° F.) or above for very long. Mammalian muscle becomes rigid with heat rigor at about 50° C., and obviously should this temperature be reached the sudden rigidity of the whole body would render life impossible. H. M. Vernon has recently done work on the death temperature and paralysis temperature (temperature of heat rigor) of various animals. He found that animals of the same class of the animal kingdom showed very similar temperature values, those from the Amphibia examined being 38·5° C., Fishes 39°, Reptilia 45°, and various Molluscs 46°. Also in the case of Pelagic animals he showed a relation between death temperature and the quantity of solid constituents of the body, Cestus having lowest death temperature and least amount of solids in its body. But in the higher animals his experiments tend to show that there is greater variation in both the chemical and physical characters of the protoplasm, and hence greater variation in the extreme temperature compatible with life.

Regulation of Temperature.—The heat of the body is generated by the chemical changes—those of oxidation—undergone not by any particular substance or in any one place, but by the tissues at large. Wherever destructive metabolism (katabolism) is going on, heat is being set free. When a muscle does work it also gives rise to heat, and if this is estimated it can be shown that the muscles alone during their contractions provide far more heat than the whole amount given out by the body. Also it must be remembered that the heart—also a muscle,—never resting, does in the 24 hours no inconsiderable amount of work, and hence must give rise to no inconsiderable amount of heat. From this it is clear that the larger proportion of total heat of the body is supplied by the muscles. These are essentially the “thermogenic tissues.” Next to the muscles as heat generators come the various secretory glands, especially the liver, which appears never to rest in this respect. The brain also must be a source of heat, since its temperature is higher than that of the arterial blood with which it is supplied. Also a certain amount of heat is produced by the changes which the food undergoes in the alimentary canal before it really enters the body. But heat while continually being produced is also continually being lost by the skin, lungs, urine and faeces. And it is by the constant modification of these two factors, (1) heat production and (2) heat loss, that the constant temperature of a warm-blooded animal is maintained. Heat is lost to the body through the faeces and urine, respiration, conduction and radiation from the skin, and by evaporation of perspiration. The following are approximately the relative amounts of heat lost through these various channels (different authorities give somewhat different figures):—faeces and urine about 3, respiration about 20, skin (conduction, radiation and evaporation) about 77. Hence it is clear the chief means of loss are the skin and the lungs. The more air that passes in and out of the lungs in a given time, the greater the loss of heat. And in such animals as the dog, who do not perspire easily by the skin, respiration becomes far more important.

But for man the great heat regulator is undoubtedly the skin, which regulates heat loss by its vasomotor mechanism, and also by the nervous mechanism of perspiration. Dilatation of the cutaneous vascular areas leads to a larger flow of blood through the skin, and so tends to cool the body, and vice versa. Also the special nerves of perspiration can increase or lessen heat loss by promoting or diminishing the secretions of the skin. There are greater difficulties in the exact determination in the amount of heat produced, but there are certain well-known facts in connexion with it. A larger living body naturally produces more heat than a smaller one of the same nature, but the surface of the smaller, being greater in proportion to its bulk than that of the larger, loses heat at a more rapid rate. Hence to maintain the same constant bodily temperature, the smaller animal must produce a relatively larger amount of heat. And in the struggle for existence this has become so.

Food temporarily increases the production of heat, the rate of production steadily rising after a meal until a maximum is reached from about the 6th to the 9th hour. If sugar be included in the meal the maximum is reached earlier if mainly fat, later. Muscular work very largely increases the production of heat, and hence the more active the body the greater the production of heat.

But all the arrangements in the animal economy for the production and loss of heat are themselves probably regulated by the central nervous system, there being a thermogenic centre—situated above the spinal cord, and according to some observers in the optic thalamus.

Authorities .—M. S. Pembrey, “Animal Heat,” in Schafer's Textbook of Physiology (1898) C. R. Richet, “Chaleur,” in Dictionnaire de physiologie (Paris, 1898) Hale White, Croonian Lectures, Lancet, London, 1897 Pembrey and Nicol, Journal of Physiology vol. xxiii., 1898-1899 H. M. Vernon, “Heat Rigor,” Journal of Physiology xxiv., 1899 H. M. Vernon, “Death Temperatures,” Journal of Physiology, xxv., 1899 F. C. Eve, “Temperature on Nerve Cells,” Journal of Physiology, xxvi., 1900 G. Weiss, Comptes Rendus, Soc. de Biol., lii., 1900 Swale Vincent and Thomas Lewis, “Heat Rigor of Muscle,” Journal of Physiology, 1901 Sutherland Simpson and Percy Herring, “Cold and Reflex Action,” Journal of Physiology, 1905 Sutherland Simpson, Proceedings of Physiological Soc., July 19, 1902 Sutherland Simpson and J. J. Galbraith, “Diurnal Variation of Body Temperature,” Journal of Physiology, 1905 Transactions Royal Society Edinburgh, 1905 Proc. Physiological Society, p. xx., 1903 A. E. Boycott and J. S. Haldane, Effects of High Temperatures on Man.

‘Winners and losers’

Gabriel d'Eustachio, a bush firefighter in Australia, said in 2014 that he has witnessed mass movements of small invertebrates fleeing blazes. “You get overrun by this wave of creepy crawlies walking ahead of the fire,” he said. (Find out what it's like to be on the front line of the fight against wildfires.)

Fires can benefit predators that prey on these fleeing animals. Bears, raccoons, and raptors, for instance, have been seen hunting creatures trying to escape the flames. Several species of birds may even help spread fires in Australia, some research suggests, as doing so may help flush out small animals for them to eat.

“In those short-term situations,” such as when creatures flee from flames, says Sullivan, “there’s always winners and losers.”

A moderate level of fire in areas where it naturally occurs may also increase the “patchiness” of forests and create a wider variety of microhabitats, from open meadows to re-growing forest, research shows. Having a diversity of biomes supports multiple species of animals and the ecosystem as a whole.

Scientists don't have any good estimates on the number of animals that die in wildfires each year. But there are no documented cases of fires—even the really severe ones—wiping out entire populations or species. (Watch: “Fish skin bandages help burned bears and cougar heal”)

Of course, some animals do die in the smoke and fire—those that can't run fast enough or find shelter. Young and small animals are particularly at risk. And some of their strategies for escape might not work—a koala’s natural instinct to crawl up into a tree, for example, may leave it trapped.

Heat can kill too—even organisms buried deep in the ground, such as fungi. Jane Smith, a mycologist with the U.S. Forest Service in Corvallis, Oregon, has measured temperatures as high as 1,292 degrees Fahrenheit beneath logs burning in a wildfire, and 212 degrees Fahrenheit a full two inches below the surface.