The heart
The heart
- The heart is a double pump with four chambers.
- Its walls are thicker where it must push blood further.
- And it sets its own rhythm — no outside command needed.
Structure
- The two upper chambers are the atria — thin walls, because they only push blood down into the chambers below.
- The two lower chambers are the ventricles — thick muscular walls, because they pump blood out of the heart.
- The left ventricle wall is the thickest: it pumps blood all the way round the body (the right side only reaches the nearby lungs).
Practice
Ventricles have thicker walls than atria because they:
Atria only push blood into the ventricles below; ventricles pump blood out, so need thick muscular walls.
Practice
The left ventricle wall is the thickest because it:
The left ventricle drives the systemic circulation (whole body), so it needs the most muscle.
The cardiac cycle
- One heartbeat is the cardiac cycle: systole (muscle contracts) and diastole (relaxes and fills).
- When a chamber contracts, the pressure inside rises.
- These pressure changes open and close the valves, so blood flows one way only.
Practice
In the cardiac cycle, systole is when the heart muscle:
Systole = contraction (pressure rises); diastole = relaxation and filling.
Controlling the heartbeat
- The sinoatrial node (SAN) in the right atrium is the pacemaker — its wave of excitation makes the atria contract.
- The atrioventricular node (AVN) picks up the wave and delays it (so the atria empty first).
- The Purkyne tissue carries the wave down and up the ventricle walls, so the ventricles contract from the bottom up.
Practice
The pacemaker that sets the heart's rhythm is the:
The SAN in the right atrium generates the wave of excitation that triggers each heartbeat.
Practice
Put the electrical conduction pathway in the correct order.
SAN (pacemaker) → AVN (delay) → Purkyne tissue (ventricles contract from the bottom up).
You've got it
Key idea
- atria = thin walls (push blood down); ventricles = thick walls (pump out)
- left ventricle is thickest — it pumps to the whole body
- cardiac cycle = systole (contract) + diastole (relax/fill); pressure works the valves (one-way flow)
- rhythm: SAN (pacemaker) → atria; AVN (delay) → Purkyne → ventricles contract bottom-up