Metabolic rate B = B₀ · M^(3/4) (Kleiber 1932)
Vascular: r_{k+1}/r_k = n^(-1/3) (Murray's law → area preserving)
The ¾ exponent arises from optimization of hierarchical fractal branching networks (West, Brown, Enquist 1997). Minimizing energy dissipation with volume-filling constraint yields exactly α = 3/4. Heartbeat ~ M^(-1/4), lifespan ~ M^(1/4) — all clocks tick at the same rate per heartbeat.