Flowers bloom, birds fly, and our hearts beat. All of these processes only occur when the various organs or organelles in a lifeform work together. And every process can be explained, to some point, with a series of molecular reactions. From microscopic interactions like oxygen exchange to visible motions such as walking, physiology covers it all

At its most basic, physiology is the study of the functions and mechanisms of organisms. Whereas anatomy looks at the parts of something, physiology focuses on how those parts work. Certain metabolic conditions – known as the physiological state – are necessary for ideal homeostasis and growth. Any metabolic deviation from the physiological state is called a pathological state. An infection is an example of a pathological state. The study of the pathological state is known as pathophysiology.

micrograph of stained neuron
Micrograph of a stained neuron with visible axon and axon terminals

Animal and human physiology

Many of the metabolic processes in an animal are controlled by chemical messengers called hormones. Examples of hormones include insulin, which regulates the absorption of glucose, and adrenaline (epinephrine), which stimulates the fight-or-flight response during stressful situations. 

Hormones are sometimes referred to by different names depending on where in the body they are found. For instance, neurotransmitters are hormones that function in the nervous system. Some neurotransmitters that may be familiar to you are adrenaline and serotonin. However, both of those examples are also used as hormones in other organ systems! 

Orexin (hypocretin) is a small neurotransmitter (neuropeptide) that regulates wakefulness and appetite. An autoimmune reaction that damages the cells that produce the neuropeptide causes a deficiency in orexin that leads to narcolepsy type 1 (NT1). Narcolepsy type 1 is a chronic neurological disorder featuring excessive daytime sleepiness, frequently interrupted sleep, sleep paralysis, hallucinations upon falling asleep and/or waking up, and brief episodes of loss of muscle tone (cataplexy). Among the other symptoms of NT1 are abnormal REM sleep patterns, not restful sleep, insomnia, trouble staying asleep, depression, and decreased appetite. This condition is a good demonstration of the wide-reaching effects that a lack of a single neurotransmitter can have. 

As their name may suggest, neurotoxins are molecules that are destructive to the nervous system. Often, neurotoxins inhibit the release of a neurotransmitter, which in turn prevents essential nerve signals from being sent. Yet other neurotoxins work by destroying nerve cells themselves, or by over- or understimulating ion channels which are necessary for the transmission of action potentials (known as nerve impulses when in between cells of the nervous system) and other important signals. 

Red tides are a type of harmful algal bloom (HAB) that occur in marine waters. Most of the algae species responsible for red tides produce some type of toxin; the types of neurotoxins produced by the red tide-causing dinoflagellate Karenia brevis are called brevetoxins. Normal neurological processes are disrupted when brevetoxins bind to voltage-gated sodium channels. Brevetoxin poisoning (brevetoxicosis) is fatal in marine animals, especially birds and fish. In humans, brevetoxicosis is non-fatal and is called neurotoxic shellfish poisoning, as it is contracted by eating shellfish affected by brevetoxicosis. 

body of water with red tide (dinoflagellate algae bloom)
red tide off the coast of La Jolla, California

When surrounding conditions become abnormal, physiological processes shift to cope as much as possible. One example of a set of innate changes that occur in the body is the mammalian dive reflex. Although apparently named for mammals, the diving reflex occurs in all vertebrates when their nostrils are submerged. The three main changes that happen as a part of the dive reflex are the cessation of breathing (apnea), a slowed heart rate (bradycardia), and decreased blood flow to the extremities (peripheral vasoconstriction). The purpose of the mammalian diving reflex is to ration the limited available oxygen most efficiently. 

Although all vertebrates have the diving reflex, it is most pronounced in marine mammals. While humans use the mammalian dive reflex as an occasional survival mechanism, marine mammals must use it constantly. One of the most adept divers, the Weddell seal, achieves its great feats thanks to the help of the diving reflex. Weddell seals have been recorded taking dives as long as 80 minutes. Aquatic animals including seals carry more oxygen in their blood, and they also have larger relative blood volumes than humans do. These adaptations allow them to use the mammalian dive reflex for much longer periods of time than a human can! 

Physiology in non-animal life

In different organisms, physiology is prioritized differently and completely disparate physiological events exist! Plants, for example, have evolved to be able to withstand a number of seasons, temperatures, rain levels, and so on. Unlike animals, plants can’t move after being rooted in place. However, though the purpose of plant physiology isn’t so similar to that of humans, the ways in which metabolism is achieved are a bit more alike. Both animals and plants have to absorb water and regulate chemical reactions that are constantly happening.

Trees drop their leaves in cold or dry weather under the command of a number of phytohormones (plant hormones). One phytohormone called auxin is particularly influential in plant growth, and it is thought that smaller amounts of auxin are sent to leaves in times of low nutrient availability. Without adequate levels of auxin, leaves cannot survive and eventually die and fall off the tree – a process known as abscission. Abscission serves to conserve water and nutrients, which there are smaller amounts of during the winter.  

But before a tree can decrease production of auxin, it must first detect changes in its surrounding environment. Most trees (and other plants that undergo abscission) use a combination of senses that detect temperature, light, and availability of nutrients and water. In plants, light is detected by specialized photoreceptor proteins. There are a variety of types of photoreceptors, which each detect a different range of wavelengths of light. 

many bare trees amidst a sea of orange leaves on the ground against a blue sky with clouds
deciduous trees that are low in auxin, as seen in the autumn in Michigan

Some life forms are dependent on photoreceptors for much more than just the detection of light. Prokaryotes such as archaea, bacteria, and fungi that detect blue light have photoreceptors that stimulate the repair of photodamaged DNA or the production of protective molecules. Although photoreceptors are especially important to phototrophic species (one that synthesizes its food from light), they also serve some purpose in chemotrophic species (those which synthesize energy from electron donors). 

Although often thought of as solitary due to their unicellular nature, bacteria and other prokaryotes demonstrate a number of “social” cooperative behaviors. Biofilms are conglomerates of microorganisms stuck together by extracellular polymeric substances (ESPs) – sticky products that are only made under certain environmental conditions. Dental plaque is an example of a biofilm. The production of ESPs is triggered by the uptake of autoinducers (signal molecules akin to hormones) when they are transferred between cells in a population, such as in a bacterial colony. This process of signal transduction in bacteria is known as quorum sensing, and is a physiological event mostly unique to bacteria.

The first commercially synthesized antibiotic, penicillin, was derived from a fungal compound. Penicillin is in the beta-lactam class of antibiotics – which all bind to a protein needed for the formation of cross-links in bacterial cell walls. By inhibiting cross-link formation, β-lactams inhibit the synthesis of strong cell walls. Without strong cell walls, bacteria have nothing to protect them from the environment, so bacterial cells exposed to β-lactams die (rather, they are supposed to, but some bacteria are resistant to many antibiotics including the β-lactams). 

There are thousands of physiological processes – we have explored just a few of them. What metabolic event interests you? 


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