This is an exclusive excerpt from my book, The Physiology of Yoga.
The hollow passageway of blood vessels through which blood flows is called the lumen, meaning opening in Latin. The walls of all blood vessels have three distinct tissue layers, called tunics. The innermost tunic is lined with a specific type of tissue called endothelium. The substantial middle tunic consists of layers of smooth muscle, a specific type of involuntary muscle, while the outermost layer contains collagenous and elastic fibers.
Blood vessel tunics
The arteries closest to the heart have the thickest walls and are known as elastic arteries because of their high percentage of elastic fibers, allowing them to distend as needed. Farther from the heart, where the surge of blood has diminished, the percentage of elastic fibers in an artery’s wall decreases and the amount of smooth muscle increases. The artery at this point is described as a muscular artery.
There are minute nerves, known as nervi vasorum, within the walls of the blood vessels that control the contraction and relaxation of smooth muscle. As smooth muscle contracts, it causes the lumen of the blood vessel to narrow; this is termed vasoconstriction. As smooth muscle relaxes, it causes the lumen of the blood vessel to widen; this is termed vasodilation.
There is a gradual transition as the vascular tree repeatedly branches. Muscular arteries branch to distribute blood to the vast network of arterioles, which eventually lead to capillaries. A capillary is a microscopic channel, sometimes so small that there is only a single cell layer that wraps around to contact itself. At the capillary level, oxygen and nutrients diffuse from the blood into the surrounding cells and their tissue fluid, known as interstitial fluid, in a process called perfusion. Waste products that have been produced by the surrounding tissues then enter the capillary system.
Blood vessels
Next, deoxygenated blood passes through a series of venules, which gradually become larger veins, and eventually returns to the heart. The flow of blood back to the heart is known as venous return. Many veins have one-way valves that prevent the backflow of blood. These valves are present most commonly in veins situated in the limbs or below the level of the heart. In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately two-thirds of the entire blood volume at any given time. When blood flow needs to be redistributed to other portions of the body, the smooth muscle in the walls of the veins constricts. The constriction of smooth muscle in veins is specifically known as venoconstriction.
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 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. Venoconstriction is also much less dramatic than the vasoconstriction seen in arteries and arterioles and may be likened more to a stiffening effect of the vessel wall, rather than significant constriction per se. The venous system is also normally working against gravity to return blood from the lower limbs to the heart.
Yoga can help to improve venous return in three main ways: (1) by improving the efficiency of the skeletal muscle pump; (2) by improving the efficiency of the respiratory pump; and (3) by inverting the body.
The muscles of the lower legs and feet play a crucial role in venous return (Masterson et al. 2006). Contraction of these muscles compresses the underlying veins and promotes the flow of blood back toward the heart. This is often referred to as the skeletal muscle pump. The calf muscles, situated at the back of the lower leg, are the largest muscles in this region and can be thought of as the heart of the legs because of the considerable part that they play in this process. When we stand for a long period without moving much, our feet can start to feel heavy as venous return slows down. One of the reasons you are encouraged to move around during a long flight is to promote this heart-of-the legs action to prevent deep vein thrombosis, which we look at in more detail later in the chapter. The practice of asanas in yoga can improve the strength and tone of the muscles in our lower legs and feet and may increase venous return as a result (Parshad, Richards, and Asnani 2011).
Calf raises in yoga
Respiratory activity also has a huge impact on venous return. In fact, a study by Miller and colleagues (2005) concluded that respiratory muscle pressure production is the predominant factor modulating venous return from the lower limb. This respiratory activity can be thought of as the respiratory pump. The mechanics of breathing can also affect the diameter of the venae cavae and cardiac chambers, which both directly and indirectly affect venous return. Byeon and colleagues (2012) reported that diaphragmatic breathing increases the efficiency of venous return and that the effect is maximized during slow respiration. Intrathoracic pressure becomes progressively more negative by deep and slow inspiration during pranayama, and this also increases venous return (Parshad, Richards, and Asnani 2011). Dick and colleagues (2014) stated that slow breathing toward a rate of six breaths per minute results in increased venous return. The many yoga practices that focus on deep and slow diaphragmatic breathing can therefore increase venous return.
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.
References:
Byeon, K., J. Choi, J. Yang, J. Sung, S. Park, J. Oh, and K. Hong. 2012. “The Response of the Vena Cava to Abdominal Breathing.” Journal of Alternative and Complementary Medicine 18 (2): 153-157.
Dick, T., J. Mims, Y. Hsieh, K. Morris, and E. Wehrwein. 2014. “Increased Cardio-Respiratory Coupling Evoked by Slow Deep Breathing Can Persist in Normal Humans.” Respiratory Physiology and Neurobiology 204:99-111.
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.
Masterson, M., A. Morgan, C. Multer, and D. Cipriani. 2006. “The Role of Lower Leg Muscle Activity in Blood Pressure Maintenance of Older Adults.” Clinical Kinesiology 60 (2): 8-17.
Miller, J., D. Pegelow, A. Jacques, and J. Dempsey. 2005. “Skeletal Muscle Pump Versus Respiratory Muscle Pump: Modulation of Venous Return From the Locomotor Limb in Humans.” Journal of Physiology 563: 925‐943.
Parshad, O., A. Richards, and M. Asnani. 2011. “Impact of Yoga on Haemodynamic Function in Healthy Medical Students.” West Indian Medical Journal 60 (2): 148-152.