This blog post of an exclusive excerpt from my new book, The Physiology of Yoga.
There is widespread belief that inverting the body in asanas such as Headstand (Sirsasana) has many potential physiological benefits including increasing blood flow to the brain, stimulating the pineal gland and lowering our blood pressure.
The brain has the very important ability to maintain relatively constant blood flow despite changes occurring elsewhere in the body. In healthy adults, large changes in blood pressure result in little or no change in cerebral blood flow (Paulson, Strandgaard, and Edvinsson 1990). This mechanism of autoregulation of cerebral blood flow is vital since the brain is very sensitive to too much or too little blood flow. Only in severe head injury or acute ischaemic stroke do we lose this autoregulation, leaving surviving brain tissue unprotected against the potentially harmful effect of blood pressure changes. So, it is reassuring to know that whether you regularly invert your body or not, your brain is receiving just the right amount of blood supply to meet its demands.
The pineal gland was once referred to as the third eye because of its location in the geometric center of our brain. The French philosopher, mathematician, and scientist Descartes regarded the pineal gland as the principal seat of the soul and the place in which all thoughts are formed. It is roughly the size of a soybean and is considered a somewhat mysterious organ because it was the last of the endocrine glands to have its function discovered. In addition to producing melatonin, the pineal gland also produces extremely tiny amounts of N,N-dimethyltryptamine, a potent psychedelic. While it has been proposed that the pineal gland excretes large quantities of N,N-dimethyltryptamine during extremely stressful life episodes, notably in the event of birth and death, to produce out of body experiences, there is a lack of evidence to back up this claim (Nichols 2018). This gland has the highest calcification rate among all organs and tissues of the human body. Pineal calcification is thought to jeopardize the amount of melatonin that can be produced by the gland and may be associated with a variety of neuronal diseases (Tan et al. 2018). There is, however, no research on the effect that yoga can have on the pineal gland. Maybe one day we will discover that Headstands decalcify the pineal gland, but until then just enjoy the literal change in perspective.
The pineal gland
Does inverting the body lower our blood pressure? Our bodies have a sophisticated way of constantly monitoring blood pressure with specialized mechanoreceptors called baroreceptors, which then provide information about blood pressure to the autonomic nervous system. While there is widespread belief that inverted postures in yoga specifically stimulate the baroreceptor reflex and lower our blood pressure over time, there is surprisingly very little research on this topic. A few studies several decades ago began to explore this (Cole 1989; Razin 1977; Tai and Colaco 1981) but it is challenging to find more recent literature that expands upon this topic.
Location of baroreceptors in the body
Blood is easily pumped to our lower limbs from the heart via large arteries; however, returning blood to the heart is not such an easy process. The walls of the veins are considerably thinner, and their hollow passageways (lumens) are correspondingly larger in diameter compared to arteries, allowing more blood to flow with less vessel resistance. But by the time blood has passed through capillaries and entered venules and then veins, the pressure initially exerted on it by heart contractions has diminished significantly. While arteries can constrict quite dramatically, veins really only stiffen. The venous system is also normally working against gravity to return blood from the lower limbs to the heart. Inverting the body causes a transient increase in venous return (Haennel et al. 1988). Incorporating yoga asanas such as Supported Bridge Pose (Setu Bandha Sarvangasana) or Legs-Up-the-Wall (Viparita Karani) into a yoga practice can have a significant impact on venous return.
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References:
Cole, R. 1989. “Postural Baroreflex Stimuli May Affect EEG Arousal and Sleep in Humans.” Journal of Applied Physiology 67 (6): 2369-2375.
Haennel, R., K. Teo, G. Snydmiller, H. Quinney, and C. Kappagoda. 1988. “Short-Term Cardiovascular Adaptations to Vertical Head-Down Suspension.” Archives of Physical Medicine and Rehabilitation 69 (5): 352-357.
Nichols, D. 2018. “N,N-dimethyltryptamine and the Pineal Gland: Separating Fact From Myth.” Journal of Psychopharmacology 32 (1): 30-36.
Paulson, O., S. Strandgaard, and L. Edvinsson. 1990. “Cerebral Autoregulation.” Cerebrovascular and Brain Metabolism Reviews 2 (2): 161-192.
Razin, A. 1977. “Upside-Down Position to Terminate Tachycardia of Wolff-Parkinson-White Syndrome.” New England Journal of Medicine 296 (26): 1535‐1536.
Tai, Y., and C. Colaco. 1981. “Upside-Down Position for Paroxysmal Supraventricular Tachycardia.” Lancet 2 (8258): 1289.
Tan, D., B. Xu, X. Zhou, and R. Reiter. 2018. “Pineal Calcification, Melatonin Production, Aging, Associated Health Consequences and Rejuvenation of the Pineal Gland.” Molecules 23 (2): 301.