Septum: shared wall between the two ventriclesBellows: right and left ventricle, filling and ejectingBladder: pericardial effusion, drained by the syringeDial: pericardial pressure Pp, mmHg; red band 15–25A rigid frame is the fibrous pericardium, and its end spring gives a little and then goes stiff, like the sac's J-shaped pressure–volume curve. The effusion, both ventricles and the septum share that fixed length. Once the fluid takes up the slack, every extra mL raises Pp steeply. Each ventricle then fills only as far as venous pressure beats Pp, so the left column (CVP, JVP) climbs to meet Pp, stroke volume falls and the heart rate climbs. On inspiration the right bellows takes more room, the septum shifts left and left stroke volume drops; that drop in systolic pressure is pulsus paradoxus (over 10 mmHg). Because the curve is steep, draining 60 mL from 210 mL drops Pp from 14 to 9 mmHg and lifts stroke volume from 26 to 45 mL. Readout: effusion · Pp · CVP (mmHg) · LV stroke volume (mL) · pulsus (mmHg). Teaching model, beats shown at half speed.