Your cells continually use oxygen. The right half of the heart sends oxygen-poor blood to the lungs; the left half sends oxygen-rich blood to the body. Follow one heartbeat to see how muscular walls and one-way valves keep both journeys moving.
How can blood travel in two circuits without mixing?
This is an anterior view: the person’s right is on your left. Dark red means less oxygen; bright red means more oxygen. Both are blood. Open the heart to inspect four chambers. Moving discs represent enlarged red blood cells. The teaching cutaway exposes the four chambers and extends the connected vessels into view. The pale valve leaflets open and coapt; fine chordae connect the inlet valves to papillary muscles.
Anatomical view
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Inspect a structure
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Oxygen-poor bloodOxygen-rich bloodValve leaflets
One beat is a pressure sequence
Muscle contraction makes a chamber smaller and raises its pressure. Blood moves from higher to lower pressure through an open valve. Relaxation allows filling. The right and left ventricles contract together; blood does not complete the lung circuit in a single beat.
Read the ECG without confusing electricity with force
P: atrial depolarisation. QRS: ventricular depolarisation. T: ventricular repolarisation. The electrical signal precedes the mechanical response. The transparent trace is an idealised teaching ECG; it is not a diagnostic recording.
Check your understanding · Why does the left ventricle have a thicker wall?
It must produce enough pressure to drive blood through the systemic circulation, which offers greater resistance than the lung circuit.