Dhatus are the tissues which nourish the body and support it. They protect our body from external encounters and are responsible for our immune mechanism. If dhatus vitiated then the body construction collapses and ultimately life ends.
Dhatus which are of seven different types work in coordination with one another. They are life sap (rasa), blood (rakta), muscles (mansa), fatty tissue (med), bones (asthi), bone marrow and nervous tissue (majja) and semen and reproductive system (shukra). It works in such a way that one tissue or dhatu transforms to the next from the nourishment they get from the digestive sysytem.
So from rasa (life sap) the body develops rakta (blood), from rakta it develops mansa (muscles), from mansa is developed med (fat), from med is developed asthi (bones), from asthi the body develops majja (nervous system) and from majja is generated shukra (semen).Thus according to ayurveda the seven dhatus are responsible for the immune system. These seven dhatus are in turn made of five mahabhutas.When there is a slight disturbance or imbalance in the equilibrium of these dhatus it will result in ailments or diseases of the human body.
According to ayurveda rasa is the first dhatu which is the primary constitution of human body is otherwise known as sap or juice. It basically represents the fluids of the extra cellular and intracellular portions of the body and it mainly nourishes and strengthens the blood. Once the food gets digested it is converted into rasa which is further transformed into blood.
When the rasa dhatu gets refined then it is called as rakta dhatu which mainly nourishes the body by circulating to all cells and tissues of the body through the blood vessels.Rakta Dhatu is said to be constituted from the metabolic refinement of the Rasa Dhatu. The primary function of the Rakta Dhatu is the nourishment of the body. It is also said to be the preserver of life. It also aids in gaseous exchange.
Mamsa is the muscle which makes up the internal organs of the body and is derived from the rasa and rakta dhatu.They constitutes the basic cover of bone and structure of body.
They are the refined part of the muscle dhatu which mainly helps in lubrication of the body organs. Meda dhatu also constitutes the brain, spinal cord and nervous tissue. It also helps in maintaining right internal body temperature.
The Asthi dhatu are the main tissues responsible for the basic structure to the body. They are the most solid form among all the other dhatus.
Majja (Bone marrow)
The Majja tissues takes the colour of yellow or red. These tissues are said to be found inside the brain and spinal cord. It is the finer essence of the asthi dhatu.
Shukra (Reproductive fluid or Semen)
This is the dhatu which is highly responsible for the reproductive elements of the body.It is the most refined product or essence of all the seven dhatus.