The Cardiac Cycle

What is the cardiac cycle?

the events that happen in a single heartbeat

Roughly how long does the cardiac cycle last?

0.8 seconds

What is systole?

contraction of the heart

What term describes the relaxation of the heart?

diastole

State the 3 stages of the cardiac cycle

cardiac diastole, atrial systole, ventricular systole

In cardiac diastole, describe the state of the atria and ventricles, and the movement of the blood

the atria and ventricles relax and fill with blood

Explain, in terms of pressure and an adaptation of the heart, how blood moves into the atria from the veins and from the atria to the ventricles in cardiac diastole

Elastic recoil in the atria causes the chambers to increase in volume, so pressure decreases and blood flows into the atria from the veins. As blood fills the atria the pressure increases, forcing open the atrioventricular valves to open so blood flows into the ventricles.

When do the semi lunar valves close and why?

In cardiac diastole as the pressure in the ventricles drops (due to elastic recoil) below the pressure in the arteries

Why do the atria contract in atrial systole?

to slightly increase pressure, pushing the remaining blood into the ventricles

What happens to the atrioventricular valves in ventricular systole and how does this happen?

they close to prevent back flow into the atria as the contraction of the ventricles increases the pressure, forcing the valve shut

What is the apex of the heart?

the base

In ventricular systole, where does the contraction start and why?

the apex of the heart so that blood is pushed upwards towards the arteries

When do the semi-lunar valves open?

in ventricular systole when the pressure in the ventricles exceeds the pressure in the arteries (due to filling with blood and increased further by the atrioventricular valves closing)

What part of the heart makes the ‘lub-dub’ sound of the heartbeat?

the closing of the valves

ADD STUFF ON GRAPH

Fill in the blanks:

Cardiac muscle is ____ as it is self-stimulating, initiating itself rather than receiving a nervous message

myogenic

Fill in the blanks:

The basic rhythm of the heart is maintained by a __________

wave of electrical excitation

Where is the heartbeat initiated?

the Sino-atrial node (SAN)

Where is the Sino-atrial node found?

at the top of the right atrium, near the vena cava

What is the function of the Sino-atrial node?

to initiate a wave of excitation at regular intervals

What is a node?

a region of specialised muscle tissue

What happens to the wave of excitation once it has been initiated by the SAN?

it travels along the membranes of muscle tissue to spread over the walls of both atria, causing the cardiac muscle to contract in atria systole

Why doesn’t the wave of excitation spread immediately to the ventricles?

there is a layer of non-conducting tissue at the bottom of the atria

What node is found at the top of the inter ventricular septum?

atrio-ventricular node (AVN)

Why is there a slight delay before the AVN stimulates the bindle of His?

to allow the atria to finish contracting and for the blood to flow into the ventricles

What does the atrioventricular node stimulate?

the bundle of His

What is the bundle of His and where is it found?

a bundle of specialised conducting tissue made up of Purkyne fibres found in the septum

What happens to the wave of excitation once it reaches the apex of the heart?

It splits (left and right) into Purkyne fibres causing the muscles of the ventricles to contract from the apex upwards

What do electrocardiograms (ECGs) measure?

the electrical activity of the heart

How can ECGs be used to diagnose heart problems?

the trace of a health person has a particular shape, so an irregular shape shows problems

What is arrhythmia?

an irregular heartbeat