Heart function: pressure, timing and blood flow
Tracing the route of blood is a starting point. Pressure differences, coordinated electrical activation and the relationship between rate and volume explain how that route becomes a repeating circulation. The quantities here remain educational examples, not personal measurements.
What makes heart valves open and close?
Valves respond to pressure differences across them. Blood can move forward when upstream pressure is sufficient to open a valve; a reversal in that pressure difference helps close it. The valves do not each need a separately timed muscle contraction. The detailed pressure sequence below explains why there are brief intervals when all valves are closed.
Why does the left ventricle have a thicker wall?
The left ventricle supports the higher pressure needed for systemic circulation, while the right ventricle sends blood through the lungs. This difference in muscular structure does not mean that the two sides normally deliver different volumes per minute. In steady conditions their outputs are approximately matched because the circuits are connected in series.
How do heart rate and stroke volume determine cardiac output?
Output is a volume per unit time, while heart rate counts beats. Multiplying the illustrative 75 beats per minute by 70 milliliters per beat gives 5.25 liters per minute. Holding stroke volume constant is useful for learning the arithmetic, but a real change in rate need not leave the volume per beat unchanged.
How is an electrical heartbeat different from a contraction?
Electrical activation coordinates the sequence that precedes the mechanical response. Atrial activation comes before ventricular activation, with the conduction system organizing that timing. The exhibit groups motion into filling and pumping so the relationship is readable. It does not reproduce every phase or provide a diagnostic ECG, and its cyan pathway never implies that human blood is blue.
Dive deeper
Cardiac output and stroke volume
Stroke volume is the amount ejected by a ventricle per beat. Multiplying it by heart rate gives output per minute. In steady conditions the two sides have approximately matched output. Stroke volume does not stay fixed when heart rate changes; the calculator holds it constant only to illustrate the relationship.
Q = HR × SV = 75 beats/min × 70 mL/beat = 5.25 L/minPressure controls the valves
During filling, ventricular pressure is low relative to atrial pressure, allowing atrioventricular valves to open. Ventricular contraction raises pressure, closes those valves, then opens the outflow valves when pressure exceeds the pulmonary artery or aorta. Brief intervals exist when all valves are closed.
An electrical event is not a muscle contraction
Electrical activation precedes the mechanical response. The sinoatrial node, atrioventricular node and ventricular conduction system coordinate timing. The faint trace below the diagram is an illustrative motif, not a diagnostic ECG recording.
Good questions.
Is blood really blue before it reaches the lungs?
No. Human blood is red, with color varying by oxygenation. Cyan and coral are diagram colors chosen to help distinguish the two pathways.
Do all four chambers contract at the same moment?
No. The electrical conduction system coordinates atrial activation before ventricular activation. The ventricles pump together into separate circuits; the simplified filling and pumping control does not resolve every phase.
Can I use this to interpret my heart rate?
This exhibit teaches general anatomy and physiology. Its adjustable rate and output calculation are illustrative and do not assess an individual’s health or replace a clinical measurement.