Heart

The study of the heart at A level. For all of the examination boards at AS they require a knowledge of the heart.

We need to be able to label a heart first of all:

heart_cut_labeled

Heart image supplied by Alila Medical media. Click on the link to see more images of the heart and circulatory systems.

Normally on the UK national curriculum the Aortic valve and the Pulmonary valve are both called the semilunar valves.   The cardiac cycle is the sequence of events during one heart beat. This is normally made up of three stages:

Atrial Systole (atria contracting)

Ventricular Systole (Ventricles contracting)

Diastole (Heart relaxing)

This heart video is supplied by Alila medical images to see more videos for heart and circulation click the link. The cardiac cycle is literally a cycle so you could start at any point in the cycle. However, I’m going to start at atrial systole.

Atrial systole

Both atria contract together which forces blood into the ventricles below through the open atrioventricular valves.

Ventricular systole

As the ventricles start to contract (ventricular systole) it will raise the pressure in the ventricles. At this point the pressure in the ventricles is greater than the pressure in the atria. This will cause the atrioventricular valves to snap shut. As the pressure continues to rise it will cause the pressure in the ventricles to be greater than the pressure in the arteries and so the semilunar valves will open. This will allow blood to flow out of the ventricles and into the arteries above.

Diastole

During diastole the whole heart muscle relaxes and then blood flows into the atria to refill them. If you are on OCR you will need to know that during this stage some blood drains into the venricles below because the atrioventricular valves are open.

Valves in the heart.

Valves in the heart prevent the backflow of blood. There are two main valves Semilunar valves (between the arteries and ventricles) Atrioventricular valves (between atria and ventricles) On AQA the atrioventricular valves are called Tricuspid and Bicuspid. Valves in the heart open and close due to pressure changes. The Atrioventricular valves open when the pressure in the atria is greater than the pressure in the ventricles. These valves shut when the pressure in the ventricles is greater than the pressure in the atria. The semilunar valves open when the pressure in the ventricles is greater than the pressure in the arteries. The semilunar valves close when the pressure in the ventricles is less than the pressure in the arteries.

Heart pressure diagramThe red line shows the pressure in the atria.

The green line shows the pressure the Aorta

The black line shows the pressure in the ventricles.

I have labelled the points as 1,2,3 and 4.

At point 1 the pressure in the ventricles is increasing and so it becomes greater than the pressure in the atria. This causes the atrioventricular valve to close. At point 2 the pressure in the ventricles exceeds the pressure in the arteries and so the semilunar valve opens. At point 3 the pressure in the ventricles falls below the pressure in the arteries and so the semilunar valve closes. At point 4 the pressure in the ventricles goes below the pressure in the atria and so the atrioventricular valves open.

Control of the heart activity.

The heart has a pacemaker and this is called the Sinoatrial node (SAN). This heart shows how the SAN sends out a wave of excitation and how the AVN conducts the wave to the apex of the heart and then up the walls of the ventricles.

heart_labeled

Alila Medical media.

1. This sends out a wave of excitation across the walls of the atria causes them to contract (Atrial systole)

2. The wave of excitation cannot pass directly to the ventricles below due to a layer of non conductive tissue.

3. After a short delay to allow the ventricles to fill with blood, the wave of excitation passes through the trioventricular node and down the Purkyne tissue to the apex of the heart. It will now pass up the ventricle walls causing the ventricles to contract. This is known as atrial systole.

The electrical activity of the heart can be measured using a Electrocardiogram (ECG).

ecg

More images of the heart and ECG diagrams can be viewed at Alila Medical media.

You need to identify the PQRST points

P = 1 = Atrial systole

QRS = 3 = Ventricular systole

T = 5 = Diastole

The following video shows an ECG for a normal heart rhythm and one below for atrial fibrillation

 

Video supplied by Alila Medical media. Click on the link to see other heart videos.

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