8.3.1—Mammalian heart structure

The mammalian heart is a hollow muscular pump with two atria above two ventricles. A septum separates the right and left sides, while valves open and close with pressure differences to keep blood moving forwards.

1. Right-side inflow: blood returning from the body enters the right atrium through the vena cava. It passes through the right atrioventricular (tricuspid) valve into the right ventricle.
2. Pulmonary outflow: the right ventricle sends blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs for gas exchange.
3. Left-side inflow: blood returning from the lungs enters the left atrium through the pulmonary vein. It passes through the left atrioventricular (mitral/bicuspid) valve into the left ventricle.
4. Systemic outflow: the left ventricle sends blood through the aortic valve into the aorta, which carries it to the body tissues.
5. Why the route stays one-way: the atrioventricular valves control atrium→ventricle flow and the semilunar valves control ventricle→artery flow; their closure prevents backflow. The septum keeps the two sides separate.

Route cue: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary vein → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.

Artery and vein are named by direction relative to the heart, not by an absolute oxygen rule: the pulmonary artery leaves the heart and the pulmonary vein returns to it. In a diagram, identify the sides by the vessel/chamber route rather than page position. No image generated or bound.