The most fundamental and characteristic principle of Ayurveda is called "Tridosha" or the Three Humors.
All matter is thought to be composed of five basic elements (Panchamahabhuta) which exhibit the properties of Earth (Prithvi), Water (Jala), Fire (Agni), Air Vayu) and Space (Akasha).
The structural aspect of our body is made up of five elements, but the functional aspect of the body is governed by three biological humors. Ether and air together constitute VATA; fire and water, PITTA; and water and earth, KAPHA. They govern psycho-biological changes in the body and physio-pathological changes too. Vata-Pitta-Kapha is present in every cell, tissue and organ.
Dosha are to be seen as all pervasive, subtle entities, Vata regulates movement and is represented by the nervous system. Pitta is the principle of biotransformation and is the cause of all metabolic processes in the body. Kapha is the principle of cohesion and functions through the body fluids. In every person these three Dosha differ in permutations and combinations and thereby determine the physiologic constitution (Prakriti) of an individual. Vata, Pitta and Kapha are distinctly present in every individual and express in each human being differently according to predominance of their different qualities (gunas).