Do yoga inversions reverse the flow of blood?
Although yoga teachers sometimes say this, the answer is no.
The blood in your body only flows in one direction. From your heart’s left ventricle, it’s pumped through a network of arteries to the capillary beds that surround the cells of your body. That’s where your cells take up oxygen and nutrients and your blood absorbs carbon dioxide. From your capillaries, de-oxygenated blood flows through a network of veins back to the right side of your heart. This loop is called the systemic circulation.
From the right ventricle of the heart, the blood is pumped to your lungs, where oxygen and carbon dioxide are exchanged between air and blood. Freshly oxygenated blood flows back to the left side of your heart, completing the pulmonary circulation. From there, the cycle begins again.
That’s the path that any single blood cell takes through the system. Since both sides of the heart contract at the same time, blood actually circulates through the systemic and pulmonary loops simultaneously. Nevertheless, the flow is always in the same direction. In fact, if you did reverse the flow of blood, your cells would soon be starved of oxygen and nutrients, so it’s a good thing you can’t. (For a primer on the circulatory system, check out this rap song.)
So why do yoga teachers say such things?
What those teachers are trying to convey is that turning yourself upside down changes the effect of the pull of gravity on your blood. When you stand, gravity tends to draw blood toward your feet. When you invert yourself in a pose such as headstand or shoulder stand, blood tends to be pulled in the opposite direction, toward your head.
However, gravity doesn’t drive the circulation of your blood. If it did, you wouldn’t get any blood to your brain when you’re upright. Blood flow is driven by blood pressure generated by the pumping of your heart. The blood in your arteries is under high pressure–high enough to overcome the pull of gravity, which ensures a constant supply of oxygen to your brain.
Once blood reaches your capillaries, the pressure drops dramatically, so that your veins are a low-pressure system. Veins are also stretchy. They can expand to store blood. In fact, at any given time, most of the blood in your body is stored in your veins.
Because of low venous pressure and the effects of gravity, blood can tend to pool in the veins of your lower legs, particularly if you stand for long periods of time. However, if you’re healthy, you won’t accumulate an excessive amount of blood in your legs. If you did, you wouldn’t have enough coming back to your heart to maintain circulation.
How does blood get back to your heart?
For one thing, veins have muscles within their walls. When blood pressure drops too low, those muscles contract to narrow the space within the veins, helping to boost venous pressure.
The muscles of your legs, particularly your calf muscles, act as a muscular pump—sort of a second heart. When you walk, for instance, your calf muscles contract and relax. By rhythmically squeezing and releasing the veins in your lower leg, they help pump blood back to the heart. Within the veins are one-way valves, which prevent backflow, ensuring that blood always flows in the right direction.
Breathing also helps return blood to the heart, a mechanism called the respiratory pump. When you inhale, you create a negative pressure in your chest, which pulls blood from the inferior vena cava, the large vein in your abdomen, into the chest and toward your heart.
Finally, turning yourself upside down can help return blood to the heart. When you’re inverted, gravity pulls blood from your legs. That’s partly why an inversion—even a simple pose like putting your legs up the wall—can feel so good after a long day on your feet.
As yoga teachers, we probably shouldn’t make too much of this. You don’t need to turn upside down to keep your blood circulating. Your body has plenty of other ways to get blood from your feet to your heart. Nevertheless, inversions can help.
So what should yoga teachers say?
If you’re a yoga teacher, what could you say about this that would be physiologically accurate?
Perhaps something like, “When you’re standing, gravity tends to pull blood toward your feet. But when you invert, and your feet are higher than your heart, the normal pull of gravity on your body is reversed, which helps return blood to your heart.”
Not as dramatic as saying that yoga inversions reverse the flow of blood. But if we yoga teachers can find clearer and more accurate ways to describe the benefits of the practice, ultimately we’ll be taken more seriously.
Illustrations:
Andy Polaine via Flicker under Creative Commons license
National Library of Medicine (public domain)
Perdoem-me meus desenhos rápidos. Aqui o tempo é curto pra caprichar mais que isso e a vontade de compartilhar é grande. 
Ilustração © Fabiana Rodrigues Barbosa 
Dr. Geeta S. Iyengar 
IYENGAR YOGA BLOG 
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