10.3 - Mechanism of Heartbeat

Myogenic Nature of Cardiac Muscle

• Cardiac muscle cells intersect and connect, allowing electrical impulses to spread rapidly and the cells to contract simultaneously and uniformly.
• Cardiac muscles are myogenic: the heart contracts and relaxes rhythmically without receiving impulse signals from the nervous system.

Conduction System of the Heart

The Pacemaker (Sinoatrial Node)

• The pacemaker is a group of specialised cardiac muscle cells in the right atrium wall that initiates and coordinates the rate of heart contraction.
• The main pacemaker is the sinoatrial node (SA node).

Sequence of Electrical Impulse Transmission

1. The sinoatrial node generates electrical impulses.
2. Impulses spread rapidly through both atria; the atria contract simultaneously and pump blood into the ventricles.
3. Impulses reach the atrioventricular node (AV node), then spread through the bundle of His and Purkinje fibres to the apex.
4. Impulses spread from the apex through the ventricular walls; the ventricles contract and pump blood to the lungs and body.

The Heart Sounds: “Lub-Dub”

The “Lub” Sound

• The first “lub” sound is produced when the tricuspid and bicuspid valves close.

The “Dub” Sound

• The second “dub” sound is produced when the semilunar valves close.

Forces That Cause Blood to Circulate in Humans

Pumping of the Heart

• The heart pumps blood through arteries, arterioles and blood capillaries.
• This force alone is insufficient to continue blood flow through veins back to the heart, especially against gravity.
• Valves in veins maintain one-way flow towards the heart.

Contraction of Smooth and Skeletal Muscles

• Smooth muscles in venule and vein walls contract.
• Skeletal muscles around veins contract, press and constrict the veins.
• Valves open to allow blood towards the heart, then close to prevent backflow towards the feet.