According to Ashtang Ayurveda, there are seven dhatus or tissues in the human body, which form the body`s foundation thus ensuring growth and survival. These dhatus are composed of the five mahabhootas or supreme powers----- earth (prithvi ), water (aap), divine fire (tej), air (vayu) and universal space (akash). Rasa, a flowing dhatu, is the lowest in the hierarchical order of dhatus and is the preliminary source of the other tissues. A nutritious meal, comprising all the six rasas or tastes when properly digested and absorbed by the digestive system generates the body fluid or rasa. It consists of dhatus essential for the growth of the children, dhatus that are damaged in the old people, and maintains body stability in the middle-aged persons.

Water, the most important and the most abundant of the rasas, constitutes about 70 percent of the total body weight. The major cations (positive ions) like sodium, potassium, calcium, hydrogen, magnesium and anions (negative ions) like chloride, bicarbonate and protein are present in water in the dissolved state. Without water there would be no form of life because water is the medium for all metabolic reactions within the cells and tissues. Total body water in an average human being, weighing about 70 kg ranges from 40 litres to 45 litres. In males, water constitutes 65 percent of the body weight while in females, the percentage is about 10 percent less. But these values may vary depending upon the leanness or fatness of the individual. It has been seen that 50 percent of water is present within the cells and the rest outside the cells. The water that is present within then cells varies in concentration according to whether it is present within the cytoplasm, nucleus, mitochondria or microsomes. The extracellular fluid, which mainly contains water can be divided into the following subcategories : transcellular water- 2.5 percent, dense connective tissue and cartilage water- 7.5 percent, plasma water that is confined within the vascular system- 7.5 percent, interstitial fluid and lymph- 20 percent, inaccessible bone water- 7.5 percent. Among these, lymph is the most important because it supplies nutrition and oxygen to those parts of the body where blood cannot reach and also helps in drainage, transmission of proteins and absorption of fats. Moreover, the lymphocytes and monocytes of lymph act as defensive cells of the body.

Tissue fluid, an important rasa or body fluid, is formed from the plasma by the processes of diffusion and filtration. This fluid occupies spaces within the cells and acts as a connecting medium in the transport of nutrition, gases and the metabolic end products between blood capillaries, tissue cells and the lymph. It constitutes the internal environment of the body, which surrounds the tissue cells. The exact composition of the tissue fluid cannot be determined as a pure sample can hardly be obtained. Roughly it can be said that tissue fluid contains a good number of lymphocytes, a small number of granulocytes and may contain a few erythrocytes. Blood proteins and nutrients are quite low and platelets are absent. The composition and volume of the tissue fluid is regulated by constant interchange with blood and lymph. This rasa provides the internal medium in which the tissue cells are bathed. The cells absorb the oxygen and nutrient from the tissue fluid and excrete their metabolites into it. Hence tissue fluid may be regarded as a medium which supplies all the immediate requirements of the cell.

Thus, we can conclude that the body fluids mainly constitute the transporting medium of the body. They are responsible for proper nourishment of each and every part of the body. Moreover, Ashtang Ayurveda considers rasa to be the source of the other dhatus. So, according to the Ayurvedic principle, every disease must have its source in the rasa which should be investigated for proper cure.