Ayurveda's foundational compendia, the Charaka Samhita and Sushruta Samhita, set ojas at the top of a seven-stage refinement process: food becomes rasa (plasma), then in turn rakta (blood), mamsa (muscle), medas (fat), asthi (bone), majja (marrow) and finally shukra (reproductive tissue), with ojas distilled as the subtlest essence of that entire chain, described as a small, stable quantity, often likened to a few drops, seated in the heart. The Charaka Samhita names it explicitly as the basis of bala, strength, and links its loss directly to ojakshaya, a wasting condition marked by fear, weakness, discoloration and susceptibility to disease, effectively an early technical account of what would now be called immune compromise.
Texts distinguish para ojas, the fixed reserve in the heart whose total loss is said to be fatal, from apara ojas, the circulating portion that ebbs and flows with digestion, stress, sleep and season, and can be depleted by overwork, excess sexual activity, fasting, anger, grief or fatigue. Because ojas is the end product of digestion rather than a substance taken in directly, Ayurvedic texts treat it as built rather than consumed, chiefly through agni, sound digestive fire, and protected through rasayana, the branch of Ayurveda devoted to rejuvenative tonics and regimens.