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Homeostasis: A balancing act

Maintaining a constant internal environment

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There are many processes going on inside our bodies which you may never know about, yet they are vital to our survival. Chemical reactions, filtering, absorbing, diffusing, digesting, making, breaking - all these bodily functions are necessary to keep us alive. Homeostasis utilises these processes. It ensures we maintain a constant internal environment because, without the processes deployed by homeostasis, you wouldn't live long! 

Red-bellied turtles, raised from eggs, bask in an incubator to keep their body temperatures constant. OXLAM

Humans require all sorts of maintenance in order to function normally, and the homeostatic system is controlled predominantly by hormones released by the endocrine system.

Warm- or cold-blooded?

Maintaining a constant 37 degrees Celsius is a very energy-demanding process. That's why we have to eat as many as three meals every day just to supply the energy to fuel our biological processes.

Biological catalysts, called enzymes, that allow vital chemical reactions to happen every second inside our cells work most effectively at 37 degrees. If the temperature of the human body varies even a few degrees from that number, it could be mortally dangerous. All birds and mammals are the same in this respect, as they are exothermic (warm-blooded).

Reptiles and other endothermic (cold-blooded) animals do not maintain an internal body temperature, and so they regulate their balance by basking in the sun. This is very energy-efficient, which is why many reptiles only need to eat occasionally.

I had a python in Australia that went seven months without eating and yet maintained its healthy condition. It was because of how cold the environment was during the winter months.

Some huge snakes eat only once a year, as do many Nile crocodiles that await the annual wildebeest migration. One huge meal can take a very long time to digest, based on the reptile's temperature, and can provide more than enough energy for a year's lying around!

This thermographic image shows a cold-blooded tarantula on an exothermic human hand. ARNO & COEN

Thermoregulation

One aspect of homeostasis is thermoregulation. Thermoregulation is the ability of an organism to keep its internal temperature within certain limits even if the external temperature is far above or below those limits.

If an animal is unable to maintain its body's normal temperature and its temperature decreases significantly below normal, hypothermia results. The opposite, or hyperthermia, results if the body's temperature rises significantly above normal.

Have you ever noticed that when you are cold, your hands, feet, ears and nose (your extremities) feel especially cold? And conversely, when warm, these extremities feel warm?

This is to ensure that the major organs in your trunk and your brain are always at 37 degrees. Your extremities are expendable, as anyone with frostbite would know.

You produce heat through the contraction of body muscles and through the action of large or energy-expensive organs, such as the liver and the brain. A region of the brain called the hypothalamus is the thermostat for the body, triggering responses if the body is too hot or too cold.

When you are too hot, the blood vessels near the surface of your skin expand in diameter, a phenomenon known as vasodilatation. This enables more blood to flow nearer the surface of the skin, which allows heat to be lost through radiation, hence resulting in warm extremities.

Hairs on your body lie flat so as to not trap any air that might insulate the skin and prevent radiation. Sweat glands are also stimulated, releasing sweat onto your skin. The sweat then evaporates, taking the heat with it.

When it is too cold, the opposite happens. The blood vessels close to the skin reduce in diametric size, thereby reducing the blood flow and reducing heat loss through radiation. This condition is known as vasoconstriction and is why your extremities get cold.

Hairs connected by muscles under the skin stand on end, giving rises to what are commonly called goose pimples. The idea is that the erect hairs will trap a layer of air, which is a very good insulator, thereby reducing heat loss. Humans have lost much of their body hair and so this process is not very effective anymore, but in our mammalian cousins and ancestors it is an effective heat-retaining method.

Sweat production is also stopped, thus not allowing evaporative cooling to take place. If you get really cold, your muscles can twitch, which, in turn, causes you to shiver. This makes the muscles contract and relax quickly, and heat is generated as a result of muscle respiration, which is an exothermic chemical reaction.

I'm sure you've seen dogs pant. As they don't sweat, that is one way they thermoregulate - by means of evaporative cooling. And have you wondered why elephants have big ears? It is not for hearing. It is also for thermoregulation. Hot elephants can radiate heat from their ears with increased blood flow, or reduce blood flow (and therefore radiation) when cold. And when birds roost, they puff up their feathers, trapping a layer of air, keeping them nice and warm overnight.

Blood sugar

The glucose in our blood is known as blood sugar, and it is controlled by two hormones produced in the pancreas: insulin and glucagon. After meals, especially high-carbohydrate meals, the food is broken down into glucose by the action of enzymes such as amylase in your saliva, and so the level of sugar in your blood can be extremely high.

This excessive amount of sugar needs to be removed as it can lead to many health-related problems, including renal (kidney) failure, which, if untreated, is lethal. The hormone insulin is released by the pancreas. It makes the liver convert the glucose in your blood into glycogen, which is stored in the cells in your muscles.

People with diabetes produce little or no insulin, meaning they can't convert glucose into glycogen. This means they have to inject themselves with insulin and regulate their own blood sugar levels, which is not always easy. Thankfully, it is manageable and such people can live a relatively normal life.

When you exercise, you require glucose as your body needs it for respiration to produce energy. This means the stored glycogen in your muscles is converted into glucose by means of the action of glucagon, thereby increasing your blood sugar levels and maintaining the balance.

More regulation

Homeostasis doesn't stop at temperature and blood sugar regulation. Osmoregulation is the regulation of the water concentration in your blood and extracellular fluid, controlled primarily by the kidneys, which determine how diluted or concentrated urine is as a result of salt and other dissolved ions in the blood. It also acts to remove toxic nitrogenous wastes produced in the liver.

Carbon dioxide, toxic at high levels, is regulated by the lungs. Deep breathing is more to remove carbon dioxide, which makes blood levels acidic, than to get oxygen into your bloodstream. There are also iron levels, acidity, calcium levels and more, all of which need regulating in order to maintain a balanced, constant internal environment and ensure a healthy body.

Dave Canavan has an MSc in Behavioural Ecology and is the Principal of Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You may contact Dave at davidc@gardenbangkok.com.

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