~ The Heart. Ultimately this posture is excellent for the heart. It sends blood rushing back into the abdomen and chest, nourishing all the internal organs. However, this posture is especially beneficial for those with hypertension or other heart issues. Personally I believe that people with hypertension and or heart issues benefit greatly from this pose not only because of the physical nourishment that the heart gets but also the mental and emotional benefits of slowing down and being restful.
~ The Lymph System. The lymph system is unique in the body in that it does not have any mechanism of it's own for movement throughout the body. Therefore it relies on physical activity and inversion for movement. Obviously physical activity is the main mode for movement of lymph fluid in the body, but why not put gravity to work while you rest? By inverting the legs upside down, gravity pulls all of the lymph fluid that has drained into the lower regions of the body throughout the day back into circulation.
~Swollen tired legs, edema and varicose veins. For years I worked standing for 6-8 hours on hard marble floors and I developed varicose veins. This pose is God sent for tired legs. 10-20 minutes can make the legs feel rejuvenated and ready to go. With any kind of swelling, be it from edema or varicose, the inverting of the legs helps the swelling go down and has both short term and long term benefits if practiced regularly.
~Calms the mind. This posture has a nourishing effect on the whole system. Because it is an inversion all of your energy (prana/qi) goes to an areas in the body called the dantians. The areas are located below the belly button - lower dantian, in the center of the chest - middle dantian, and between the eyebrow center - upper dantian. Dantian is also known as "energy center" and during rest or meditation energy accumulates in these 3 areas having profound restorative and rejuvenating effects on the whole system.
~Menopause, Stress, and Exhaustion. I want to be clear about this. These are three separate complaints but I grouped them together because the mechanism of relief is the same. Legs up the wall helps balance the nervous system and therefore relieves nervous exhaustion. This in turn mitigates the effects of stress hormones (cortisol and adrenaline).