The Dhatus are the basic varieties of tissues which support and sustain the living body. The word “Dhatu” comes from a Sanskrit word which means “that which enters into the formation of the body”; the root Dha means “support” i.e. that which bears.
The primary Dhatus are seven in number. They are:
- Rasa dhatu (Nutrient fluid)
- Rakta dhatu (Blood tissue)
- Mamsa dhatu (Muscle tissue)
- Medas dhatu (Adipose tissue)
- Asthi dhatu (Bony tissue)
- Majja dhatu (Bone marrow and nervous tissue)
- Sukra dhatu (Reproductive tissues including sperm and ovum)
The most unique feature of Ayurvedic histology (concept of tissue formation) is that each human tissue is formed from the previous tissue in ascending order of complexity.
Each tissue type has its own Agni (Fire), which determines metabolic changes in the tissues. Thus when food is ingested, it is digested until, in the small intestines, it becomes a liquid, chyme-like material known in Ayurveda as ‘ahara rasa’, or food essence or nutritive fluid.
With the help of ‘ahara rasagni’ (each dhatu has its own specific metabolic fire) this ahara rasa is converted into Rasa dhatu (nutrient fluid or blood plasma) – the first and most simple tissue.
Now, Rasa dhatu catalyzed by Rasagni is transformed into Rakta dhatu (formed blood cells), the second fundamental bodily tissue. Rakta dhatu in turn, with the help of raktagni, becomes mamsa dhatu (muscle); and so on formation of majja, asthi and shukra dhatu takes place.
Together, the dhatus and updhatus (accessory tissue) make up the physical bulk of the body. The upa-dhatus include skin, tendons, ligaments, etc.; they are important structurally but usually are not implicated in disease conditions of the body.
Each dhatu consists of countless infinitesimal paramanus (cells) which are units of structure and function. Each paramanu contains innumerable suksma srotas (channels, pores) through which it receives nutrients and subtle energies and eliminates waste materials. Because dhatus are saturated with pores, the human body can also be said to be filled with pores (srotomaya). The srotas of each dhatu are unique in their structure and function and in the materials which move through them.
Dhatus have inherent tendency to get vitiated or diseased hence they are called dushya. In contrast to doshas, the dushya have no tendency to vitiate other factor of body. They are only a passive component of disease where as the doshas are active counterparts in disease.
The state of health of each dhatu as well as its relative vriddhi/kshaya (excess/deficiency; increase/decrease) is assessed by the physician.