The cardiac cycle, heart action coordination, and ECGs Flashcards
Outline the movement of electrical signals through the heart
- 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
Why can the wave of depolarisation not travel directly to the ventricles?
There is a ring of non-conducting tissue between the atria and ventricles
Why is it important that the AVN imposes a slight delay?
- Ensures the atria contract before the ventricles
- all atrial blood empties into ventricles before the ventricles contract
What does AVN stand for?
Atrial-ventricular node
What does SAN stand for?
Sino-atrial node
Why do ventricles contract from the apex up?
- blood needs to be moved upwards
- ensures complete emptying of ventricles
What is the effect on the heart of the sympathetic nerve releasing noradrenaline?
Heart rate increases
What is the effect on the heart of the vagus nerve releasing acetyl choline?
Heart rate decreases
What does ECG stand for?
Electrocardiogram
What does an ECG show?
Shows the spread of electrical excitation through the heart as a way of recording what happens when it contracts
What are is the x axis on an ECG?
Time
What are is the y axis of an ECG?
Electrical potential (mV)
What does the P wave on an ECG show?
Depolarisation of atria in response to SAN triggering the atria to contract
What does the QRS wave on an ECG show?
- Ventricular systole
- wave of depolarisation in ventricle walls
- ventricles contract
- AV valve closes
What does the T wave on an ECG show?
-ventricles relax
How can heart rate be measured using an ECG?
-heart rate is time between peaks (R)
What is the cardiac cycle?
The sequence of events that occur in the heart during one heartbeat
What occurs during atrial diastole?
- 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
What occurs during atrial systole?
- atria contract
- contract almost simultaneously
- atrial pressure higher than ventricular pressure so AV valves open
- the semi-lunar valves are still closed
What occurs during ventricular systole?
- 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
What occurs during ventricular diastole?
- pressure in the aorta and pulmonary artery increases
- ventricles relaxed
- semi-lunar valves close to prevent backflow
What causes the heart sounds?
Blood hitting the heart valves
What causes the ‘lub’ sound?
Blood hitting AV valves as ventricles contract
What causes the ‘dub’ sound?
Backflow of blood hitting the semilunar valves
