Charak has described, that the Human body is made-up of infinite numbers of microscopic and sub-atomic sized (Paramanu) units. These units (cells) form seven important tissue elements that make the whole structure of living body. The function of the dhatu is to support (deha-dharanat) and nourish (poshanat) the body. Thus dhatus not only form the structure of the body, but also supports all the functions in body. They are seven in number. Each dhatu has its own Agni known as Dhatwagnis.
The food ingested is digested by the Jatharagni (fire in stomach). Jatharagni is the chief metabolic fire of the body, which is reffered to in Upnishadas as garhyapatya vahni which keeps constantly burning. All the other types of Agnis (thirteen ) depend upon it. Jatharagni is located in Gastrointestinal tract.
Dhatuagnis help in converting the atoms in to molecules to form the tissues or dhatus, thus form the basic molecular biochemistry of body. In other words this metabolic fire through the process of combustion break the atomic bonds in food and release the energy necessary for forming the substrate for development of different tissues. These tissues are constantly formed in order to replace the dead and degenerating ones. Therefore dhatus are found in two states. First variety is known as Poshak that which nourishes the dhatu to help its formation and second is Poshya which maintains and sustains the body after being formed. Dhatwagnis causes conversion of dhatus from one another. Thus all the dhatus are maintained in the state of dynamic equilibrium in health.