Sushruta says :

“Samdosha Samagnischa Samdhatu Malkriya.
Prasanna Aatma Indriya Mana Swastha iti Abhidhiyate.”

Meaning :
Optimal Health or “Swastha” in Ayurveda is a state of equilibrium of the Doshas, Agni (metabolic fire), function of Dhatus (body tissues), Malas (excretory waste products) along with a cheerful mind, intellect and senses. Any disturbance in this equilibrium due to internal or external factors leads to a diseased state.
For a healthy body one needs to keep the tissues (dhatus) healthy. There are seven dhatus within our body, and the health of each dhatu is dependent upon the health of the dhatu that is formed and developed before it.

The Sapta ( seven ) dhatus are : 

- Rasa dhatu ( plasma )
- Rakta Dhatu (blood cells)
- Mamsa Dhatu ( muscles)
- Medha dhatu ( fat tissue)
- Asthi dhatu ( bone)
- Majja dhatu ( Bone marrow)
- Shukra Dhatu ( sexual fluids)

The health of one main bodily tissue, i.e Rasa dhatu is responsible for the quality and health of all other tissues. Rasa dhatu is the plasma or the circulatory fluid present in the blood that provides development support and nutrition to blood cells, muscles, fat, bone, bone marrow and our sexual & reproductive fluids. All dhatus go through a regular wear and tear process and are responsible for major core functions of the body. This straw coloured liquid, Rasa or the plasma in the blood is responsible for the nourishment of all cells in the body. It is composed of the very essence of food that we intake. In many ways it is very true that - ‘we are what we eat’, and that all health issues originate from the stomach.

The fact is that the result of what you eat on day 1 is visible or felt holistically in the body only on day 5 or later. This is because it takes time for that food to transform into rasa dhatu and circulate throughout the body. For example, if you consume alcohol on a certain day, it’s assimilated impact can only be experienced on the 5th day and an accumulated impact on the 35th day,  since it takes at least 5 days for each dhatu to develop. Ultimately, the overall impact is produced on the 35th day, when all sapta dhatus have been formed. Therefore, rasa dhatu is prime for building immunity ( Ojas) and radiance in the body.

The seat of rasa dhatu is the heart. After the food is digested, the essence of the food goes to the heart from where it branches out through the aorta and arteries into the rest of the body.

Rasa Dhatu is the ultrafine form of ahara rasa. ‘Ahara rasa’ is formed from ‘Ahara’ (food) with the action of ‘Jatharagni’ (digestive fire), and when metabolised further with Rasa Dhatvagni (every tissue has its own fire), it divides into two parts, one that carries out the physiological activities of nourishment and maintenance in the whole body, and the other part is utilised for the formation of the next dhatu, i.e Rakta dhatu.

Rasa dhatu is brought to the heart with the help of Samana Vata ( Vayu), and is ejected into the channels of arteries and capillaries with the help of Vyana Vata, that are spread out through the body. Rasa is in constant motion providing nourishment to each and every cell at all times.

Breast milk and menstrual blood are formed as the metabolic byproducts of rasa dhatu. During pregnancy, it is the rasa dhatu that is the only source of nutrition to the fetus in the womb. It is said that the heart of the fetus is connected with the heart of the mother through the channels carrying the rasa.

During the metabolic process, this rasa is transformed into rakta dhatu by action of ranjaka pitta. It acquires a red colour during this process. This metabolism takes place at yakrit (liver) and pliha (spleen).

Rasa ( plasma ) forms 55% of the blood, of which 91% is water, while the rest consists of protein, essential nutrients, ions, some wastes and gases. 'Rasa' holds all the blood cells in suspension. It helps transport nutrients & wastes to all parts of the body, maintains the blood volume and also balances electrolytes that are essential for the functioning of the nervous system. Most importantly, it provides a strong defence mechanism against foreign bodies.

Antibodies like immunoglobulins are found in plasma that fight against bacteria and also ‘fibrinogen', yet another type of protein that helps platelets to form blood clots. Therefore, we see how Rasa Dhatu plays a critical role in defending the body and boosting immunity.

Being watery in nature, rasa dhatu is associated with kapha dosha. A healthy ‘Rasa’ that translates into the sap of the body is associated with a healthy state of mind , also with the feeling of romantic love and joy, which is very beneficial to the body.

Quality food intake as per the season, a healthy pitta ( digestive fire) and a disciplined lifestyle is what improves the quality of Rasa dhatu, thereafter improving the nutrition and nourishment of the whole body.