Physiology · शरीरविद्या
What it actually does, and what it does not
This section exists to stop the site from overclaiming. Every topic below states the mechanism where one is known, names the strength of the evidence honestly, and says plainly where yoga is a support and where it is not a substitute for medicine.
The Nervous System
The one system every practice on this site is actually talking to
Almost every benefit claimed anywhere on this site — calmer, clearer, less anxious, better sleep, lower blood pressure — routes through one mechanism: the balance between the sympathetic nervous system (fight-or-flight) and the parasympathetic (rest-and-digest), mediated largely by the vagus nerve.
Modern life keeps the sympathetic branch mildly, chronically active — not enough to feel like an emergency, but enough to never fully rest. Yoga's specific contribution is that it gives you several direct, voluntary levers on a system that is otherwise entirely automatic. That is the whole case for the practice, stated plainly.
Strength of evidence
Randomised trials support meditation and slow-breathing practices for anxiety and stress with moderate-to-strong effect sizes; the mechanism (vagal tone, heart-rate variability) is measurable, replicated and not seriously disputed. What remains genuinely uncertain is the size of the effect for any one person on any one day — which is exactly why it is practised daily rather than once.
How it works
The vagus nerve is the main handle
The vagus carries about 80% of its fibres from body to brain, not the other way round — meaning what happens in the chest, throat and gut genuinely informs mood and mental state, not just the reverse. Slow exhalation, humming, chanting and gargling all stimulate it directly.
Heart rate variability is the scoreboard
A healthy nervous system does not keep a metronome-steady heartbeat — it speeds up slightly on the inhale and slows on the exhale. That variability, not resting heart rate, is what researchers use to measure how well the two branches are balanced, and slow breathing at roughly six breaths a minute raises it more reliably than almost any other single intervention.
The gate control mechanism
Touch — a mudra held, a hand resting on the belly — travels on fast nerve fibres that reach the spinal cord before slower pain and discomfort signals do, and can partly close the gate on them there. This is a real, well-established mechanism, not a metaphor.
Practices for this
Humming Bee Breath
भ्रामरी
Fastest measured calming effect of any practice here
Alternate Nostril Breathing
नाडी शोधन
Best-evidenced effect on blood pressure and sleep
Gyan Mudra
ज्ञान मुद्रा
A single sustained sensory anchor for attention
Corpse Pose
शवासन
Where the nervous system absorbs everything practised before it
Heart & Circulation
Blood pressure is where yoga has its best evidence
Of every health claim made for yoga, the one with the strongest research support is its effect on blood pressure. Multiple randomised trials and meta-analyses show meaningful reductions in both systolic and diastolic pressure from regular practice — comparable, in some studies, to a modest dose of medication.
The heart also benefits indirectly: better sleep, lower stress hormones and improved circulation to the extremities all reduce the background load the cardiovascular system carries every day, independent of any single dramatic effect.
Strength of evidence
Strong: multiple meta-analyses of randomised trials show yoga reduces both systolic and diastolic blood pressure in people with hypertension. Effects on hard endpoints like heart attack and stroke are less directly studied and should not be overstated — yoga is a support alongside medical care, never a replacement for prescribed treatment.
How it works
The baroreflex
Stretch receptors in the aorta and carotid arteries continuously report blood pressure to the brainstem, which adjusts heart rate in response. Breathing at resonance frequency — close to six breaths a minute — synchronises breath with this reflex loop and appears to retrain its sensitivity over weeks of practice.
Endothelial function
The lining of the blood vessels releases nitric oxide in response to relaxation and to nasal breathing in particular, which dilates vessels and reduces resistance. This is a slower-acting mechanism than the baroreflex, which is part of why blood pressure benefits accumulate over months rather than appearing after one session.
Practices for this
Alternate Nostril Breathing
नाडी शोधन
The single most consistently studied practice for blood pressure
Bridge Pose
सेतु बन्धासन
A gentle inversion that improves venous return without strain
Apana Vayu Mudra
अपान वायु मुद्रा
The tradition's own first-aid gesture for the heart
Corpse Pose
शवासन
Measurably lowers heart rate and cortisol within minutes
Digestion & Metabolism
The gut has its own nervous system, and yoga speaks to it directly
The gut contains its own semi-independent nervous system — sometimes called the second brain — with more neurons than the spinal cord. It communicates constantly with the brain along the gut-brain axis, largely via the vagus nerve, which is why stress produces real digestive symptoms and not merely imagined ones.
Twists, forward folds and specific mudras act mechanically, massaging the abdominal organs and improving blood flow to them; the breath acts on the same organs through the autonomic nervous system that controls peristalsis and digestive secretions.
Strength of evidence
Moderate: good evidence for yoga easing symptoms of irritable bowel syndrome and functional dyspepsia, largely explained by the stress-digestion link above. Evidence for yoga altering nutrient absorption or curing structural gut disease does not exist and should be treated with suspicion wherever it is claimed.
How it works
Rest-and-digest is not a figure of speech
Digestion is a parasympathetic-dominant process — saliva, stomach acid and peristalsis all slow or stop under sympathetic activation, since the body redirects blood toward muscle for a fight-or-flight response it does not need at the dinner table. Practices that raise vagal tone before and after eating measurably improve digestive comfort.
Mechanical massage of the organs
Twisting postures compress one side of the abdomen and stretch the other, which several small studies suggest transiently increases blood flow to the liver, spleen and kidneys on release — comparable to the effect of manual abdominal massage, though the evidence base here is thinner than for the nervous-system mechanisms above.
Practices for this
Thunderbolt Pose
वज्रासन
The only asana recommended immediately after eating
Half Spinal Twist
अर्ध मत्स्येन्द्रासन
The classical twist for the liver, kidneys and pancreas
Apana Mudra
अपान मुद्रा
The mudra of elimination and downward-moving energy
Vayu Mudra
वायु मुद्रा
The first-response mudra for gas and bloating
Immunity & the Endocrine System
Chronic stress is measurably immunosuppressive — which is where yoga's claims on immunity actually come from
Cortisol, the primary stress hormone, suppresses immune function when chronically elevated — this is well established, not speculative. Practices that reliably lower cortisol therefore have a genuine, if indirect, effect on immune resilience, which is the honest version of the claim that "yoga boosts immunity."
The endocrine glands most often named in the tradition — thyroid, adrenal, pancreas — do respond to specific postures through mechanical stimulation (inversions and the chin lock affecting the thyroid) and through the same stress-hormone pathway that affects the immune system.
Strength of evidence
Moderate for stress-hormone reduction, which is well established; weaker and more preliminary for any direct effect on immune cell counts or thyroid hormone levels, where studies are smaller and less consistent. Treat specific-gland claims as plausible hypotheses worth watching, not settled fact.
How it works
The HPA axis
The hypothalamus-pituitary-adrenal axis governs cortisol release, and is itself regulated partly by vagal input from the gut and lungs. This is the direct chain by which slow breathing, several hours old, ends up lowering a stress hormone measured in blood the next morning.
Inversions and the thyroid
The chin lock created in shoulder stand and fish pose compresses and then releases blood flow through the throat, which some small studies associate with short-term changes in thyroid hormone levels. The effect size is modest and the studies are small — worth knowing, not worth overselling.
Practices for this
Shoulder Stand
सर्वाङ्गासन
The strongest mechanical stimulation of the thyroid in the collection
Prana Mudra
प्राण मुद्रा
Traditionally associated with immune resilience and vitality
Bellows Breath
भस्त्रिका
Raises metabolic rate and internal heat
Shankh Mudra
शङ्ख मुद्रा
Directs vibration to the throat, over the thyroid
Bones, Joints & Muscles
Where yoga does the most obvious, least disputed good
This is the least controversial section on this page. Regular asana practice reliably improves flexibility, balance and functional strength, and multiple trials show it reduces chronic low back pain about as effectively as standard physiotherapy.
The mechanism needs no special pleading: joints that move through their range regularly stay more mobile, muscles that are loaded regularly stay stronger, and balance work genuinely reduces fall risk in older adults — this is orthopaedic medicine, not mysticism.
Strength of evidence
Strong for chronic low back pain, flexibility and balance — among the best-evidenced claims in the field, with effect sizes comparable to conventional physiotherapy in several trials. Yoga is not a substitute for treating an acute injury or a structural condition diagnosed by a doctor.
How it works
Proprioception
Balance poses train the sensory feedback loop between muscle spindles, joint receptors and the brain — the sense of where a limb is without looking at it. This sense degrades with age and inactivity, and it is directly trainable, which is most of why balance poses feature so heavily here.
Load-bearing without impact
Standing poses and arm balances place genuine mechanical load on bone, which is the stimulus bone needs to maintain density, without the joint impact of running or jumping — a real advantage for older practitioners and anyone with joint sensitivity.
Practices for this
Tree Pose
वृक्षासन
The clearest balance and proprioception trainer here
Mountain Pose
ताडासन
Corrects the postural collapse behind most chronic back pain
Triangle Pose
त्रिकोणासन
Loads and stretches the whole side chain in one shape
Prithvi Mudra
पृथ्वी मुद्रा
The tradition's practice for bone and tissue strength
Mind, Mood & Obstacles
Patanjali named the obstacles; here is what addresses each one
Yoga Sutra 1.30 names nine antarāya — obstacles to a settled mind: illness, dullness, doubt, carelessness, laziness, craving, false perception, failure to progress, and instability. Naming them precisely is itself a kind of treatment, since a named problem is easier to address than a vague sense of being unwell.
The evidence base for yoga and meditation in mild-to-moderate anxiety and depression is genuinely good — comparable in several trials to structured exercise programmes, and useful as an adjunct to, though not a replacement for, therapy and medication where those are needed.
Strength of evidence
Good for mild-to-moderate anxiety and depressive symptoms, and for stress-related sleep disturbance. Yoga is not established as a treatment for severe depression, psychosis or acute suicidality — those need professional mental health care, and a teacher who suggests otherwise should not be trusted.
How it works
Interoception and emotional regulation
Emotions are experienced partly as bodily sensations, processed in the insula, which also receives the bulk of interoceptive signalling from breath and posture. Practices that improve the accuracy of interoception appear to improve emotional regulation as a direct consequence — you get better at naming a feeling when you get better at sensing its bodily signature.
GABA and the calming amino acid
A small but notable body of research using MR spectroscopy has found increased brain GABA levels after yoga practice compared with a walking control of similar duration and exertion — GABA being the primary inhibitory neurotransmitter, and the target of most anti-anxiety medication. The finding is real but from small studies, and should be read as promising rather than conclusive.
Practices for this
Humming Bee Breath
भ्रामरी
The fastest-acting practice for acute agitation
Child's Pose
बालासन
A safe place to stop, in any practice or any day
Abhaya Mudra
अभय मुद्रा
An expansive posture that directly counters the shape of fear
Ganesha Mudra
गणेश मुद्रा
Active practice for the physical constriction of grief and dread
A note on health
Yoga supports health, it does not replace medical care. If you are pregnant, recovering from surgery, or managing a heart, spinal, eye or blood-pressure condition, speak to your doctor and to a teacher before beginning — and never force a posture or hold the breath past comfort.