ng/L
TnC N-lobe: the one regulatory Ca²⁺ site (site II) in heart muscleTroponin: TnT collar, TnC dumbbell, TnI latch; loose pellets: the free pool in the cytoplasmTray: the myocyte; vial: blood, its level on a log scale of hs-cTnTRed tick: 14 ng/L, the 99th percentile of healthy peopleAt rest the TnI arm holds the rod over each bead's binding site, so myosin heads can't attach. Each beat the reservoir (the sarcoplasmic reticulum) releases Ca²⁺, which drops into TnC's N-lobe; TnC grabs TnI, the latch lets go of actin, and the rod rolls up and back, uncovering the sites. Heads bind, stroke and pull the Z-disc toward the M-line. When SERCA pumps Ca²⁺ back up, the rod rolls back and the muscle relaxes. Cardiac TnI is made only in heart muscle and cardiac TnT almost only there, so blood troponin points at heart damage. Hover to the right to block the coronary artery: the beat stops, and after about 30 min the membrane gives way (the valve opens). The free pool in the cytoplasm (the pellets) leaks first, then the bound pool as the filaments break down. The vial drains at its tap (about a 3 h half-life), so its level is the blood concentration: above 14 ng/L by 2 h, 1,315 ng/L at 13 h, a second rise near day 3–4, and still above 14 ng/L at day 12. Teaching model, not for clinical use.