The endocrinal glands supplement the nervous system, in particular the sympathetic and parasympathetic nerves. Together, they make up a single neuroendocrine system that integrates and coordinates the metabolic activities of the body, and controls the ability of the body to face changes in the internal and external conditions. The two systems work hand in hand. However, the important difference between them is that the nervous reactions are quick and short lived, whereas endocrinal responses are slow and protracted to give permanent changes to the body. That is, the nervous system produces speedy changes in the body, while the endocrinal system gives continuity to these changes.
The glands are distributed throughout the body and secrete specific chemicals called hormones into the bloodstream. In this way, the hormones are distributed to all parts of the body. These trigger definite organs that react to specific hormones. In this manner changes are induced within the body according to the dictates of the brain, which is also the master controller of the endocrinal system.
The glands do not work independently each gland reacts with and modifies the influence of the other glands. Thus any disorder of the endocrinal system in general or in one of the glands alone can have negative repercussions on the health of the whole body. Yoga practices in general help to harmonize the entire system and render individual glands efficient.
The main glands in the endocrinal system are the pituitary, pineal, thyroid, parathyroids, adrenals, ovaries and testes. It is the two glands in the brain, the pituitary and the pineal, that we are interested in briefly describing here for it is these two glands that are influenced directly by sirshasana. The other glands are also influenced, but less directly.