The human brain requires an enormous amount of oxygen which is transported to the brain via the blood system. To continuously supply large amounts of oxygen the blood flow to the brain has to be correspondingly large. The brain weight is only 2% of the total body weight, yet it requires about 20% of the total blood supply coming from the heart to keep it in good condition. This also implies that 20% of the oxygen taken into the body by the lungs goes to the brain. This huge supply is necessary in order to meet the demand of the highly active neurons.
There is little storage of oxygen in the brain. Therefore, if nerves are totally deprived of blood for more than a few seconds, then they cease to function. If this continues for about five minutes then the nerves will die. This can lead to severe disturbance of the neurological functions. If the impoverished nerves can obtain enough oxygen from nearby blood vessels, they may be able to function for some hours or days, and eventually regain their working capacity. If the entire blood supply to the brain is stopped, then one will become unconscious in less than ten seconds. These facts clearly indicate the importance of an adequate blood supply to the brain and how sirshasana helps by supplementing the brain with an enriched blood supply.
Let us give a few more interesting facts. The average blood flow to the brain is about 750 millilitres per minute (according to Human Neuroanatomy by R. Truex and M. Carpenter). This perhaps does not sound very much, but if you do a short calculation you will find that this amounts to approximately 1080 litres per day, which is a lot of blood.
The number of blood vessels in the brain is colossal. Most of them are so thin that they can only be seen under a microscope. It is said that the total length of the blood vessels in the brain is more than 50,000 miles (though there are various other differing estimates). Can you imagine this length? It is twice the distance around the world at the equator. Perhaps now you are beginning to appreciate the density and complexity of the blood system of the brain, and how vital the blood supply is for perfect health and functioning. In certain regions of the brain, the network of blood vessels is so dense and the capillaries so fine that some penetrate between enmeshed individual nerve cells and some even pierce them (remember that the individual cells are very small, there being billions in the brain). This system of nerve cells and blood vessels must be nourished by a constant flow of fresh, oxygenated blood. If this nourishment does not reach the brain cells then brain power and efficiency will diminish.