The Cardiac Cycle

• describe the cardiac cycle, with reference to the action of the valves in the heart;

• explain how heart action is coordinated with reference to the sinoatrial node (SAN), the atrioventricular node (AVN) and the Purkyne tissue;

Exam tip

• When referring to the bicuspid (on the left) and tricuspid (on the right) valves, use the word atrioventricular valves- that way, you don’t have to remember which is which!

Sequence of Contraction

• The sequence of events involved in one heartbeat is called the cardiac cycle

• It is important that the chambers of the heart contract in a co-ordinated fashion, if not, this leads to inefficient pumping

Task

• Using this diagram, outline the main stages of diastole, atrial systole and ventricular systole during the cardiac cycle

Phase 1: Diastole• Both the atria and ventricles are relaxing

• the internal volume increases and blood flows into the heart from the major veins

•The blood flows first into the atria, then through the atrioventricular valves into the ventricles

Phase 2: Atrial Systole• The atria contract- right and left together

• The small increase in pressure pushes blood into the ventricles

• This stretches the walls of the ventricles and ensures they are full of blood

Phase 3: Ventricular Systole• Once the ventricles are full they begin to contract

• Blood fills the atrioventricular valve flaps causing them to snap shut preventing blood returning to the atria

• There is now a short period where all 4 valves are closed

• The walls of the ventricles continue to contract raising the pressure

• The contraction starts at the base of the ventricles pushing blood upwards towards the arteries

• The semi-lunar valves open and blood is pushed out of the heart

•The ventricles walls then relax and the cycle begins again

Valves

• The valves ensure blood flows in the correct direction

• They are opened and closed by changes in the blood pressure in the various changes in the heart

Task: You will be given a series of statements about the blood pressure and valves, arrange them in a sequence and stick them in your book

Atrioventricular Valves• When the ventricular walls relax and recoil after contracting, the

pressure in the ventricles drops below the pressure of the atria- this causes the atrioventricular valves to open

• Blood entering the heart flows straight through the atria and into the ventricles

• The pressure in the atria and the ventricles slowly rises as they fill with blood

• The valves remain open while the atria contract

• As the ventricles begin to contract, the pressure of the blood in the ventricles rises. When the pressure rises above that in the atria, the blood starts to move upward.

• This movement fills the valve pockets and keeps them closed • This prevents blood flowing back into the atria

Semilunar Valves• When the ventricles start to contract, the pressure in the major arteries is higher than the

pressure in the ventricles- this means the semilunar valves are closed

• As the ventricles contract, the pressure rises very quickly because the blood cannot escape

• Once the pressure in the ventricles rises above the pressure in the aorta and pulmonary arteries, the semilunar valves are pushed open

• The blood is under very high pressure, so it is forced out of the ventricles in a powerful spurt

• Once the ventricle walls have finished contracting, the heart muscles start to relax

• Elastic tissue in the walls of the ventricles recoils to stretch the muscle out again and return the ventricle to its original size

• This causes the pressure in the ventricle to drop quickly

• As it drops below the pressure in the major arteries, the semilunar valves are pushed closed by blood starting to flow back towards the ventricles and collecting in the pockets of the valves

• This prevents blood returning to the ventricles

Control of Heart Beat

1. What is the patch of muscle called that controls the heart beat? (2)

2. Name the 2 nerves that carry impulses from the brain to the heart (2)

3. How do acetylcholine and noradrenaline affect the heart? (5)

4. What does digoxin do to the heart? (3)

5. What does propanolol do to the heart? (3)

Control of Heart Beat1. What is the patch of muscle called that controls the heart beat? (2)

pacemaker (1) or sino-atrial node (1)

2. Name the 2 nerves that carry impulses from the brain to the heart (2) vagus nerve (1) sympathetic nerve (1)

3. How do acetylcholine and noradrenaline affect the heart? (5) acetylcholine released from the vagus nerve (1) slots into receptors on the plasma membrane of the heart cells (1) this makes the heart beat more slowly (1) noradrenaline released from the sympathetic nerve (1) makes the heart cells beat more rapidly (1)

4. What does digoxin do to the heart? (3) inhibits and slows the Na-K pump in the plasma membrane (1) sodium ions accumulate in the cells (1) this increases calcium ions which increases the force of muscle contraction (1)

5. What does propanolol do to the heart? (3) a beta blocker (1) which decreases the effect of noradrenaline on the SAN (1) reducing heart rate (1)

Pressure Practice Questions

• Try to answer SAQ Q5 on p67

• Leave any you cannot do blank

• Remember to answer in full sentences to help with revision