Postures deal with the mind, body and spirit. The easiest of these is to understand how they work on the body.
Stretching and elongating is the very first quality of postures.
Every day tensions build up and cause hardness of the muscles. This, in turn, blocks circulation. All yoga postures work on circulation in two ways: by stretching and by blocking it in one part and increasing it in another part. Blood is life: it carries oxygen, glucose, and takes waste matter away. When a muscle works, it uses nutrition and releases uric acid. Therefore, blood is the provider and the purifier.
The circulatory system is connected to the lungs, to the respiratory system, which help clean the blood.
Moreover, the digestive system has to work in order to provide the nutrients e.g. liver which stores the nutrients.
All in all, in order for health to be, all the organs, including the brain, have to work and be in the state of balance, homeostasis.