The adrenal glands sit as small paired caps directly atop each kidney, and despite their modest size each one is really two glands fused together. The outer cortex is a slow hormonal system, manufacturing steroid hormones from cholesterol, cortisol regulating blood sugar, immune activity and the body's response to sustained stress, and aldosterone controlling sodium and fluid balance through the kidneys below. The inner medulla is a fast neural one, embryologically closer to nerve tissue than to the cortex surrounding it, releasing adrenaline and noradrenaline within seconds of a threat, the surge behind a racing heart, widened pupils and blood diverted toward the muscles that early twentieth century physiology named the fight or flight response. One small organ therefore answers both a passing stressor, on a hormonal timescale of minutes, and a sudden one, on a neural timescale of seconds, from two different kinds of tissue sharing a single capsule.
Hatha yoga's subtle body maps that same urgency onto the base of the spine. In Swami Satyananda Saraswati's synthesis in Asana Pranayama Mudra Bandha, the adrenal correspondence sits with Muladhara, the Root chakra, the center the tradition reads as survival, physical security and the raw will to keep living, the same register the gland's own hormones serve biologically. The two accounts, the endocrine one and the subtle body one, describe the same gland from within two different traditions of knowledge, and neither is presented here as evidence for the other.