Inversions - What is it about?

Both Salamba Sirsasana (head stand) and Salamba Sarvangasana (shoulder stand) are classified as inversions. While many benefits are attributed to these poses, the main benefits result from the effect of gravity on circulation within the lymphatic and cardiovascular systems in the inverted state. The lymphatic system can drain more efficiently and blood from the veins in the leg can move back toward the heart with less resistance, resting the heart. These asana also cultivate the mind, by building the awareness, coordination and restraint required to stay and balance in the pose.
Sarvangasana and Sirsasana are considered complementary asana. Both are inversions, however each has a different effect on the nervous and musculoskeletal systems.

The main effects of Sirsasana are on the brain. The pose stimulates oxygenated blood supply to the brain, promoting clarity and sharpness of intellect. The position also revitalises the pituitary and pineal glands that regulate growth and health in the body. Sirsasana is classified as a stimulating and invigorating pose. On the contrary, Sarvangasana soothes the nervous system, and promotes patience and emotional stability. The inverted state in this asana stimulates oxygenated blood supply to the chest area, which is considered to assist the healing of ailments in this region (e.g. bronchitis and asthma). The chin lock of Sarvangasana stimulates the thyroid and parathyroid glands, which are involved in the regulation of the nervous and muscular systems (parathyroid) and metabolism (thyroid).

BKS Iyengar says that "Regular practice of Sirsasana makes healthy pure blood flow through the brain cells. This rejuvenates them so that thinking power increases and thoughts become clearer.” He goes on to say that "Regular and precise practice of Sirsasana develops the body, disciplines the mind and widens the horizons of the spirit. One becomes balanced and self reliant in pain and pleasure, loss and gain, shame and fame and defeat and victory." (Light on Yoga pp 151-152)  For these reasons it is a pose which becomes essential to daily practice. 

Sirsasana can exaggerate or shed light on physical problems apparent in standing poses including (such as the tendency for the lumbar spine to dig, the floating ribs to jut forward) and can be a battle ground for dealing with thoughts, emotions, fears. Initially, it requires intense concentration. Over time the pose provides a key opportunity to sustain a level of effort and simultaneously observe one’s capacity for the renunciation of extraneous work, both physical and mental.
Geeta Iyengar also points out that "[t]he upside-down position counteracts the effects of the normal upright position on the internal organs, which have a tendency to drop and to sag and thus become sluggish. The Sirsasana position gently coaxes them to a new life." (Gem for Women p. 188). For this reason the pose is refreshing, energising and invigorating. 

BKS Iyengar says of Salamba Savangasana that "[i]t is a panacea for most common ailments. There are several endocrine organs or ductless glands in the human system which bathe in blood, absorb nutrients from the blood and secrete hormones for the proper functioning of a balanced and well enveloped body and brain. If the glands fail to function properly, the hormones are not produced as they should be and the body starts to deteriorate. Savangasana has a direct effect on thyroid and parathyroid glands which are situated on the neck region, since due to the firm chinlock their blood supply is increased, helping them to function properly."  (Light on Yoga, p. 171) For these reasons the pose is vital to the maintenance of general physical and mental health.
Both Sirsasana and Savangasana and their variations, by virtue of the inverted nature and particularly the chinlock of the Savangasana, which regulates glands located in the neck region, are useful in reducing premenstrual tension and other hormone related disorders.