What is physiology? Physiology is often described as the ways living things do work. In turn, "work" involves any expenditure of energy which puts mass into motion, does mechanical work or, more broadly, any activity which imposes orderliness on otherwise disorderly matter. In this sense, physiology can include any biological manipulation of flows of energy, matter and information. For example, the epithelial cells of Malphighian tubules use energy stored in ATP to transport solutes against their concentration gradients. Similarly, the transmission of a bit of information in an action potential depends upon a biologically-controlled sequence of changes in membrane permeabilities to sodium and potassium ions.

"Reductionist" vs "Integrationist" Physiology

Physiology is a materialistic science. That is, physiology looks to fundamental laws of chemistry and physics, and no further, for its explanatory tools. One interesting feature of modern physiology is a pervasive reductionism. Thus, the trend in modern physiological research has been to look to ever finer scales of resolution, starting at organisms, and progressing inward to organs, tissues, and cells, ending ultimately at the molecular level. This type of physiology works to explain how systems and components work within a particular level of organization work, seeking to explain how individual nerve cells work, or how gastric motility functions. However, physiology is also concerned with how the different levels of organization in an organism interact in a coordinated way. For example, the various tissues in an organ must act in a coordinated way for the organ to function properly. The proper functioning of the intestine, for example, requires the coordinated action of muscle tissues, nerve cells, secretory cells and transport epithelia. This type of integration is required at each level of organization. Just as tissues in an organ must work together properly, so too must the workings of the various organ systems be coordinated for the organism to function properly. This type of physiology is materialist - that is it seeks explanation in well-known principles of physics, thermodynamics and chemistry - but it is not reductionist, it is integrationist.

"External" vs "Internal" Physiology

Physiology today is concerned largely with energy that flows through the "ATP economy", that is transactions of chemical energy that involve ATP as a sort of energy currency. Thus, metabolic foodstuffs, like glucose, are converted by catabolism into waste products, like CO2 and H2O, and energy, some of it dissipated as heat and some fraction stored in ATP. The ATP then goes on to fuel the various kinds of physiological work. Despite the prevalence of the ATP energy economy, there is no reason to suppose that other energy sources could not be tapped to do physiological work. For example, the heating of a honeybee's flight muscles on a cool morning involve an expenditure of ATP for heat production, derived from shivering of the flight muscles. If a bee could sun bask while warming, it could absorb some of the stream of solar energy intercepting the Earth, and use this energy to heat its flight muscles. Similarly, parachute spiders use kinetic energy in wind to do the physical work of locomotion, energy that would otherwise involve an ATP expenditure in muscles. These types of interactions with external energy sources are as much physiology as any energy transaction carried out by a muscle, transport epithelium or nerve cell. The only difference is that these occur outside the outer boundaries of the animal's integuments. Thus, the physiological work done by animals involves really two physiologies: an internal physiology, with work done by cells, tissues and organs, fueled mostly by the ATP energy economy, and an "external physiology", with work done by energy sources external to the animal, such as wind, solar radiation, rainfall, and so forth.

Animal-built Structures as Organs of External Physiology

For external physiology to work, there must be a structure that captures energy from an external source and channels its flow so that it does physiologically useful work. Many animals, insects included, construct edifices which appear to do just this. For example, the web nests of tent caterpillars and fall web worms help to retain solar energy within their structures. This provides a locally-warmed microclimate for the larvae, keeping them warm on cool days. The heating of the larval bodies is physiological work which would normally require an expenditure of ATP energy for the production of heat. The web can do this because it admits light to the web interior, but excludes wind which would otherwise waft absorbed solar heat away by convection. Many other examples of such "external organs" of physiology can be found among insects, including: tuned singing burrows of mole crickets, web "aqualungs" used by aquatic beetles and spiders as accessory plastron gills, and so forth.

External Physiology and Emergent Homeostasis

Perhaps the most dramatic examples of animal-built structures serving as accessory organs of physiology may be found among the social insects. Among these insects, the phenomenon of "social homeostasis" is fairly common, particularly among the more advanced bees and ants, and among the advanced termites. In bee colonies, for example, the temperature of the hive is a regulated property, just as the body temperature of a mammal would be regulated. The regulation comes about because of social interactions among the worker bees, with some serving as "heater bees" and others serving as "insulator bees". Less well-known is the extent to which social homeostasis of the hive atmosphere depends upon the architecture of the nest the insects build. Among honeybees, for example, regulation of hive carbon dioxide and oxygen concentrations depends crucially on a particular orientation of openings to the hive: the hive can breathe only if there is a single hive opening located below the combs where fanning workers can station themselves for ventilation of the nest. Perhaps the most spectacular example of such emergent physiology can be found among the fungus-growing macrotermitine termites of southern Africa. These termites construct massive mound nests that can extend several meters high. These mounds are devices for capturing wind energy to power ventilation of the hive. Remarkably, the capture of wind energy appears to be finely tuned to the ventilatory requirements of the hive. The "tuning" is accomplished through building the mound to a height where wind energy in the surface boundary layers are sufficient to ventilate the nest, but not excessive. The result of this remarkable interaction between mound structure and capture of wind energy is social homeostasis of the nest atmosphere. Despite substantial variations of metabolic demand, nest oxygen partial pressure varies by only a few hundred pascals.

Copyrights: The copyrights of this work belong to the author, J. Scott Turner.

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