So a pool of blood courses slowly through the veins in the lower part of the body waiting to be pumped back toward the heart. So if you are chronically inactive fluids move more sluggishly out of this region, the processes of cellular nourishment and elimination are compromised.
This gravity induced congestion can affect any organ or tissue below the heart. Lying down for the night's sleep helps to correct the situation, but still the effects of gravity can be seen in varicose veins, hemorrhoids, etc. That's why nurses try to get people up and about as soon as possible after surgery, and in yoga we do inverted postures.
The benefits: Shivananda calls it "a panacea, a cure for all diseases". If you remain in this asana for just 3-5 min, blood will not only drain quickly to the heart from the lower extremities and the abdominal and pelvic organs, but tissue fluids will flow more efficiently into the veins and lymph channels, and this will make for a healthier exchange of nutrients and waster between cells and capillaries.
It increases blood pressure in brain's hypothalamus which regulates the autonomic nervous system and pituitary gland which in its turn regulates many other endocrine glands, and the brain itself which carries out all the aspects of mental functioning. This asana lifts your spirits wonderfully, it is also a great morning wake-up. It increases digestive fire, counters depression, fills you with enthusiasm for meeting your day.
Anyone who is serious about doing the headstand should be aware of the danger signals from cranial bones and sutures. Any time you are not comfortable you should come down. If you feel any pain in shoulders you are making too much effort. In such case learn to balance, build strength and flexibility. Pain in neck refers to imbalances, tension or inflammation in vertebral joints. Stop doing headstand until this pain has gone away.
Any extensive practice of shirshasana should be followed by sarvangasana. Practicing headstand alone results in an imbalance and manifests as edginess and irritability. If you do headstand for more than 20 min it is good to have some food afterwards at least a cup of hot milk, if you do not you can experience a raw uncomfortable feeling in the abdomen atet in the day. If you are going to hold the headstand for more than 5 min, the softer the surface the better.
There are two kinds of the headstand depending on which region of the skull you are standing:
Bregma headstand, it is characterized by having hands more underneath the head, placing the weight at the bregma (the intersection of parietal and coronal sutures). The lumbar region is arched to remain balanced.
Crown headstand. Behind the crown the cranium immediately rounds and that is the spot where we place the weight of the body. Sometimes there is a tendency to somersault in the crown headstand. The body is straight.
The bregma headstand is more dynamic while the crown headstand is calm and poised. The crown headstand is highly recommended for people which have problems with vertebrae.
Please remember that shirshasana is a balancing pose and so it is not designed to correct side-to-side imbalances. Indeed, if you have any, shirshasana will increase them.