Our body constitutes water in major quantities i.e. 60% of our total body weight. That means, a man who weighs 100 kg has about 60 L of water in him. This entire water is divided into two components:

1. Extracellular fluids : It is found outside the cells and forms about 45% of total body fluid weight.

2. Intracellular fluids : It is found inside the cells and forms about 55% of total body fluid weight.

As we know that water is an essential element for our living, these bodily fluids are also associated with the same. Proper hydration of the body is essential for normal functioning of cells.

In Ayurveda, there are five elements namey – Aap, Agni, Prithvi, Vayu, and Aakash. These are known as Panchamahabhutas and our body is entirely composed of these elements. The elements within the body which have Aap (water) as predominant element are known as Apya Dravyas, for example – Kapha, Pitta, Rakta, Mutra, Sweda, etc. The term “Udaka” in Ayurveda refers to the physiological aspects of Aap Mahabhuta. The meaning of the word Udaka is water and it includes both compartments i.e. intracellular and extracellular fluids. Udaka is mentioned in 10 Anjali and Anjali Pramana is the amount of the liquid present in the body.

The extracellular compartment of a body fluid includes:

1. Blood plasma

2. Lymph and interstitial fluid

3. Mesenchymal tissue fluid

4. Transcellular tissue fluid

1. Blood Plasma:

It is the Rasa Dhatu and comprises 25% of the total fluid weight of the body. The volume of blood in an average adult body is 5L and that of plasma is 3.5 L. It is made up of water element and is cold, heavy, moist, soft, stable, smooth, flowing, gross, cloudy and dull. These qualities of Rasa Dhatu are similar to Kapha Dosha whose main constitutions are Earth and water. Thus, the health of Kapha Dosha is dependent on Rasa Dhatu. When Rasa Dhatu is depleted, the water element of Kapha Dosha also gets depleted, thus, skin becomes dry and rough, produces bowel constipation and dry mucous membranes lose the ability to resist disease.

This dryness of Rasa Dhatu is caused by Vata and Pitta Dosha. As Vata Dosha is made of air and space, it enters Rasa Dhatu and dries it. Pitta Dosha, which is made of fire element, enters Rasa Dhatu and burns it. Thus, increase in Vata and Pitta Dosha causes depletion of Rasa Dhatu.

Further, increase in Rasa Dhatu leads to excessive water and Kapha Dosha. Thus causes water retention, swelling and mucus formation within the body. The Rasa Dhatu usually increases due to the increase in Kapha Dosha, and when Kapha Dosha increases, a person becomes more lazy and quiet.

Furthermore, the quality of Rasa Dhatu depends on the health of Agni. Rasa Dhatu is the first Dhatu formed after the metabolic process initiated by Agni i.e. Rasagni. The health of Rasagni depends on the health of digestive fire viz. Jatharagni.

2. Interstitial fluid:

It is termed as Lasika in Ayurveda. It is the main component of extracellular fluid and forms the internal environment of the body in which cells bath. It fills in the vacant spaces within the body and is formed by filtration through blood capillaries. It contains less proteins than the blood plasma. It is the supporting medium or channel for the exchange of materials across the cells. It transports nutrients and waste products between the blood capillaries and cell, signals between the cells, cytokines and antigens to drain lymph nodes to maintain immune regulation.

It is said to be the seat of Pitta, and is considered a waste product of Rasa Dhatu. It is thrown out via skin after any type of injury like burns, wounds, etc. When there is an injury, inflammation occurs and this is the response to wound healing. In the process of inflammation, Lasika plays a very important role. The colloid osmotic pressure increases in Lasika as compared to plasma, leading to signals of buffering capacity against edema formation. Simultaneously, there is formation of pro-inflammatory mediators to a systemic inflammatory stimulus. This stimuli results in decrease of pressure of Lasika, which leads to increase in filtration process and edema formation.

3. Mesenchymal fluid:

The fluid originated from mesenchymal tissue and forms about 6% of total fluid weight of the body. It is termed as Majja Dhatu in Ayurveda. It is similar to bone marrow, the soft material that fills the bond cavities. It is composed of Aap (water) Mahabhuta. During the metabolic process, Vata Dosha creates hollow cavities between the bones and that cavities are filled in by this fluid. People having a high quality of Majja Dhatu have soft body parts, strong complexion and sweet voice. It is responsible for providing strength and nourishment of bone cavities.

4. Transcellular fluid:

It is the smallest portion of the total extracellular fluids. It is present between epithelial lined spaces. It is formed from transport activities of cells like cerebrospinal fluid, pericardial fluid, lacrimal fluid, luminal fluid, urinary tract, peritoneal fluid, etc. The main component of transcellular fluid is Pitta Dosha. Besides water as a main component, they are also made up of electrolytes. Thus, they have different functions depending upon the electrolyte present and their location within the body. For example, the synovial fluid is used as lubricant. Bladder fluid like urine, containing by-products of metabolism, maintains homeostasis when eliminated through urination.

Udaka not only represents the water in the body, but also represents various bodily fluids like fraction of water in Dhatus (Rakta, Rasa, Mamsa, Meda, Shukra) , Upadhatus (Stanya and Vasa) and Malas (Mutra, Sweda, and Dravyansha) which have their own compositions, properties and functions. This entire network of matrices is useful to bind together and carry out various cellular functions like adhesive, migration, proliferation and differentiation.