Unit 3 - 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