Blood follows one continuous closed loop, in series: left ventricle → aorta → body tissues (oxygen delivered) → vena cava → right atrium → right ventricle → pulmonary artery → lungs (oxygen picked up) → pulmonary vein → left atrium → left ventricle. The scene colors it directly — red where blood is oxygenated, blue where it isn't — with the two capillary beds (body, lungs) as the transition points.
The cardiac cycle alternates systole (ventricles contract, semilunar valves — aortic & pulmonary — open, AV valves — mitral & tricuspid — shut) and diastole (ventricles relax and fill, AV valves open, semilunar valves shut). Flow speed surges during systole as the ventricles eject their stroke volume.
T = 60 / HR (cycle period, s)
CO = SV × HR / 1000 (cardiac output, L/min)
systole ≈ 0.35 × T, diastole ≈ 0.65 × T
- Heart rate (HR) — beats per minute; shortens the whole cycle, mostly at the expense of diastolic filling time.
- Stroke volume (SV) — blood ejected per beat; scales how far the ventricles visibly contract and how much flow surges each systole.
- Cardiac output (CO) — total blood pumped per minute, the product of the two; a normal resting adult is roughly 5 L/min.