A dhatu can be considered an ''anatomical classification of the human body'', ¨that which supports the body´´
Dhatus are seven basic supporting structures of the human body formed by food. Our bodily tissues are continuously formed, destroyed and reformed in the body, using materials derived from the food and drinks we take in. When all the stages of digestion are functioning properly, and our digestive fire (agni) is balanced, we are able to build the tissues that make up the structure of our body and which support and maintain us.
The dhatus consist of Rasa (which can be considered a vital fluid resembling plasma), Rakhta (blood), Mansa (muscle), Meda (adipose tissue), Asthi (bone and cartilage), Majja (bone marrow, the fatty part), and Shukra (reproductive tissue, ovaries, eggs, semen). These dhatus must not be considered to correspond completely with plasma, blood etc., but should be considered on their own terms.
Dhatu are formed by aharasa, the substance first created after food has been metabolised. Ahar literally means food or ´intake´. Food or ´intake´ is metabolized by ´agni´, ´fire´, an Ayurvedic concept which I will consider in detail in another post. First, food is metabolised by Jatharagni (literally the ´fire´ in the stomach which breaks down raw materials into rasa), and second, by the five Bhootagni in the liver (ie the ´fires´ of ether, air, fire, water and earth which metabolise the corresponding property of foodstuffs). Finally, each dhatu has its own corresponding dhatvagni (therefore 7 dhatvagni) which sorts the previous dhatu´s apportation into useful nutrition for its own dhatu and mala (waste) to be expelled.
These seven major dhatu form sequentially, each building on the one that came before it, (although in principal each dhatu may also able to take what it needs from circulating rasadhatu). In other words the dhatu are nourished in two ways. The first way, the principle of overflow, suggests that aharasa must be of sufficient quantity and quality to satisfy the requirements of circulating plasma before the subsequent dhatu, raktadhatu can be nourished. Then raktadhatu must be well nourished, in order for mamsadhatu to form properly. We can imagine the classic image of abundance of the pyramid of champagne glasses, where the champagne overflows from the top glass into the second, third and subsequent layers. All layers depend on the previous one to be filled to capacity and overflow.
However we must remember that this process is ongoing as the body continues to ingest food and use it to make aharasa, allowing all cellular processes to take place. Rasa is in each cell, as is ´agni´. Each dhatu is made up of innumerable paramanu (cells) which both receive nutrients and expel waste. Sukshma strota are tiny pathways throughout the body (for example leading to the pores of the skin)
How does this work exactly? Each dhatu generates ' prasada' , literally food or nutrition which forms the subsequent dhatu. Each is produced sequentially, one after another, so if there is a breakdown early on in the process of tissue formation, then it affects all other tissues in the body. Because our vitality, immunity and strength (ojus, the essence of nourishment, or what is left when aharasa has been absorbed by lesser dhatu and all wastes have been removed) is only produced at the end of the cycle, it is essential for our overall health that the tissue formation process is functioning properly. You may also note that the reproductive system is the last tissue system to be nourished, so when someone is having problems conceiving, ultimately, an improper digestive system or lack of proper nourishment may be the root of the problem.
Ahar forms Aharasa, the raw material of nutrition and preliminary Rasadhatu within a day or two. Raktadhatu takes betwen 7 and 10 days to form, Mamsadhatu between 14 and 20 days, Medadhatu 5 to 7 days, Asthidhatu 7-10 days, Majjadhatu 7-0 days and Shukradhatu between 30 and 72 days. Marma, Asthi , Sandhi (the latter ´joints´) being the deepest tissues, take longer to be nourished. Note the order of the depth of the tissues. If you scratch the skin, first simple fluid is seen, then later blood. If you were to penetrate further into the body you would pass through fat and muscle to the tendons and bones. At the deepest levels we find bone marrow and reproductive tissue.
If rasadhatu is malnourished you will see the consequences quickly, in terms of dehydration, skin, eye and tongue dryness, weakness, dizziness etc. If shukradhatu is affected it will take much longer. For example skin will dry out in a matter of hours due to a lack of fluids, but amenhorrea tends to occur after months of extreme malnutrition, eating disorders etc.
Medadhatu circulates as fat globules in aharasa and has a unique blocking quality, tending to block the strota, causing later malnutrition as nutrients cannot get through to support deeper tissues. It can be seen as cholesterol in the blood, yellow motes in the eyes, fatty liver, etc and explains why someone can appear well nourished, overweight or obese but when nutritional levels are checked they are in fact malnourished. When medavahastrota is blocked by ama we tend to see overweight, alongside badly nourished dhatu, so feelings of heaviness, tiredness, lack of appetite and energy, and cravings are common.