8.3 The Heart

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.

Cardiac muscle thickness is an adaptation to the pressure and distance that each chamber must supply. Thicker muscle can generate a greater pressure when it contracts; wall thickness is not defined by the oxygen content of the blood.

• Atria — thin walls: thin muscular walls → only a short push is needed → atrial contraction generates enough pressure to move blood into the ventricles, but does not need to drive it around a circuit.
• Ventricles — thicker walls: thicker, more muscular walls → a stronger squeeze is possible → ventricular contraction raises pressure and ejects blood through the semilunar valves.
• Right ventricle — thinner than the left: blood travels from the right ventricle to the nearby lungs → a lower pressure is sufficient → its wall can be thinner while still powering the pulmonary circuit.
• Left ventricle — thickest wall: blood must travel from the left ventricle through the systemic circuit to the body → high pressure is required → the much thicker muscle generates the force for systemic delivery.
• Septum: the muscular septum separates the right and left sides → the two circuits remain distinct and blood does not mix between them.

Boundary: this card explains structural pressure demand. The timed systole/diastole and valve sequence belong to the cardiac-cycle card.

The left ventricle is thicker because it pumps farther and needs higher pressure, not because its blood is “more oxygenated”. Do not infer a cardiac-cycle time point from wall thickness alone. No image generated or bound.

One cardiac cycle is one heartbeat: atrial systole, ventricular systole and diastole. Muscle contraction decreases chamber volume and raises pressure; relaxation increases volume and lowers pressure. Valves respond to these pressure differences to keep flow one-way.

1. Atrial systole: atrial muscle contracts → atrial volume falls and atrial pressure rises above ventricular pressure → atrioventricular (AV) valves open → blood is pushed into the relaxed ventricles. Semilunar valves remain closed.
2. Ventricular systole: ventricular muscle contracts → ventricular volume falls and ventricular pressure rises above atrial pressure → AV valves close, preventing backflow into the atria. When ventricular pressure exceeds pressure in the aorta and pulmonary artery, semilunar valves open and blood is ejected.
3. Diastole: atria and ventricles relax → ventricular pressure falls below arterial pressure → semilunar valves close, preventing blood returning from the arteries. The atria fill from the vena cava and pulmonary veins; when atrial pressure rises above ventricular pressure, AV valves open and blood flows passively into the ventricles.
4. Repeat: filling restores chamber volume, the pressure sequence begins again with atrial systole, and blood continues to move without a pause between cycles.

Valve rule: a valve opens when pressure behind it exceeds pressure in front; it closes when the pressure relationship reverses. The closure prevents backflow rather than actively pushing blood.

Do not assign valve states from “systole” alone without checking which chamber pressure is changing. This card covers mechanical pressure and valves; SAN, AVN, bundle of His and Purkyne conduction belong to 4614. No image generated or bound.

The heart is myogenic: its own conduction system starts and distributes the excitation wave. The sequence is SAN → atria → AVN delay → bundle of His → Purkyne tissue → ventricular muscle, so atrial contraction precedes coordinated ventricular contraction.

1. SAN starts the wave: the sinoatrial node in the right atrium generates a wave of excitation, causing the atria to contract.
2. AVN delays the wave: non-conducting tissue between atria and ventricles prevents the wave travelling straight through; the atrioventricular node receives it and delays it, allowing the ventricles time to fill after atrial contraction.
3. His bundle carries it through the septum: after the delay, the wave passes down the bundle of His.
4. Purkyne tissue spreads it from the apex: the bundle divides into Purkyne fibres that spread through the ventricular walls, initiating excitation from the apex upward.
5. Ventricles contract together: this coordinated wave makes the ventricles contract and forces blood into the pulmonary artery and aorta.

The key control relationship is electrical excitation → muscle contraction → coordinated pumping. The AVN delay is essential for atrial-then-ventricular timing. This card covers the conduction pathway, not ECG interpretation, drug effects or other unsupported clinical detail; the mechanical valve-pressure sequence is covered in 4613. No image generated or bound.