Physiology is the study of normal function within living creatures. It is a sub-section of biology, covering a range of topics that include organs, anatomy, cells, biological compounds, and how they all interact to make life possible.
From ancient theories to molecular laboratory techniques, physiological research has shaped our understanding of the components of our body, how they communicate, and how they keep us alive.
Merrian-Webster defines physiology as:
“[A] branch of biology that deals with the functions and activities of life or of living matter (such as organs, tissues, or cells) and of the physical and chemical phenomena involved.”
The study of physiology is, in a sense, the study of life. It asks questions about the internal workings of organisms and how they interact with the world around them.
Physiology tests how organs and systems within the body work, how they communicate, and how they combine their efforts to make conditions favorable for survival.
Human physiology, specifically, is often separated into subcategories; these topics cover a vast amount of information.
Researchers in the field can focus on anything from microscopic organelles in cell physiology up to more wide-ranging topics, such as ecophysiology, which looks at whole organisms and how they adapt to environments.
The most relevant arm of physiological research to Medical News Today is applied human physiology; this field investigates biological systems at the level of the cell, organ, system, anatomy, organism, and everywhere in between.
In this article, we will visit some of the subsections of physiology, developing a brief overview of this huge subject. Firstly, we will run through a short history of physiology.
The study of physiology traces its roots back to ancient India and Egypt.
As a medical discipline, it goes back at least as far as the time of Hippocrates, the famous “father of medicine” – around 420 BC.
Hippocrates coined the theory of the four humors, stating that the body contains four distinct bodily fluids: black bile, phlegm, blood, and yellow bile. Any disturbance in their ratios, as the theory goes, causes ill health.
Claudius Galenus (c.130-200 AD), also known as Galen, modified Hippocrates’ theory and was the first to use experimentation to derive information about the systems of the body. He is widely referred to as the founder of experimental physiology.
It was Jean Fernel (1497-1558), a French physician, who first introduced the term “physiology,” from Ancient Greek, meaning “study of nature, origins.”
Fernel was also the first to describe the spinal canal (the space in the spine where the spinal cord passes through). He has a crater on the moon named after him for his efforts – it is called Fernelius.
Another leap forward in physiological knowledge came with the publication of William Harvey’s book titled An Anatomical Dissertation Upon the Movement of the Heart and Blood in Animals in 1628.
Harvey was the first to describe systemic circulation and blood’s journey through the brain and body, propelled by the heart.
Perhaps surprisingly, much medical practice was based on the four humors until well into the 1800s (bloodletting, for instance). In 1838, a shift in thought occurred when the cell theory of Matthias Schleiden and Theodor Schwann arrived on the scene, theorizing that the body was made up of tiny individual cells.
From here on in, the field of physiology opened up, and progress was made quickly:
- Joseph Lister, 1858 – initially studied coagulation and inflammation following injury, he went on to discover and utilize lifesaving antiseptics.
- Ivan Pavlov, 1891 – conditioned physiological responses in dogs.
- August Krogh, 1910 – won the Nobel Prize for discovering how blood flow is regulated in capillaries.
- Andrew Huxley and Alan Hodgkin, 1952 – discovered the ionic mechanism by which nerve impulses are transmitted.
- Andrew Huxley and Hugh Huxley, 1954 – made advances in the study of muscles with the discovery of sliding filaments in skeletal muscle.
The major systems covered in the study of human physiology are as follows:
- Circulatory system – including the heart, the blood vessels, properties of the blood, and how circulation works in sickness and health.
- Digestive/excretory system – charting the movement of solids from the mouth to the anus; this includes study of the spleen, liver, and pancreas, the conversion of food into fuel and its final exit from the body.
- Endocrine system – the study of endocrine hormones that carry signals throughout the organism, helping it to respond in concert. The principal endocrine glands – the pituitary, thyroid, adrenals, pancreas, parathyroids, and gonads – are a major focus, but nearly all organs release endocrine hormones.
- Immune system – the body’s natural defense system is comprised of white blood cells, the thymus, and lymph systems. A complex array of receptors and molecules combine to protect the host from attacks by pathogens. Molecules such as antibodies and cytokines feature heavily.
- Integumentary system – the skin, hair, nails, sweat glands, and sebaceous glands (secreting an oily or waxy substance).
- Musculoskeletal system – the skeleton and muscles, tendons, ligaments, and cartilage. Bone marrow – where red blood cells are made – and how bones store calcium and phosphate are included.
- Nervous system – the central nervous system (brain and spinal cord) and the peripheral nervous system. Study of the nervous system includes research into the senses, memory, emotion, movement, and thought.
- Renal/urinary system – including the kidneys, ureters, bladder, and urethra, this system removes water from the blood, produces urine, and carries away waste.
- Reproductive system – consisting of the gonads and the sex organs. Study of this system also includes investigating the way a fetus is created and nurtured for 9 months.
- Respiratory system – consisting of the nose, nasopharynx, trachea, and lungs. This system brings in oxygen and expels carbon dioxide and water.
There are a great number of disciplines that use the word physiology in their title. Below are some examples:
- Cell physiology – studying the way cells work and interact; cell physiology mostly concentrates on membrane transport and neuron transmission.
- Systems physiology – this focuses on the computational and mathematical modeling of complex biological systems. It tries to describe the way individual cells or components of a system converge to respond as a whole. They often investigate metabolic networks and cell signaling.
- Evolutionary physiology – studying the way systems, or parts of systems, have adapted and changed over multiple generations. Research topics cover a lot of ground including the role of behavior in evolution, sexual selection, and physiological changes in relation to geographic variation.
- Defense physiology – changes that occur as a reaction to a potential threat, such as preparation for the fight-or-flight response.
- Exercise physiology – as the name suggests, this is the study of the physiology of physical exercise. This includes research into bioenergetics, biochemistry, cardiopulmonary function, biomechanics, hematology, skeletal muscle physiology, neuroendocrine function, and nervous system function.
The topics mentioned above are just a small selection of the available physiologies. The field of physiology is as essential as it is vast.