The Cardiac System
Cardiac Muscle: Key Properties
Cardiac muscle is myogenic — it initiates its own contraction from within the tissue itself.
This means contraction is self-controlled, not directly determined by the brain.
Cardiac muscle cells are called myocytes.
Myocytes hold a slight electrical charge across their membrane — at rest they are polarised.
When this charge reverses, the cells become depolarised.
Depolarisation is what triggers contraction.
Artificial Pacemakers
Situations requiring a pacemaker:
The SAN (sinoatrial node) fails to initiate contraction.
There is a blockage or disruption of the impulse between the SAN and AVN (atrioventricular
node).
There is a blockage or disruption of the impulse travelling down the Bundle of His.
How a pacemaker functions:
It continuously monitors the heart's electrical activity.
It stimulates the ventricles or atria to contract whenever necessary.
Impulses are transmitted via electrodes implanted directly in the muscular walls of the heart.
The Cardiac Cycle: Overview
Phase Also Known As What Happens
Relaxation phase Diastole Heart fills with blood
Contraction phase Systole Blood is pushed out and pumped around the body
Contraction and relaxation directly affect both the volume and the pressure inside the heart
chambers.
These pressure changes are what drive the whole cycle forward, stage by stage.
, Stage 1 — Diastole
All chambers of the heart are relaxed at once.
The chamber walls have recoiled, so chamber volume increases.
Pressure is higher in the veins than inside the heart itself.
Blood always moves from an area of high pressure to low pressure — so blood enters the
heart.
As more blood flows into the atria, atrial pressure gradually increases.
Atrial pressure becomes higher than ventricular pressure — this pressure difference forces
the AV valves open.
This allows a completely passive flow of blood straight into the ventricles, with no muscular effort
required.
Key fact: Around 70% of the blood in the atria enters the ventricles this way — passively, before the
atria even contract.
Stage 2 — Atrial Systole
The atria contract.
Contraction of the atrial walls reduces atrial volume.
As volume falls, atrial pressure increases further.
This rise in pressure forces the remaining blood out of the atria and into the ventricles.
This completes ventricular filling — the final ~30% not delivered by passive flow.
Stage 3 — Ventricular Systole
Once the atria are completely emptied, the ventricles contract.
Contraction spreads from the apex (bottom) upwards — this helps push blood efficiently
towards the exit valves.
Contraction causes pressure inside the ventricles to increase.
Once ventricular pressure is higher than atrial pressure, this forces the AV valves to close —
preventing blood flowing backwards into the atria.
Pressure continues to build inside the ventricles until it becomes higher than in the arteries
(pulmonary artery / aorta).
This forces the semilunar valves open, and blood is ejected into the arteries.
Cardiac Muscle: Key Properties
Cardiac muscle is myogenic — it initiates its own contraction from within the tissue itself.
This means contraction is self-controlled, not directly determined by the brain.
Cardiac muscle cells are called myocytes.
Myocytes hold a slight electrical charge across their membrane — at rest they are polarised.
When this charge reverses, the cells become depolarised.
Depolarisation is what triggers contraction.
Artificial Pacemakers
Situations requiring a pacemaker:
The SAN (sinoatrial node) fails to initiate contraction.
There is a blockage or disruption of the impulse between the SAN and AVN (atrioventricular
node).
There is a blockage or disruption of the impulse travelling down the Bundle of His.
How a pacemaker functions:
It continuously monitors the heart's electrical activity.
It stimulates the ventricles or atria to contract whenever necessary.
Impulses are transmitted via electrodes implanted directly in the muscular walls of the heart.
The Cardiac Cycle: Overview
Phase Also Known As What Happens
Relaxation phase Diastole Heart fills with blood
Contraction phase Systole Blood is pushed out and pumped around the body
Contraction and relaxation directly affect both the volume and the pressure inside the heart
chambers.
These pressure changes are what drive the whole cycle forward, stage by stage.
, Stage 1 — Diastole
All chambers of the heart are relaxed at once.
The chamber walls have recoiled, so chamber volume increases.
Pressure is higher in the veins than inside the heart itself.
Blood always moves from an area of high pressure to low pressure — so blood enters the
heart.
As more blood flows into the atria, atrial pressure gradually increases.
Atrial pressure becomes higher than ventricular pressure — this pressure difference forces
the AV valves open.
This allows a completely passive flow of blood straight into the ventricles, with no muscular effort
required.
Key fact: Around 70% of the blood in the atria enters the ventricles this way — passively, before the
atria even contract.
Stage 2 — Atrial Systole
The atria contract.
Contraction of the atrial walls reduces atrial volume.
As volume falls, atrial pressure increases further.
This rise in pressure forces the remaining blood out of the atria and into the ventricles.
This completes ventricular filling — the final ~30% not delivered by passive flow.
Stage 3 — Ventricular Systole
Once the atria are completely emptied, the ventricles contract.
Contraction spreads from the apex (bottom) upwards — this helps push blood efficiently
towards the exit valves.
Contraction causes pressure inside the ventricles to increase.
Once ventricular pressure is higher than atrial pressure, this forces the AV valves to close —
preventing blood flowing backwards into the atria.
Pressure continues to build inside the ventricles until it becomes higher than in the arteries
(pulmonary artery / aorta).
This forces the semilunar valves open, and blood is ejected into the arteries.
