Combine the past and present knowledge to get the overall health.
Blood flow to the cranium or head region will be more. Hence the sinuses and ventricles in the brain get replenishment of nutrients through fluid exchange. Opens up the small blood vessels called collateral circulation in the brain. Improves the sense of balance (Basal Ganglia) and coordination (cerebellum). Much self-awareness, less stress and clarity of mind (Amygdala & Limbic system) are the benefits we can observe after every session this pose. So, it is not advisable for hypertension situation. But as mentioned earlier, this practice is very good for THYROID, PARATHYROID issues through activation of VISHUDDHI chakra.
Respiration: Thoracic cavity will be pressurized more than the abdominal and pelvic cavity. Inhalation will be difficult because of Diaphragm has to contract against gravity. Hence it strengthens the diaphragm, external intercostals muscles.
Circulation: Vyana of lower limbs eased up like the venous return to the heart increases because of gravity. This allows the detoxification of feet and pelvic organs.
Digestive system: Improves the digestive reflexes, peristalsis, and enzyme reactions. This middle place between the pelvic and thoracic cavity can descend a bit and gives pressure to the neck region. So this should be avoided after the meal.
Musculoskeletal systems: Ventricle of heart get assistance from gravity and the ejection fraction will be easy. But the supply for lower limbs will be against gravity and that is how fewer pulses, coldness, numbness will be felt after few seconds. Reverse pressure on joint cartilage, synovial membrane initiates the cartilage growth in a revered way. The proprioception about the reversed joint positions also releases the tension on ligaments, tendons and the bursa than standing.
Excretory organs like the anus, urethra will get resting time with reversal pressure. Hence it is not advisable during pregnancy, menstruation, filed bowels. But it replenishes the circulation and which strengthens the uterus make it get ready for menstruation and pregnancy. So this pose is useful as a preparatory for menstruation and pregnancy.
All these changes are comfortable for any practitioner. This can be supported if required for a beginner or patient. But if still the variation is needed, it may lead to varying degrees of injuries from muscle, bursa, tendon, ligaments and joint. It is just the variation and flexibility to show to the world. If a practitioner considers this as a wide BOS, the weight shift can be done but that will not be a Sarvangasana. Variation can be done by (1) reducing the gap between the elbows, (2) lifting the arms off the ground.