The cardiac cycle, heart action coordination, and ECGs

Question 1

Outline the movement of electrical signals through the heart

Answer 1

• wave of depolarisation begins at SAN causes atria to contract
• wave of depolarisation travels to AVN
• AVN imposes slight delay before stimulating the bundle of His
• AVN stimulates bundle of His
• Bundle of His splits into two branches and conducts the wave of excitation to the apex
• at apex Purkyne fibres spread out through ventricular walls
• spread of excitation triggers the contraction of ventricles

Question 2

Why can the wave of depolarisation not travel directly to the ventricles?

Answer 2

There is a ring of non-conducting tissue between the atria and ventricles

Question 3

Why is it important that the AVN imposes a slight delay?

Answer 3

• Ensures the atria contract before the ventricles

- all atrial blood empties into ventricles before the ventricles contract

Question 4

What does AVN stand for?

Answer 4

Atrial-ventricular node

Question 5

What does SAN stand for?

Answer 5

Sino-atrial node

Question 6

Why do ventricles contract from the apex up?

Answer 6

• blood needs to be moved upwards

- ensures complete emptying of ventricles

Question 7

What is the effect on the heart of the sympathetic nerve releasing noradrenaline?

Answer 7

Heart rate increases

Question 8

What is the effect on the heart of the vagus nerve releasing acetyl choline?

Answer 8

Heart rate decreases

Question 9

What does ECG stand for?

Answer 9

Electrocardiogram

Question 10

What does an ECG show?

Answer 10

Shows the spread of electrical excitation through the heart as a way of recording what happens when it contracts

Question 11

What are is the x axis on an ECG?

Answer 11

Time

Question 12

What are is the y axis of an ECG?

Answer 12

Electrical potential (mV)

Question 13

What does the P wave on an ECG show?

Answer 13

Depolarisation of atria in response to SAN triggering the atria to contract

Question 14

What does the QRS wave on an ECG show?

Answer 14

• Ventricular systole
• wave of depolarisation in ventricle walls
• ventricles contract
• AV valve closes

Question 15

What does the T wave on an ECG show?

Answer 15

-ventricles relax

Question 16

How can heart rate be measured using an ECG?

Answer 16

-heart rate is time between peaks (R)

Question 17

What is the cardiac cycle?

Answer 17

The sequence of events that occur in the heart during one heartbeat

Question 18

What occurs during atrial diastole?

Answer 18

• Both atria and ventricles are relaxed
• blood enters the right atrium via the vena cava and the left atrium via the pulmonary vein
• atria fill with blood
• AV valves are closed
• pressure in atria increases as they fill with blood
• semi-lunar valves are closed

Question 19

What occurs during atrial systole?

Answer 19

• atria contract
• contract almost simultaneously
• atrial pressure higher than ventricular pressure so AV valves open
• the semi-lunar valves are still closed

Question 20

What occurs during ventricular systole?

Answer 20

• ventricles fill with blood and contract
• AV valves close so blood isn’t forced back into the atria
• semilunar valves open with forces blood into the aorta/pulmonary artery

Question 21

What occurs during ventricular diastole?

Answer 21

• pressure in the aorta and pulmonary artery increases
• ventricles relaxed
• semi-lunar valves close to prevent backflow

Question 22

What causes the heart sounds?

Answer 22

Blood hitting the heart valves

Question 23

What causes the ‘lub’ sound?

Answer 23

Blood hitting AV valves as ventricles contract

Question 24

What causes the ‘dub’ sound?

Answer 24

Backflow of blood hitting the semilunar valves

Question 25

When does the ‘lub’ sound happen?

Answer 25

During ventricular systole

Question 26

When does the ‘dub’ sound happen?

Answer 26

During ventricular diastole

Question 27

What happens when ventricular pressure is greater than atrial pressure?

Answer 27

AV valve closes as systole is entered

Question 28

Why do the semilunar valves open?

Answer 28

Because pressure in the aorta is lower than pressure in ventricles

Question 29

Why do the semilunar valves close?

Answer 29

The pressure in the aorta is greater than in the ventricles

Question 30

Why do AV valves open?

Answer 30

Atrial pressure is greater than ventricular pressure

Question 31

Why does ventricular pressure increase after the closing of the AV valve?

Answer 31

Due to the thicker ventricle wall

Question 32

As the ventricles contract what happens to the ventricular volume and pressure?

Answer 32

Volume decreases

Pressure increases

Question 33

What is arrhythmia?

Answer 33

An abnormal rhythm of the heart

Question 34

What is bradycardia?

Answer 34

The heart beats more slowly than normal rhythm

Question 35

What is Tachycardia?

Answer 35

A fast heart rate (pver 100bpm)

Question 36

What is ecotopic heartbeart?

Answer 36

Extra heartbeats that are out of the normal rhythm

Question 37

What is atrial fibrilation?

Answer 37

Abnormal rhythm of the heart when the atria beat very fast and incompletely

Question 38

How are ECGs measured?

Answer 38

• electrodes stuck on skin
• measure electrical differences in the skin
• signal fed into machine which produces an ECG