Control of The Cardiac Cycle

—> the heart is myogenic : the muscles can contract on its own.

—> it needs coordinating to ensure it contracts in a rhythm.

—> this is the job of the Sino-Atrial Node (SAN), Atrio-Ventricular Node (AVN), Bundle of His and Purkinje fibres.

—> found in the wall of the right atrium.

—> peacemaker : initiates the heartbeat and sets the heart rate at 70-80 beats-min but this can be altered during activity or rest.

—> generates an electrical impulse that propagates across both atria walls causing them to contract at the same time.

—> electrical impulse from SAN is picked up by the AVN in the septum.
—> this is a gateway for electrical impulses to pass to the ventricles.

—> AVN imposes a slight delay in sending the wave of excitation to the ventricles
—> allowing the atria to finish contracting and to fully empty is blood before the ventricles contract
—> meaning they don’t contract at the same time as the atria.

—> Bundle of His are specialised in muscle fibres
—> consists of the right and left bundle branches.
—> conduct electrical impulse down the interventricular septum down to the apex.

—> the Purkinje fibres the spread up the ventricle walls transmitting the electrical impulse
—> causes contraction of the ventricles from the apex upwards.
—> pushes the blood up and out of the heart which means that the ventricles can completely empty.

Atrial Systole :

• atria contract, ventricles relax
• atria volume decreases, pressure increases
• blood is pushed through the AV valves down into ventricles

—-

• SAN initiates heartbeat and spreads impulse across atria wall causing them to contract.
• impulse doesn’t spread to ventricles by picked up by AVN which imposes a delay.

Ventricular Systole

• ventricles contract, atria relax
• ventricle volume decreases, pressure increases higher than atrial pressure
• blood pushes the AV valves shut and opens the SL valves

—-

• Bindle of His picks up the impulse from AVN and spreads down septum to the apex.
• Purkinje fingers branch and spreads up the ventricle wall to cause contractions of ventricles from the apex upwards.

Diastole

• whole heart relaxes
• SL valves shut as ventricular pressure starts to drop
• atria begins to fill with blood which increases the pressure pushing the AV valves open
• blood trickles down into the ventricles

—-

• electrical activity of the heart recovers so nothing contracts