There is an old yogic saying that goes something like this, "The secret to a long life is a flexible spine and the digestion of a lion." Unfortunately, many of the people I see as a nutritional counselor and yoga teacher have serious digestive issues. This is often due to years of eating refined and processed food, taking antibiotics without restoring healthy microflora back to the intestines and living a high stress lifestyle. With 80 percent of the immune system located in the digestive system a compromised colon can weaken the whole body.
When the mind and body are under pressure, excess hydrochloric acid is released in the stomach and the core of the body becomes rigid with tension. Your digestive system is only reflecting what is going on in your mind, and what you are putting into your mouth. Both stress and refined, processed, sugar-laden foods create an acid condition in the digestive system. "Dis"-ease grows in an acid condition, whereas optimal health requires a more alkaline/acid balance. I compare it to how the afflictions of our thoughts can create an acid mental condition.
A strong, healthy digestive system requires physical rest, mental calm and fibrous foods. High amounts of stress can cause intestinal disorders by reducing the circulation of blood to the absorptive areas of the bowels. There are a number of things that can be done to heal the digestive system including improving the diet, taking certain herbs that heal intestinal tissue, detoxifying the filtering organs and cleansing the blood. Meditation and yoga have been shown to aid in the treatment of many digestive problems, including colitis, irritable bowel syndrome, constipation, acidity, and gastric ulcers.
While meditation helps to calm the stomach, reducing acidity and releasing tension in the large and small intestine, yoga poses help to stimulate the peristaltic action in the intestines, increasing blood flow to the area. At the same time specific poses massage all the internal organs enhancing the body's ability to better absorb nutrients and eliminate waste more effectively. This is due to twisting the body, bending forward, doing backbends, going upside down and sitting on the heels and rounding forward. Seems simple, but according to B.K.S. Iyengar in his book, The Path To Holistic Health, "Health is not a commodity to be bargained for. It has to be earned through sweat."
He goes on to say that each posture is "aimed at purifying and strengthening each organ, bone, and cell of the body." Somewhat different than taking an hour out of your day to exercise; yoga integrates the holistic aspects of mind, body, and spirit into that same hour, with the added benefit of healing your digestive system.
Do you need to focus on any particular poses to target the digestive system? Yes and no. If you are including a variety of poses in your daily practice you should be fine. Make sure to listen to what your body needs rather than trying to force more than your body is willing or able to do. In this way you can unite the body, mind and spirit in a focused moment with balance and integrity.