Oxygen cylinders store compressed medical-grade oxygen at 2,000-2,200 psi, delivering life-sustaining gas through regulators to patients with hypoxemia from COPD, pneumonia, or post-surgical recovery. Seamless aluminum or steel construction (Type 1-3) ensures durability under high pressure; pin-indexed valves prevent misconnections. Regulators reduce pressure to 50-70 psi for flowmeters (1-15 LPM), connecting to masks, cannulas, or ventilators. E-cylinders (660L at STP) suit hospital crash carts; larger M-cylinders (3,450L) serve ICUs. Portable units with shoulder straps enable homecare mobility.