What is Yoga Physiology
Physiology is the area of biology that deals with the normal functions of living things, and the functions of their organs or systems. When we refer to the physiology of yoga, we are talking about how yoga practices affect the functioning of the human body.
It’s important to understand that when scientific research investigates whether yoga has a certain physiological effect, it doesn’t simply ask people whether they feel better - although it may do that as well. Modern science has found ways to measure physiological effects. Common scientific measurements include familiar tests like heart rate and blood pressure measurements, but researchers often collect meaningful details from a variety of scans and blood tests.
Blood tests for studies into stress might assess the levels of dozens of hormones and chemical markers in the blood. Some of the most tested markers are cortisol, catecholamines, glucose, HbA1c, triglycerides, cholesterol, prolactin, oxytocin, dehydroepiandrosterone sulfate, and interleukin - 6 and 8. Research related to inflammation will commonly monitor the inflammatory markers C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and plasma viscosity (PV) - but there are other possibilities as well.
Such scientific details of physiology are too enormous to cover fully in a blog (or even in an entire master's degree). This article will therefore address the practical and functional effects of yoga on our physiology and outline the pathways leading to these effects.
Yoga philosophy and physiology
Yoga philosophy offers lifestyle guidance, which naturally influences physical health and wellbeing. For example, practicing the Niyama (habits) leads to positive changes in our diet and exercise, our hygiene and sleep habits and more. Changes in these areas are well known to improve overall health and wellbeing.
The connection here is that our health and wellbeing depend on our physiology. They are, in essence, the same thing. Improved diet, exercise and sleep will improve the function of our digestive system, brain, muscles, skeletal system and more.
Yoga philosophy also encourages mental health strategies such as gratitude, kindness and generosity. Modern research has confirmed that these practices improve brain health and are associated with lower rates of brain dysfunctions such as dementia. The exact physiological pathway for these effects is still being researched, but it is known that positive mental health strategies do have a physical effect on the structure and function of the brain.
Both the physical practices of yoga and the philosophy of yoga contribute to yoga’s proven ability to interrupt the negative cycle of chronic stress and promote holistic pathways toward health and well-being.
Physical yoga practices and physiology
The physical practice of yoga asana was invented by monks as a way to improve the function of their organs and glands. This means that the original purpose of yoga asana was to influence the physiology of those who practiced it. Ancient practitioners believed asana improved the function of organs and glands by various means including applying pressure and the squeeze-and-release effect, and altering blood flow. Modern scientific testing is slowly uncovering the physiological pathways that produce the long-recognized beneficial effects of yoga.
Known benefits of yoga
Research in recent decades has focused on the physiological effects of asana, breathing exercises and meditation. Many of the studies are small, but over time the results of many small studies can be combined to produce stronger evidence.
Within the many studies conducted, yoga has been found to help or ease symptoms of:
- urological disorders
- pulmonary tuberculosis
- chronic back pain
- rheumatoid arthritis
- chronic obstructive pulmonary disease
- stress and psychological outcomes
- mood state
- symptoms of depression, anxiety and negative mood
- sleep quality
- physical fitness and quality of life in older adults
- These health outcomes are due to physiological effects of the yoga practices used in the studies.
The Physiology of Breathing Exercises
Although pranayama and breathing exercises are not exactly the same thing, breathing exercises are often used in pranayama practice and in many styles of yoga classes. A common feature in many breathing exercises is deep, slow diaphragmatic breathing. There is extensive evidence confirming the physiological benefits of this type of breathing. It appears that a slow exhalation phase of breathing is particularly relevant to achieving the beneficial results.
Physiological effects of deep slow breathing include:
- Reducing activity in the sympathetic ‘flight or fight’ system.
- Exposing more lung tissue to air, allowing the lungs to transfer more oxygen into the bloodstream.
- Sensitizing the baroreceptors: these are sensors in blood vessels near the heart that send information about blood volume and pressure to the brain. This change helps our cardiovascular system to adjust heart rate and blood pressure more smoothly and efficiently.
- Increasing the flow of blood returning to the heart and increasing the volume of blood pumped with each heartbeat. Between these effects and the increased oxygen present in the blood, this significantly reduces the heart’s workload.
- Clustering of heartbeats during inspiration (with less heartbeats during exhalation) and synchronization of the pulse harmonics of blood flow and heart rhythm. Although the mechanisms are complex, the result is positive - the heart and blood flow become more efficient and work together in the way they were designed to work best.
- Increasing brain activity related to improved emotional control and psychological well-being, found in brain scanning research.
Some of the effects of breathing exercises are direct responses to the physical changes in lung expansion and diaphragm movement. The diaphragm is connected to and supports the heart, and the aorta and the inferior vena cava (large blood vessels leaving and entering the heart) pass through the diaphragm. Studies of diaphragmatic breathing have found improved return of blood to the heart, especially during slow respiration. This is attributed to movement of the diaphragm assisting the collapse of the inferior vena cava as blood is emptied from it into the heart.
Other changes brought about by breathing exercises are due to the influence of the breath on the parasympathetic nervous system. Measurements such as blood pressure, heart rate or heart rate variability indicate that slow and diaphragmatic breathing increase activity in the parasympathetic nervous system. Discussed in more detail below, this activation triggers ‘rest and digest’ mode in the body for a host of positive changes.
The physiological results are clear: breathing patterns that are slow, deep, and emphasize exhalations have a measurable calming effect on both body and mind. It should be noted, however, that breathing exercises must be practiced according to an individual’s ability and experience level. Hyperventilation (over-breathing) can trigger dizziness and even fainting as the body strives to return blood gases to a healthy balance. Alternatively slowing the breath too much, to the point of feeling oxygen-deprived can have its own negative effects. To benefit from the positive physiological effects of breathing exercises, you must practice exercises appropriate to you.
Effects of Yoga on the Nervous System
Many of the positive effects of yoga arise from activation of particular areas of the nervous system. The nervous system is the body’s communication system, carrying messages that control all the processes inside our body as well as the movements we consciously choose to make.
The part of the nervous system that manages our internal organs and involuntary functions is the autonomic nervous system (ANS). The ANS is regulated by a part of the brain called the hypothalamus. It controls cardiac function, respiration, digestion and other reflexes including vomiting and sneezing.
The autonomic nervous system can be further divided into sympathetic and parasympathetic nervous systems. These two systems, when working well, balance each other with opposite actions.
Sympathetic nerve fibres use the neurotransmitter noradrenaline to increase blood flow in skeletal muscles and lungs, dilate lungs and blood vessels and raise the heart rate. This is known as the "fight or flight" response and is a natural function of the body intended to help keep us safe. The physiological results prepare us to respond to or escape a threat, whether that threat is a lion or a pressing deadline at work.
The parasympathetic system regulates “rest and digest” responses such as reducing heart rate and increasing salivation, lacrimation (tear production to hydrate the eyes) and digestion. It uses the neurotransmitter acetylcholine for much of this inhibitory activity. The vagus nerve, a huge nerve network between the brain and the organs and glands, makes up a large part of the parasympathetic system.
In a balanced life the sympathetic nervous system responds to stressors in a healthy and productive way and is balanced by the parasympathetic nervous system responding to opportunities for calm. However, many people are chronically stressed or find it difficult to dampen the “fight or flight” mode even when a stressful event is over.
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Chronic stress and yoga
Chronic stress is a state of feeling pressured or overwhelmed for a long period of time. It has been linked to negative health effects including psychological and cognitive issues. There is growing evidence that these negative mental states, in turn, increase our risk for chronic health conditions such as high blood pressure, heart disease, insulin resistance and diabetes. It’s also thought that the negative emotional and mental states create a harmful stress cycle by becoming an added source of stress themselves.
Research published in Alternative Therapies in Health and Medicine (a peer-reviewed medical journal) reports that this chronic stress cycle results in increasing emotional, cognitive, structural, and physiologic impairment. Their conclusion, based on reviewing a range of research, was that yoga can break the destructive cycle by relieving stress, improving mood, and promoting positive change in function and physiology.
Many of these beneficial physiological effects of yoga result from reducing reactivity in the sympathetic nervous system, and stimulating parasympathetic activity. Damping down the “flight or fight” response and balancing it with an enhanced “rest and digest” parasympathetic response has positive effects on neuroendocrine status, metabolic function and related inflammatory responses, and can also affect our mood and energy state.
One of the mechanisms for this improvement is particularly clear in Hatha Yoga. Hatha Yoga alternates active poses with resting poses, which promotes an awareness of the difference between the contrasting states of excitement (stress) and relaxation. Feeling and understanding the difference between the two states and practicing the shift between them can increase your ability to shift from stressed to relaxed in your daily life as well.
Understanding the vagus nerve
Studies suggest that yoga’s influence on the nervous system is largely due to stimulation of the vagus nerve. The vagus nerve acts like an information web between the brain and the bodily systems. It’s a major component of the parasympathetic nervous system, and stimulation of the vagus nerve promotes the parasympathetic “rest and digest” functions.
The vagus nerve collects information from across its large network of receptors in the airways, gut, heart, blood vessels and more. Either the brain or the vagal network itself then responds to the collected information by causing responses intended to keep our systems functioning in a healthy, balanced way.
We are able to influence the activity of the vagus nerve by altering the information those receptors collect. This is called vagus nerve stimulation.
Vagus nerve stimulation is a recognized health intervention in western medicine. Artificial stimulation of the nerve using electrodes has been approved as a medical therapy for refractory epilepsy and for treatment resistant depression. Medical trials are ongoing for treatment of other conditions including heart failure, anxiety, chronic headaches and more.
Without resorting to using electrodes, we can also use activities and practices like yoga to alter the information received by the vagus nerve.
How does vagal nerve stimulation cause changes in physiology?
One of the most researched mechanisms of physiological change are the mechanoreceptors in the airways. In response to information about airway expansion from these receptors, vagus nerve activity adapts. This means that rapid or shallow breathing will cause our body to react as if preparing to deal with a threat. Slow deep breathing will cause our body to react as if we are safe and calm.
A second important pathway for vagal nerve activity involves mechanoreceptors in the heart and blood vessels. These baroreceptors collect information about blood volume and pressure. If blood pressure falls too low, the sympathetic nervous system will signal changes that cause the pressure to rise to healthier levels. If blood pressure rises above acceptable levels, the parasympathetic nervous system will trigger responses leading to lower blood pressure.
The vagus nerve and parasympathetic activity can be stimulated by the baroreceptors during yoga inversions and other poses that involve tipping the body towards an inverted state, such as Standing Forward Bend. As the inversion creates pressure at the baroreceptors, the vagus nerve responds by lowering the heart rate and reducing resistance in the blood vessels.
A third pathway for vagal stimulation relates to the location of part of the nerve. The vagus nerve has a right and left branch, and these pass along each side of the neck to connect the brain to the organs. Because it’s not part of the spinal cord, the vagus nerve isn’t encased by the bones of the spine. Its vulnerable position in the soft tissues of the neck means it might be stimulated by postures that compress the neck or throat tissues. It’s thought that postures such as Sarvangasana (shoulder stand) can compress and thereby stimulate the vagus nerve, leading to positive calming effects on the body.
Effects of Vagus Nerve Activation
The vagus nerve coordinates information from multiple systems, so we see changes in one system reflected in other systems.
When the vagus nerve activates the parasympathetic ‘rest and digest’ response, the body slows its production of stress hormones like cortisol, and increases production of rest-related chemicals like the parasympathietic nuerotransmitter acetylcholine. This has positive effects on our physical body, and also results in a mental perception that we feel less stressed.
As described above, vagus nerve stimulation leads to healthy balancing of our heart rate and blood pressure in response to information from receptors in our heart, lungs and blood vessels.
Studies have also found other measurable effects of vagal stimulation. Insulin sensitivity and glucose tolerance may improve. Levels of ‘bad’ cholesterol may fall, and the body may be less inclined to deposit fat around the internal organs. Known as visceral fat, these internal deposits are more damaging to our health than the superficial fat stored closer to our skin.
Parasympathetic activation also reduces the levels of inflammatory markers that can be found circulating in our bloodstream. Inflammation can be a normal response to injury or infection, and form part of the healing process. However, inflammation that becomes chronic and never-ending is counter-productive. It starts to harm our organs and systems instead of helping them heal. Diseases associated with chronic inflammation are many, including diabetes, malignancy, kidney and liver diseases and auto-immune disease.
Symptoms that can develop due to chronic inflammation are similar to those you might expect from the closely linked condition of chronic stress, and include:
- Body pain, arthralgia, myalgia
- Chronic fatigue and insomnia
- Depression, anxiety and mood disorders
- Gastrointestinal complications like constipation, diarrhea, and acid reflux
- Weight gain or weight loss
- Frequent infections
To improve health and avoid chronic diseases, it’s important to reduce the inflammatory load in our bodies. Yoga asana, meditation and breathing exercises have all been researched and found effective in alleviating stress-induced inflammation and its harmful effects on the body.
By reducing stress and inflammation, and improving a vast array of health markers, yoga activation of the vagus nerve can reduce our risk of chronic health conditions and diseases such as atherosclerosis, heart disease, high blood pressure and diabetes.
Can yoga activate “flight or fight” instead of “rest and digest”?
It’s important to note that not all yoga practices include relaxing, calming elements. Some pranayama exercises and yoga styles (e.g. Vinyasa Yoga) are designed to increase heat and energy levels, and will activate the sympathetic nervous system instead of the parasympathetic nervous system. This is safe and appropriate in the right circumstances.
Hatha Yoga is designed to alternate between activation of each response system, with the intent of improving the body’s ability to switch between modes in a smooth easy way.
Challenging poses and more active yoga styles can also raise heart-rates, quicken the breath and create feelings of stress. This is reasonable as part of a holistic approach that also includes more calming activities. Often, it’s challenging to learn a new pose, with excessive muscle activity and tension activating “flight or fight” responses in the body. As the body develops the required strength and balance, the pose becomes more comfortable. As the practitioner becomes able to hold the pose without undue strain, it may become a calming pose that activates the parasympathetic system.
Physiological Effects of Movement and Compression
Some of the body’s responses to yoga practice are the result of the physical actions rather than stimulation of the nervous system. This has been mentioned in the previous section on the effects of breathing exercises, with movement of the diaphragm influencing blood return to the heart.
Physical actions of yoga include:
- physical movement of body parts
- pressure placed on body parts by certain poses
- the ‘massage’ effect of pressure being applied and released as poses change
Applying pressure to a blood vessel will temporarily reduce blood flow through that vessel. Pressure applied around organs or glands may therefore have the effect of reducing blood flow in that area. Upon release of the pressure, blood will flow with renewed vigour to replenish the supply of oxygen and nutrients. This was the effect sought by the ancient monks who devised yoga poses to improve their health.
The lymphatic system is a network of tiny vessels throughout the body, along with a system of lymph nodes and other lymphoid tissue. The blood vessel system has the heart to keep fluid circulating, but the lymph system relies on movement, gravity, and a one-way valve system to move fluid from around the body to the lymph nodes.
The lymph system is collecting excess fluid from the tissues as well as cellular waste products and bacteria that entered through our skin. When the lymphatic system doesn’t drain fluids properly, the tissues become puffy and uncomfortable. The lymph nodes and other lymphoid tissues monitor the lymph flowing into them and produce cells and antibodies which protect our body from infection and disease.
The main roles of the lymphatic system include:
- managing the fluid levels in the body
- reacting to bacteria and cancer cells
- dealing with cell products that otherwise would result in disease or disorders
- absorbing some of the fats in our diet from the intestine
Physical movement and muscle contraction stimulate the flow of lymph along the lymph vessel pathway. Asanas provide a wide variety of muscle contractions to assist lymph flow. The movement of breathing exercises is particularly effective on the extensive lymph vessel system protecting the health of the lungs.
Yoga asanas also offer opportunities to assist lymph flow by altering the body position in relation to gravity. When any part of the body is inverted, whether that be a Legs Up the Wall or a Headstand, lymph fluid can more easily drain from areas that normally have to battle against gravity.
Fascia is a connective tissue that forms a web throughout the body. It forms sheaths around muscles and organs and connects them to each other. Within muscles, each bundle of muscle fibres is surrounded by fasica; within each bundle, individual muscle fibres are surrounded by fascia.
When a body moves, tissues and organs move and slide against one another. There is increasing evidence that this movement increases the health and hydration of the fascia. Fasica doesn’t have a blood supply - it relies on movement to deliver fluids. Lack of movement then becomes a debilitating cycle, as dehydrated and stiffened fascia prevents easy movement of the body.
It’s thought that poor fascial health, and particularly fascial dehydration, may be responsible for much of the creakiness and stiffness we attribute to old age.
Plentiful and varied movement is good for fascial health and hydration. Yoga asanas offer a tremendous variety of movement patterns and positions. This variety of movement prevents the cycle of poor movement leading to decreasing fascial health.
All the major joints of our body are surrounded by a membrane that produces a fluid the lubricates the surfaces of the bones or cartilage. This fluid, called synovial fluid, allows the bones to move against each other smoothly. It also carries oxygen and nutrients to the cartilage, which has no blood supply to keep it healthy.
The range of movements involved in yoga asana allow us to maintain the full range of motion that keeps joints healthy.
According to the Arthritis Foundation, exercise that involves joint movement has several effects including:
- Weightbearing through joints during exercise forces water molecules out of the cartilage. When the weight is lifted, the water molecules return, refreshed with oxygen and nutrients.
- Research shows that joint movement activates genes associated with rebuilding cartilage.
- Exercise triggers a waste removal process called autophagy, where damaged cells in the joint are broken down and removed.
In addition, appropriate exercise strengthens the muscles, ligaments and tendons that surround and support the joints. This protects joints, helps them stay healthier for longer and can help relieve symptoms of arthritis.
If the practice of yoga is to move you towards a state of contentment and eventual enlightenment, it must also move you towards good health. Since ancient times, yoga has included philosophy and physical practices intended to improve the function of the body and its systems, organs and glands. Modern research is slowly catching up with ancient knowledge and experience, giving scientific explanations to the effects monks discovered centuries ago.
Ultimately, the physiology of yoga is simple: yoga can transform your body and your mind.
Sources - books
Hatha Yoga for Teachers & Practitioners - https://www.amazon.co.uk/dp/9082705613
Sources - research reports
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6137615 - How Breath-Control Can Change Your Life: A Systematic Review on Psycho-Physiological Correlates of Slow Breathing
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5709795 - The physiological effects of slow breathing in the healthy human
https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD009506.pub4/full?highlightAbstract=yoga%7Ceffects%7Cof%7Ceffect - Yoga for secondary prevention of coronary heart disease
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3482773 - Physiological Effects of Yogic Practices and Transcendental Meditation in Health and Disease
https://pubmed.ncbi.nlm.nih.gov/17658122 - Chronic stress and insulin resistance-related indices of cardiovascular disease risk
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6189422 - Breath of Life: The Respiratory Vagal Stimulation Model of Contemplative Activity
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4017164 - Vagus Nerve Stimulation
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