The heart: anatomy and function
- Basic vocabulary: artery, vein, muscle.
- A general idea of how blood circulates through the body.
A muscle that never stops
The heart is a muscular organ roughly the size of a fist. It beats about 100,000 times a day, without interruption, to circulate blood through the whole body.
Four chambers, one direction
The heart is hollow and divided into four chambers: two atria above, two ventricles below. Valves separate these chambers and stop blood from flowing backward.
Two circuits, not one
The right side of the heart sends oxygen-poor blood to the lungs (pulmonary circuit). The left side sends oxygenated blood to the rest of the body (systemic circuit). Both halves beat together but never mix their blood.
An electrical signal before every beat
Every beat starts with an electrical signal from the sinoatrial node, which spreads across the atria, then passes through the atrioventricular node before activating the ventricles. This system explains why the atria contract just before the ventricles, not at the same time.
Observe the 3D model
Rotate the heart slowly. Find its conical shape and its apex, the tip that almost touches the chest wall. Notice it is not perfectly symmetrical: the two halves are not the same shape.
Trace the blood's path using the landmarks
Select, in order: right atrium, right ventricle, left atrium, left ventricle, then the aorta. This roughly follows blood's path through the heart, from venous return to its departure toward the body.
Reference annotated view
The "anatomical view" illustration attached to this lesson uses the same landmarks as the 3D model, useful for finding a structure quickly on a still diagram.
At the macroscopic scale
About the size of a fist, 250–350 g, sitting behind the sternum and shifted slightly to the left.
Three wall layers
The heart wall has three layers: the endocardium inside, the myocardium in the middle (the muscle that does the pumping), and the epicardium outside, covered by the pericardium, which reduces friction during each beat.
How blood keeps moving forward
When a chamber contracts, the pressure inside rises. The valves upstream close and the ones downstream open, forcing blood to move in only one direction with every beat.
Systole and diastole
During ventricular systole, the ventricles contract and eject blood. During diastole, the heart relaxes and the chambers refill before the next beat. This cycle repeats continuously.
The heart never works alone
It depends on the lungs to receive freshly oxygenated blood, on the autonomic nervous system to adjust its rate during effort or rest, and on the endocrine system: hormones such as adrenaline speed up its rhythm.
A common mix-up
"Left" and "right" always refer to the person's own point of view, not that of someone facing them. On a front-facing diagram, the heart's "right" side therefore appears on the left of the image.
When a coronary artery becomes blocked
A myocardial infarction happens when a coronary artery becomes blocked, depriving part of the heart muscle of oxygen. This example illustrates why the heart's own blood supply — distinct from the blood it pumps — is essential to its survival.
In summary
The heart is a four-chamber pump that sends blood to the lungs on one side and to the rest of the body on the other, using valves that enforce one-way flow and an electrical signal that triggers every beat.
Classroom activity
In pairs, one student names the blood's path aloud from the vena cava to the aorta, structure by structure, while the other selects each one on the projected 3D model.
Teacher note
Budget around 15 minutes for the guided exploration: this is the step where students most often confuse left and right (see "common mix-up" above). Projector mode is useful for this part.