Cardiac Muscle, Electrical Activity and ECGs

Question 1

Define: aerobic capacity

Answer 1

the ability to take in, transport and use oxygen

Question 2

Define: cardiac output/stroke volume

Answer 2

volume of blood pumped out of the left ventricle each time the ventricle contracts

Question 3

What is the relation between cardiac output, stroke volume and heart rate?

Answer 3

cardiac output = stroke volume x heart rate

Question 4

What description is used to describe the cardiac muscle and what does it mean?

Answer 4

Myogenic, which means it can contract wihtout external nervous stimulation

Question 5

What initiates contraction of the cardiac muscle?

Answer 5

the depolarisation of cells (when a positive charge is reversed)

Question 6

Describe in detail the contraction of the heart in relation to the electrical impulses

Answer 6

Depolarisation starts at the sinoatrial node (SAN) wihch is located beneath the opening of the superior vena cava. The SAN generates electrical impulses which spread across both atria, causing them to contract.

The impluse travels to the atrioventricular node (AVN).

After a delay the signal reaches the bundle of His which is located in the septum. This then divides into the the purkyne fibres which surround the ventricles.

The deoplarisation travels from the bundle of His to the Purkyne fibres and sets off a contraction of the ventricles, starting at the apex and squeezing the blood out.

Question 7

Give a summary in 4 steps of the cycle

Answer 7

1. Electical impulse from SAN causes contraction of the atria
2. Impulses pass to the ventricles via the AVN
3. Impulses pass down the Purkyne fibres to heart apex
4. Ventricles contract

Question 8

What is a Electrocardiogram?

Answer 8

a graphical record of the electrical activity during the cardiac cycle

Question 9

How are ECGs carried out?

Answer 9

Electrodes are attached to the chest and limbs to record electrical currents produced during the cardiac cycle.

Question 10

On an ECG trace what does the P wave represent?

Answer 10

the depolarisation of the atria, leading to contraction

Question 11

On an ECG trace what does the PR interval represent?

Answer 11

the time between the impulses sent from SAN to reach the AVN

Question 12

On an ECG trace what does the QRS complex represent?

Answer 12

a wave of depolarisation resulting in contraction of the ventricles

Question 13

On an ECG trace what does the T wave represent?

Answer 13

the repolarisation of the ventricles during the heart´s relaxation phase