CoachThe heart’s two sides (Definition 27.5) each hold two rooms: an upstairs atrium that receives incoming blood, and a downstairs ventricle — the thick-walled pumping room — that drives it out. Between and beyond them, valves — one-way doors — snap shut behind each surge; their paired snaps are exactly the “lub-dub” a stethoscope (or an ear on a chest) hears.
Examples
Example 52.5 (One drop, one full tour)
Example 27.2’s delivery round, with the map’s names: right ventricle — lung capillaries (oxygen aboard at the alveoli, bright red) — left atrium, left ventricle — artery — a thigh muscle’s capillaries (oxygen and glucose off, carbon dioxide on, darker) — vein, valves easing the uphill trip — right atrium. Two circuits, under a minute, no wrong turns available: the valves see to that.
Example 52.7 (Measurements at the roadside)
Three everyday readings of the network: the pulse — artery stretch, counting the surges (Method 27.8); blood pressure, the cuff’s two numbers — the artery pressure at surge and between surges; and the stethoscope’s lub-dub — the valves’ doors heard closing. Three windows, no incision.