Blood pressure monitor and LIGO: what they have in common

Step 1. Blood pressure monitor — evolved from — Manometer. A blood pressure monitor is a manometer connected to a cuff: it measures the air pressure that squeezes the artery.
Step 2. Manometer — works like — Barometer. Both measure pressure with a column of liquid: a barometer measures the air around us, a manometer the pressure inside a vessel.
Step 3. Barometer — uses — Vacuum. The Torricellian vacuum above the mercury.
Step 4. Vacuum — is used in — LIGO. 4 km vacuum tubes.