Scientific Concepts: Anatomy, Physiology, Chemistry, and Electricity
Anatomy and physiology
Know the organizational ladder (cell, tissue, organ, system) and the systems that matter most behind the chair: skeletal (206 bones; parietal ridge, occipital bone, mandible, clavicle), muscular (origin is the fixed attachment, insertion moves; frontalis, temporalis, occipitalis, masseter, trapezius, sternocleidomastoideus), nervous (fifth cranial/trigeminal and seventh cranial/facial serve the face; greater occipital and supraorbital nerves), circulatory (arteries carry blood away from the heart; common carotid, temporal, occipital, facial arteries), integumentary (epidermis, dermis, subcutaneous; hair, nails, glands), and endocrine (hormones explain hair loss and skin changes).
Chemistry
Matter, elements, compounds, and mixtures; solutions, suspensions, and emulsions. pH runs 0 to 14 and is logarithmic, so each whole step is a tenfold change in hydrogen ion concentration. Hair and skin sit near pH 4.5 to 5.5, which is acidic. Acids donate hydrogen ions and firm the hair; alkalis accept them, swell the cuticle, and raise porosity. Oxidation and reduction drive lighteners, neutralizers, and color; monomers link into polymers in gels and acrylics.
Electricity
Direct current flows one way; alternating current reverses direction. A rectifier converts AC to DC, and a converter steps voltage down. Volts push, amps measure flow, ohms measure resistance, and watts measure power (watts = volts x amps). Galvanic current produces a chemical effect, thermal current produces heat, and mechanical current produces motion. Water and metal raise shock risk.
Traps
Reversing origin and insertion; calling the dermis the outer protective layer; treating pH 4 and pH 6 as equally different (they differ 100-fold); confusing amps with volts.
Rule of thumb
pH 3 is ten times more acidic than pH 4 and one hundred times more acidic than pH 5.