Physiology 101

It’s probably the most intimidating title in this introductory series. Ecology and evolution may be complex, but at least they’re easy to spell. Many people don’t even know what physiology is.

In fact, you do know what physiology is. After all, it’s been a vital part of your life since you were born. If your physiological processes stopped functioning, you’d die almost instantly.

When we want to know an organism’s name or identity, we turn to classification. Morphology tells us how it’s put together and what it looks like. Physiology tells us how it works. More precisely, physiology is the study of how living things function.

Merriam-Webster defines physiology as the study of “the functions and activities of life or of living matter . . . and of the physical and chemical phenomena involved.” Wikipedia defines it as “the science of the function of living systems.”

Of course, physiology isn’t just the study of something; it’s also the thing physiologists study. Physiology can also be defined as organic processes and phenomena. Thus, we could focus on the physiology of a lily or a house cat.

Physiological Systems

The human respiratory system (See a bigger picture)

We could almost say physiology is the study of how cells function, because cells are the basic units of life. However, some physiological processes are the result of groups of cells working together.

In fact, physiology focuses primarily on organs, which are made of tissues, which are in turn composed of clls. The heart, lung, liver, kidneys and brain are among the more important organs in humans.

Just as cells work together, so do organs often work together to perform a particular physiological function. Such associations are called systems.

For example, what parts of your body are involved in respiration? If you answered lungs, you’re partly right. However, the respiratory system also includes the nose, nasopharynx and trachea - in humans, at least. Fish have gills isntead of lungs, while plants have neither. Yet respiration is a vital physiological function in plants as well as animals.

Physiological Functions

It’s easy to predict the physiological processes that make living things function - just take a close look at cells, which can be thought of as miniature organisms. To keep it simple, let’s think of cells as miniature cars.

(Above) Photosynthesis is probably the most familiar physiological process in plants.
(Below) Respiration and circulation merge with the exchange of gases in animals’ lungs.

In order to function, a car needs gas and oil, two things it can’t find in your body. Car engines also need air - or, more precisely, oxygen - in order to function. (Internal combustion could almost be thought of as a crude physiological process.) Therefore, all those little cars (cells) that make up your body are going to continue to work only as long as you can somehow obtain fuel and oxygen, then deliver it to each hunger car...er, cell.

Of course, cells are far more complex than any car. Like cars, they need fuel (in the form of food, nutrients, sun?) and oxygen (animals) or carbon dioxide (plants).

But cells have little use for pizza, ice cream or the grass and bark that many animals eat. Your body has to break down the food you eat, both physically and chemically, unlocking the nutrients your cells need and eliminating wastes (essentially things your body doesn’t need). This involves more physiological processes, including digestion and waste disposal.

It might help us understand the myriad physiological processes that occur in diverse life forms if we organize them into categories. In fact, most physiological functions can be neatly classified into one of five categories, with a sixth for anything we missed.

Core Functions

Core physiological functions are those that are most urgently needed for survival. They include respiration, circulation and digestion. You could probably survive for days without eating, as your body would essentially begin digesting itself. But if your body couldn’t process fat reserves, you’d die much sooner. And if your heart or lungs stopped functioning, you’d probably expire within minutes.

(Above) Structure of ATP (adenosine triphosphate), an organic molecule (or nucleotide) dubbed the “molecular unit of currency” of intracellular energy transfer or the “energy currency of life”
(Below) Another structural scheme, identifying the carbon, hydrogen, nitrogen, oxygen, and phosphorus atoms that make up ATP

The most familiar core functions (respiration, digestion and circulation) are better known as metabolism - a set of chemical reactions that sustain life. More precisely, metabolism refers to the chemical processes in living cells that sustain an organism by acquiring energy and assimilating new material.

There are two basic types of metabolism - catabolism breaks down organic matter, while anabolism uses energy to construct components of cells such as proteins and nucleic acids.

Universal Functions

People have noses and lungs, while fish have gills and plants have none of the above. There are therefore enormous differences between their physiological processes. Yet there are also similarities.

All living things need energy and what we might call raw materials or natural resources (like nutrients) to survive and reproduce. Thus, respiration is a vital process for plants, animals and bacteria alike.

The raw materials living things eat, absorb or whatever have to be converted into organic substances (a catabolic process). That process is called digestion in animals. We don’t normally apply the term digestion to plants - except carnivorous plants, like the Venus flytrap. However, scientists have lots to say about cell digestion, and plants are composed of cells, just like animals.

The energy and raw materials living things obtain must be distributed to their constituent cells. That’s what circulation is all about, whether we focus on animal circulation or plant circulation.

Support Functions

Some support functions may be necessary for survival, but you aren’t going to die immediately if they stop functioning. Others may not even be critical for survival, but they help your body function properly.

Think about the tears that flush foreign objects out of your eyes, the production of saliva and ear wax, or the physiological processes that induce sweating and shivering.

Sensory Functions

Most of us are familiar with the five traditional senses - sight (ophthalmoception), hearing (audioception), taste (gustaoception), smell (olfacoception or olfacception) and touch (tactioception). But even if you don’t have that magic sixth sense, you do have other senses, including temperature perception (thermoception), kinesthetic sense (proprioception), pain (nociception), balance (equilibrioception) and acceleration (kinesthesioception).

Other animals have senses that seem magical to us, including echolocation in bats and cetaceans and heat detection abilities in pit vipers.

We don’t normally think of plants as having senses, yet they respond to changes in their environment as well as competitors and enemies. Some plants even respond to being touched - none more dramatically than the Venus flytrap.

Movement

We don’t normally think of movement as a physiological function, but the soreness you feel after strenuous exercise tells us otherwise. A beating heart is an example of an involuntary movement regulated by physiological processes. Plants also exhibit a variety of movements.

We’ll learn more about movement in the next article in this series.

Reproduction

There’s probably no greater physiological challenge than the creation of life...except perhaps growth, both of which involve a wide array of physiological functions.

Defense Mechanisms

If you explore ecophysiology, you’ll discover the amazing physiological weapons diverse life forms use against their competitors and enemies. From plant toxins to a jellyfish’s stinging cells, these weapons range from nasty (e.g. poison ivy) to deadly (e.g. a stingray’s sting).

Some physiological weapons are used for offense as well as defense. In fact, more people are probably killed by venomous snakes than all other animals combined (except for people killed by animal-borne diseases).

Misc. Physiological Functions

There are probably many kinds of physiological functions that don’t fit into any of the above categories. In fact, there are many physiological functions still awaiting discovery.

For lack of a better word, I classify some of my favorite physiological tricks in the category...

Special Effects

OK, that’s not a scientific term, but it works for me. How can one not think about special effects when watching fireflies light up the night sky? Their glow is an example of bioluminescence, a magical physiological trait shared by many creatures that live in the deep sea.

Another exampe of special effects is the ability to change colors, a talent shared by chameleons, octopuses and other species. Even people have some ability to change colors; their cheeks may become reddish when they’re angry or excited, and they often look pale when frightened.

And what about the fabulous colors that sweep across temperate forests as leaves begin to fall in the season appropriately named fall?

In fact, the ability to change colors is a form of camouflage - a defense mechanism - in many species.

The article you just read was a very brief, user friendly introduction to a vast and complex science. Not surprisingly, physiology is a topic many people steer clear of. More than complex, it can be a little yucky.

Yet physiology is biology’s great equalizer. To a physiologist, the drabbest moth may be just as beautiful, or at least fascinating, as a monarch butterfly.

Series: Life 101 (12 articles)