This is the 2D cross-section companion to the 3D heart simulator, but instead of showing one chamber view it lays out both loops of the double circulation at once: the pulmonary circuit (right heart → lungs → left heart) and the systemic circuit (left heart → body → right heart), which run in series so every drop of blood alternates between them once per full lap.
Timing uses the classic two-phase model rather than a fixed percentage split: real systole (ventricular contraction) stays close to a fixed ~0.27–0.3 s regardless of heart rate, so as heart rate climbs it is diastole that shrinks almost entirely — the reason a racing heart fills less completely and can eventually pump less blood per beat even though it beats faster.
- Heart rate — beats per minute; raises cardiac output roughly linearly until diastole gets so short the ventricles can't fill, at which point stroke volume (and so output) starts to fall.
- Cardiac output CO (L/min) = HR (bpm) × SV (mL) / 1000, with stroke volume SV held near a resting 70 mL baseline and reduced automatically once filling time runs short.
- Valve lights — the AV valves (tricuspid, mitral) glow open during diastole while blood fills the ventricles; the semilunar valves (pulmonic, aortic) glow open only during systole while blood is ejected. The two sets are never open together.
- Deoxygenated (blue) blood always enters the right side from the body and leaves toward the lungs; oxygenated (red) blood always enters the left side from the lungs and leaves toward the body — the two colors never mix inside a healthy heart.