Rakta Dhatu (रक्त धातु)
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The word 'Rakta' (रक्तम्) refers to the color 'Red'. Thus Rakta dhatu literally means the body tissue which is red in color. Therefore, rakta dhatu in Ayurveda is to some extent similar to blood tissue described in western medicine. It is second in order of 7 dhatus (धातु) forming building blocks of physical body. In Ayurveda, in the context of feminine health, the word 'Rakta' is also used to denote 'menstrual blood'.
In Ayurveda Rakta dhatu has been referred by using various other terms like Shonita (शोणितम्), rudhira (रुधिरम्), asrik (असृक्), astra (अस्र), lohita (लोहितम्), kshataja (क्षतजम्).
Formation of Rakta dhatu
Rakta dhatu is formed from 'Rasa dhatu'(रस धातु). When Rasa dhatu is subjected to the process of micro-transformation (सूक्ष्मपचनम्), one processed part of it is specifically utilized to provide nourishment to the next dhatu in order i.e. Rakta Dhatu (रक्त धातु). This part is responsible for development and nourishment of rakta dhatu in body. Transformation of this part of rasa into rakta happens by the action of ranjaka pitta (रंजक पित्तम् | one of the 5 types of Pitta dosha) at sites yakrut (यकृत्) and pliha (प्लीहा). 
Location of Rakta dhatu
Rakta dhatu is present everywhere in the shariram (शरीरम्). However, the channel through which it is predominantly circulated and transformed is called as Raktavaha srotas (रक्तवह स्रोतसम्). Yakrut (यकृत्) and pliha (प्लीहा) are the chief sites of raktavaha srotas. Rakta is metabolized at these sites mainly. Also it flows through siras (सिरा) and reaches to every part of the body.
The Rakta dhatu which is red in color and serves as the seat of pitta dosha naturally shows dominance of agni mahabhuta. But Ayurveda acharyas have opined, after keenly observing its functions and nature that it shows signs of presence of every mahabhuta equally at the same time. To explain this, qualities of Rakta dhatu and their asssociation with each of the 5 elements has been narrated. See table 1. 
|Sr No||Quality||Linked Mahabhuta|
|1||विस्रता | Peculiar Odor||Prthvi (पृथ्वी)|
|2||द्रवता | Liquidity||Jalam (जलम्)|
|3||रागम् | Red color||Teja (तेजः)|
|4||स्पंदनम् | Pulsation||Vayu (वायुः)|
|5||लघुता | lightness||Akasha (आकाश)|
The most important function of Rakta dhatu has been believed to be as Jeevana (जीवनम्) i.e. giving life and sustaining vitality. 
It is said that the life of an individual is sustained because of rakta. While explaining this Acharya Charaka says,
तद्विशुद्धं हि रुधिरं बलवर्णसुखायुषा| युनक्ति प्राणिनं प्राणः शोणितं ह्यनुवर्तते ॥ (Char. Samh 24.4)
tadviśuddhaṁ hi rudhiraṁ balavarṇasukhāyuṣā| yunakti prāṇinaṁ prāṇaḥ śoṇitaṁ hyanuvartate ॥ (Char. Samh 24.4)
Meaning: That pure blood provides the individual with strength, complexion, happiness, and life. Because vitality of life depends on blood.
Therefore Rakta dhatu is also called as one of the seats of 'Prana' (life or soul) of a human being and it is called as one of the 10 "pranayatanas' ( दश प्राणायतनानि).
दशैवायतनान्याहुः प्राणा येषु प्रतिष्ठिताः | शङ्खौ मर्मत्रयं कण्ठो रक्तं शुक्रौजसी गुदम् ॥ (Char. Samh 29.3)
daśaivāyatanānyāhuḥ prāṇā yeṣu pratiṣṭhitāḥ | śaṅkhau marmatrayaṁ kaṇṭho raktaṁ śukraujasī gudam ॥ (Char. Samh 29.3)
Meaning: There are surely ten principal seats in which the life forces are established. The two temples, the three vital organs, the throat, rakta dhatu (blood), shukra dhatu (reproductive fluid), oja (the vital fluid) and the guda (anus).
Along with this, various other functions performed by Rakta dhatu are as below,
- वर्ण प्रसादनम् | enhancing complexion,
- मांस धातु पोषणम् | nourishing successor muscle tissue and other components
Upadhatu of Rakta dhatu and their function
During the process of metabolism and formation of rakta dhatu, tendons(kandara) and veins(sira) are formed as the metabolic byproducts(upadhatu).[Cha. Sa.Chikitsa Sthana 15/17]. Sharangadhara opines menstrual blood(artava) as the metabolic by-product. [Sha. Sa. Purva khanda chapter 5]
Various pigments and acidic secretions are formed as waste during this process. These are termed as malarupa pitta.[Cha.Sa. Sutra Sthana 15/5]
Pitta dosha is formed as a waste by-product of Rakta dhatu metabolism. Also, Rakta has been known to be the site of pitta dosha. Thus, Pitta dosha is closely associated with rakta dhatu.
Characteristics of purity of Rakta dhatu
The color of pure blood is like golden red, firefly, red lotus, lac-resinous material(laksha), and fruit of Gunja (Abrusprecatorius Linn.). [Cha. Sa. Sutra Sthana 24/22]
The person with pure blood has glowing complexion, well-functioning sense organs, and a cheerful state of mind. His digestion and natural urges are normal. He is endowed with happiness, satiety, and strength. [Cha. Sa. Sutra Sthana 24/24].
Characteristics of a person with pure blood
सुखान्वितं तु(पु)ष्टिबलोपपन्नं विशुद्धरक्तं पुरुषं वदन्ति||२४||
A person should be considered as having pure blood if he has glowing complexion, well-functioning sense organs, and is cheerful. A person with unvitiated blood has normal digestion and unobstructed natural urges, is happy and is endowed with saturation and strength. (https://www.carakasamhitaonline.com/index.php?title=Vidhishonitiya_Adhyaya#Characteristics_of_a_person_with_pure_blood)
The characteristics of the essence of rakta dhatu can be observed on ears, eyes, face, tongue, nose, lips, palms, soles, nails, forehead and genitals. These body parts are unctuous, red, beautiful and shining. The persons are endowed with happiness, sharp intellect, magnanimity, tenderness, moderate strength, lack of endurance, and intolerance to heat.[ Cha. Sa. Vimana Sthana 8/104]
तथाऽतिलवणक्षारैरम्लैः कटुभिरेव च||५||
पिण्डालुमूलकादीनां हरितानां च सर्वशः||६||
जलजानूपबैलानां प्रसहानां च सेवनात्|
दध्यम्लमस्तुसुक्तानां सुरासौवीरकस्य च||७||
विरुद्धानामुपक्लिन्नपूतीनां भक्षणेन च|
भुक्त्वा दिवा प्रस्वपतां द्रवस्निग्धगुरूणि च||८||
अत्यादानं तथा क्रोधं भजतां चातपानलौ|
छर्दिवेगप्रतीघातात् काले चानवसेचनात्||९||
शरत्कालस्वभावाच्च शोणितं सम्प्रदुष्यति||१०||
The blood is vitiated due to the following causes:
By habitual intake of spoiled or unsuitable food and/or liquids, overeating, acidic or pungent liquor and other similar drinks, too much salty, alkaline substances, sour and pungent food, kulatha (Dolichos biflorus Linn.), masha (Phaseolus mungo Linn.), nishpava (Dolichos lablab Linn.), tila taila (Sesamum indicum Linn. oils), pindalu (Randia uliginosa DC.), radish and all green (leafy) vegetables, meat of aquatic and marshy animals, burrowing animals and those that snatch birds, excessive intake of curd, sour whey (curd-water), vinegars and other sour fermented liquids, consuming decomposed, putrid food with opposite properties excessive sleeping during the day especially after taking liquids, unctuous and heavy food, excessive anger, excessive exposure to the sun and the wind, suppression of the natural urges like vomiting, avoidance of blood-letting in prescribed time (autumn), too much exertion, injury, heat, indigestion, taking meal during indigestion and due to natural pattern in autumn season.[5-10]
Importance of Rakta dhatu in health and well-being
The pure blood provides the individual with strength, complexion, happiness, and life. [Cha. Sa. Sutra Sthana 28/4].
It is the fundamental source of vitality and life. [Su. Sam. Sutra Sthana 14/44]
- Charaka Samhita (Chikitsasthanam Adhyaya 15 Sutra 16)
- Sushruta Samhita (Sutrasthanam Adhyaya 14 Sutra 4)
- Sushruta Samhita (Sutrasthanam Adhyaya 14 Sutra 9)
- Ashtanga Hrudayam (Sutrasthanam Adhyaya 11 Sutra 4)
- Charaka Samhita (Sutrasthanam Adhyaya 24 Sutra 4)
- Charaka Samhita (Sutrasthanam Adhyaya 29 Sutra 3)
- Available from Charakasamhitaonline.com
- Sushruta Samhita (Sutrasthanam Adhyaya 15 Sutra 5)