Until very recently, there has been little interest in the West in objectively documenting the effects of yoga on health, especially for the more advanced or esoteric practices, such as inversions. The medical doctors who have conducted the existing studies are predominantly Indian. Ralph Laforge, M.Sc., managing director at a clinic at Duke University Medical Center and an authority on the scientific foundations of hatha yoga, knows of only two clinical trials in this country designed to determine the physiological benefits of inversions, both of which were too "statistically underpowered" to draw clear conclusions.
Our understanding of how inversions benefit us, then, is built upon expert opinion, case studies, and educated reasoning. In the absence of more scientifically rigorous studies, we can cite biomechanical principles, measure indices such as heart rate or blood pressure, and witness the effects of inversions on people who practice regularly.
All the evidence points to one principal, galvanizing effect that inversions have on the practitioner: They upend one's relationship to gravity. Gravity has a profound effect on the physiological processes of the human body. As NASA discovered and Jerome Groopman reported in a New Yorker article (February 14, 2000), once humans enter zero gravity, we are subject to severe biomedical problems. Our sense of balance, determined by the vestibular system of the inner ear and calibrated to minute fluid movements, is destroyed. Blood, no longer weighted in the lower torso and legs, floods upwards and the heart speeds up, provoking dehydration and eventually anemia. Muscles atrophy and bone mass drops precipitously.
Here on earth, gravity slowly but surely weighs us down and saps our strength. We stand, sit, or walk with head above the heart, legs and pelvis underneath. As the years rack up, so do the damages. Subcutaneous fat sags. Varicose veins and hemorrhoids erupt. Weary of incessantly pumping blood through its vast circulatory network, the heart falters. According to Payne, the ancient yogis called gravity "the silent enemy." The yogi performs a martial-arts sleight-of-hand: Upend oneself and enlist gravity's power to arrest the ravages of that self-same force.
The human body is sensitive to the fluctuations of gravity because it consists of more than 60 percent water. From the skin in, the body is dense with cells, floating in a bath of intercellular fluid. A complex network of vessels weaves in and around every cell, steadily moving fluids through valves, pumps, and porous membranes, dedicated to transporting, nourishing, washing, and cleansing.
According to David Coulter, Ph.D., who taught anatomy at the University of Minnesota for 18 years, when one inverts, tissue fluids of the lower extremities drain—far more effectively than when one is asleep. Areas of congestion clear. In a 1992 Yoga International article on Headstand and the circulatory system, Coulter wrote: "If you can remain in an inverted posture for just 3 to 5 minutes, the blood will not only drain quickly to the heart, but tissue fluids will flow more efficiently into the veins and lymph channels of the lower extremities and of the abdominal and pelvic organs, facilitating a healthier exchange of nutrients and wastes between cells and capillaries."
All Systems Check
There are four major systems in the body that the practice of inversions is said to positively influence: cardiovascular, lymphatic, nervous, and endocrine.
The circulatory system is comprised of the heart, the lungs, and the entire system of vessels that feed oxygen and collect carbon dioxide and other waste products from the cells. Arteries fan out in an intricate tributary system from the heart, which pumps freshly oxygenated blood from the lungs outward. Veins return blood to the heart, and, unlike arteries, make up a low-pressure system that depends on muscular movement or gravity to move blood along. One-way valves at regular intervals prevent backwash and keep fluids moving towards the heart in a system known as "venous return."
Turning yourself upside down encourages venous return. According to Pat Layton, physiology teacher for the Iyengar Yoga Institute of San Francisco's Advanced Studies Program, "People have to do aerobics because they don't invert. You have to run really hard—get the heart pumping hard—to circulate blood down to the feet and up the back. Not that you shouldn't do aerobics, but inversions are a healthier way to get the benefits [to the circulatory system], particularly as you get older."
Layton believes that inversions also ensure healthier and more effective lung tissue. When standing or sitting upright, gravity pulls our fluids earthward, and blood "perfuses" or saturates the lower lungs more thoroughly. The lower lung tissue is thus more compressed than the upper lungs. As a result, the air we inhale moves naturally into the open alveoli of the upper lungs. Unless we take a good, deep breath, we do not raise the ratio of air to blood in the lower lungs. When we invert, blood perfuses the well-ventilated upper lobes of the lungs, thus ensuring more efficient oxygen-to-blood exchange and healthier lung tissue.
Finally, as Payne says, "Inverting gives the heart a break." The heart works doggedly to ensure that freshly oxygenated blood makes its way up to the brain and its sensory organs. When inverting, the pressure differential across the body is reversed, and blood floods the carotid arteries in the neck. It is believed that baroreceptors, mechanisms that calibrate blood flow to the brain, sense the increase in blood, and slow the flow, thus reducing blood pressure and heart rate. It has not, however, been clinically established whether the practice of inversions could lower blood pressure over the long haul, and in fact, high blood pressure is typically considered a contraindication for inversions.
The lymphatic system is responsible for waste removal, fluid balance, and immune system response. Lymph vessels arise among the capillary beds of the circulatory system, but comprise a separate system that transports stray proteins, waste materials, and extra fluids, filtering the fluid back through the lymph nodes and dumping what remains into the circulatory system at the subclavian veins, under the collarbones. The lymphatic system is analogous to a sewage system—an intricate, underground network tied to every house in town—that keeps the citizens healthy.
Inversions, then, are analogous to the sump pump in the basement, propelling sewage into the pipeline. Lymph, like the blood returning to your heart via the veins, is dependent upon muscular movement and gravity to facilitate its return. Because the lymphatic system is a closed pressure system and has one-way valves that keep lymph moving towards the heart, when one turns upside down, the entire lymphatic system is stimulated, thus strengthening your immune system. Viparita Karani is the best example of this, as it is a mild inversion that one can enjoy for at least five minutes with no stress to the body when one is fatigued or ill. It's interesting to note that for problems like varicose veins and edema (swelling) of the feet, when lymph is unable to maintain the appropriate fluid balance in the lower extremities, doctors often simply tell people to put their feet up.