5.1 The Cardiac Cycle

The Cardiac Cycle is made up of 3

Stages

1. Atrial systole

2. Ventricular systole

1. Atrial Systole

• Both the atrial walls contract

• Blood is forced through the atrio-ventricular valves into the ventricles

2.

2. Ventricular Systole

• The thick muscular walls of the ventricles contract from the bottom upwards increasing the pressure of the blood and forcing it out of the heart. Blood

rushes upwards into the aorta and pulmonary artery through the semi lunar valves.

3. Ventricular Diastole

• This is the stage where the ventricles relax.

• As the muscles relax, the pressure in the ventricles drops.

• The high pressure blood in the arteries would flow back into the ventricles, but the semi-lunar valves prevent this by snapping shut.

• Atrial Pressure - the major bump in the black

curve “a” indicates an increase in pressure –

this is when the atria

contract

• When pressure in the atria is greater than in the

ventricles the A/V valve opens. When the

pressure in the atria is lower than in the

ventricles the A/V valve closes.

Atrial Pressure Curve

Ventricular Pressure Curve

(colour over this curve in blue)

• Ventricular Pressure - increases a little when the atria

contract as blood is being forced into the ventricles (small bump).

• The pressure increases a great deal at the ventricles

• When pressure in the ventricles is greater than the aorta, the semi-lunar valves open.

• When the pressure in the ventricles is lower than the pulmonary artery and aorta, the semi-lunar valve

Ventricular Volume Curve

(colour over this curve in red)

• Ventricular volume increases to a

maximum

as

the atria contract forcing blood into the

ventricles.

• Ventricular volume decreases as blood is being

forced out by ventricles contracting.

• Ventricular volume increases when heart is in

Properties of the Heart

• The heart muscle is myogenic: electrical impulse is generated within the heart muscle and this brings about depolarisation. • The heart beat is initiated at the SAN (known as the

pacemaker)

1. The SAN sends out electrical impulses over the atria,

initiating depolarisation over the atria. This causes them to contract.

2. The electrical impulse is picked up by a second node known as the AVN (atrioventricular node)

3. After a short delay the electrical impulse is passed down specialized conducting fibres known as the

bundles of His - in the wall (septum) between the left and right ventricles

•_{This short delay at the AVN is important as it allows}

4. The bundles of His diverge into Purkinje fibres which take the excitation wave to the base of the ventricles.

5. The excitation wave passed through the wall to the base of the ventricles causing them to contract. This ensures that the ventricles contract from the base

Electrocardiogram

•Used to analyse how fast your heart is beating

•Whether the rhythm of your heartbeat is

steady or irregular

An Electrocardiograph

• P wave – contraction (depolarization) of the atria –

atrial systole

• QRS - complex – caused the contraction

(depolarization) of the ventricles – ventricular

systole

Electrocardiogram

• An electrocardiograph is a machine that

records the electrical activity of the heart.

• The heart muscle depolarises when it

contracts and repolarises (regains charge)

when it relaxes.

Tachycardia

• Exercise

• Anxiety, pain, fear • Hypotension

• Over active thyroid

Brachycardia – slow heart rate

• Normal finding in fit, athletic individuals • Drug effect e.g. beta blockers

• Hypothermia

Fibrilliation

• _{Fibrillation – a very irregular heart beat. The atria} and ventricles lose their rhythmn and stop

contracting properly. It can result in chest pain and fainting and can also lead to a lack of pulse and

Cardiac output is the volume of blood pumped

out by the left ventricle (known as the

stroke

volume

) in one minute.

Adrenaline

• Adrenaline is released from the adrenal glands and travels in the blood to various target organs including the heart. This causes the SAN to increase the

frequency of the impulses.

Class Questions

• What is systole?

• What is diastole?

• What are the three stages of the cardiac cycle

• Why the pressure curve for the ventricles so

• How the use of electrocardiograms (ECGs) can

aid the diagnosis of cardiovascular disease

Nervous System

• Somatic NS – NS under conscious control

• Autonomic NS – NS under unconscious control - Sympathetic NS Excitatory

Pupils dilate

increase heart rate

increase breathing rate

- Parasympathetic NS Inhibitory

Pupils constrict

decrease heart rate

Medulla oblongata

–

• When more frequent nerve impulses pass down the

sympathetic nerve from the cardiovascular centre in the

medulla to the heart, this stimulates the SAN. This increases more frequent depolarisation in the SAN. This increases the heart rate/ cardiac output.

• When more frequent nerve impulses down the

parasympathetic nerve from the cardiovascular centre to the heart, this inhibits depolarisation in the SAN. This decreases the heart rate/ cardiac output.

Responding to Exercise

1. Decrease in pH

2. Change in blood pressure

1. Decrease in blood pH

• A decrease in blood pH is detected by

chemoreceptors in the carotid arteries and aorta

• The chemoreceptors send impulses to the medulla • More frequent nerve impulses pass down the

sympathetic nerve from the cardiovascular centre in the medulla to the heart, this stimulates the SAN.

2. Change in blood pressure

• Detected by pressure receptors (baroreceptors) in the aorta and carotid sinuses.

• It the pressure receptors detect high blood pressure

impulses are sent to the medulla. More frequent nerve impulses pass down the parasympathetic

nerve from the cardiovascular centre in the medulla to the heart, this inhibits the SAN decreasing the

Low Blood Pressure

• If

blood pressure falls

the pressure receptors

are no longer stretched.

• When fewer nerve impulses are sent to the

3. Increase in volume of blood

returning to the heart.

• When exercise starts more blood returns to the heart. This is detected by stretch receptors in the wall of the atria.

• These causes an increase in nerve impulses being sent to the cardiovascular centre. The cardiovascular

centre causes more nerve impulses to be sent down the sympathetic nerve. This increases the frequency of depolarisations in the SAN.

• The increased stretching of the heart atrial muscle also makes the heart muscles contract harder