Environmental factors are the characteristics of the external environment that have a direct impact on our organism. Sometimes, the alteration of some of these parameters (such as temperature or humidity) can negatively affect bodies, damaging and disrupting the execution of physiological functions. It’s important for animals to be able to have control over these factors, in order to maintain a certain degree of constance and stability in their organism.

Some regular activities, such as drinking or transpiring, help achieve this stability, but there are a series of extremely complex mechanisms involved in the regulation of the body. Keep on reading to learn about the importance of homeostatic mechanisms, which are in charge of maintaining a state of steady internal conditions.

The importance of maintaining a constant internal environment

The cells that form the organs and tissues of animals are immersed in a liquid medium, a fluid compartment that Claude Bernard, father of modern physiology, called ‘internal environment’. The internal environment refers, mainly, to extracellular fluid (ECF), a section that separates blood from cells, which is, in turn, composed by interstitial fluid, plasma liquid and lymph, fluids that are crucial in the performance of physiological functions.

By researching mammals, Bernard discovered that this internal environment stayed considerably steady, even when there were fluctuations in external parameters; the variation of several environmental factors, such as temperature or environmental pressure, didn’t cause an imbalance in the composition and properties of the internal environment, which remained stable.

The discovery of the constance of the internal environment was extremely significant, as it allowed researchers to reach the conclusion that animals that were able to regulate their internal environment were also able to exploit a wider variety of potential habitats. This revelation allowed Bernard to formulate one of his most famous statements: “the constance of the internal environment is the condition for a free and independent life”, meaning that those beings that are able to maintain the constance of the internal environment can be considered an organism that is independent of the environment. In any case, and in order to make this happen, a mechanism known as homeostasis is involved.

Animals that maintain their internal environment steady are able to exploit a wider variety of habitats. By Michael Hoyt
Animals that maintain their internal environment steady are able to exploit a wider variety of habitats. By Michael Hoyt

What is homeostasis?

The term ‘homeostasis’ was coined by American physiologist Walter Cannon, and is linked to Bernard’s notion of the physiological stability of the internal environment. In 1932, Cannon defined homeostasis as the series of physiological processes that are involved in the regulation and maintenance of the state of an organism in the face of any disturbance. It’s important to note that the main destabilizing factors of the internal environment are environmental parameters and the cellular metabolism itself.

The homeostatic processes involve a series of internal sensors (sensory receptors) that can detect any kind of deviation from an optimal physiological state, and, at the same time, to initiate the appropriate actions to correct these alterations. This optimal state can be maintained

This optimal state can be maintained by set point, that is, by an appropriate reference value for each species: when a disturbance (vibrations, radiation…) is perceived by the sensory receptors, the organism checks that reference value and sets the appropriate homeostatic mechanisms, which act in consequence to maintain that value. Homeostasis includes both physiological and ethological mechanisms: sweating, panting (physiological thermoregulation), occultation, fur (ethological responses to cold), etc. In short, homeostatic mechanisms are essential for animals, as they regulate and maintain the organism in optimum conditions, even when they face adversity. For example, it has been proven that, in some rodents, the blood sugar levels remain constant, even when they have no access to food.

The methods of homeostatic regulation

Two different homeostatic mechanisms are in charge of maintaining the stability of the internal environment.

Reactive homeostasis

Reactive homeostasis is a direct response to the changes that take place in the internal environment (a variation in pH, for example); that is, it occurs when an internal parameter of the organism is subject to a variation that must be corrected. An example of reactive homeostasis is the moment when an animal drinks as a response to a dehydration caused by excessive panting or heavy sweating.

Babbling is a physiological mechanism. By Tom Hills
Babbling is a physiological mechanism. By Tom Hills

Predictive homeostasis

The internal oscillating mechanisms act as true chronometers, which can prepare a physiological response to external environmental changes in advance. This early preparation is known as ‘predictive homeostasis’, a term proposed by Martin Moore-Ede.

Predictive homeostasis is a response to changes in the external environment. It’s anticipatory, meaning that it allows to predict the appearance of an environmental stimulus, and to anticipate a proper response to any disturbance that will divert the reference value or set point. This model of homeostasis also affects the circadian system, which, aware of the disturbance, allows the deviation of the reference value, so the organism must regulate from this new, modified set point (the adaptive response acts in reference to the new set point).

Some types of macaws provide an interesting example of predictive homeostasis: this group of birds often consumes a clay mineral called ‘kaolin’, which acts as a natural drug that prevents potential intoxications by the ingestion of seeds. Another example would be the reduction of food intake by animals that are dehydrated, in order to avoid the losing water through excretion.

The red-and-green macaw (Ara chloropterus) is able to anticipate the changes of the external environment. By Zachary Spears
The red-and-green macaw (Ara chloropterus) is able to anticipate the changes of the external environment. By Zachary Spears

Types of organisms and their regulatory mechanisms

There are different types of organisms, depending on the regulatory mechanisms they use. In general, we can say that, as we ascend in the evolutionary scale, the ability to maintain the stability of the internal environment will be more and more effective, making the process of homeostasis increasingly sophisticated.

Conformer organisms

Conformer species are influenced by external factors, so the organism gradually adapts its internal parameters to the environmental parameters, thanks to the flexibility of its enzymes. Conformers have an advantage: they don’t have to invest so much energy in keeping internal characteristics stable. However, the possibilities of free life are limited, as internal cells are subject to the alterations of external conditions.

Salmons are conformers, as their body temperature adapts to the waters they inhabit. By Rocksweeper | Shutterstock.com
Salmons are conformers, as their body temperature adapts to the waters they inhabit. By Rocksweeper | Shutterstock.com

These organisms can only perform their functions satisfactorily in a narrow margin of parameters, while, outside that range, they simply try to survive. In general, conformers tolerate wide variations in the parameters of their internal environment.

Regulator organisms

Regulator organisms maintain the conditions of their internal environment stable, within narrow limits, in the face of the variation of the conditions of the external environment. Unlike conformers, the cells of regulators work independently from the external environment, tolerating extensive changes in its characteristics. The mechanisms that make this possible consume a lot of energy. Mammals, for example, are regulatory organisms.

Translated by Carlos Heras

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