When learning about the structure and function of the heart, it is useful to have a labelled diagram close to hand. Even better still would be to get to a butcher or supermarket and buy a lamb heart and investigate for yourself!
It is also worth looking for animations of the cardiac cycle on the internet to try and give you an image of how this organ functions.
Until you are comfortable with the structure of the heart, it will be very difficult to understand HOW the heart works. You should be able to identify the following structures:
- Right atrium (RA)
- Right ventricle (RV)
- Left atrium (LA)
- Left ventricle (LV)
- Sinoatrial node (SAN)
- Atrioventricular node (AVN)
- Tricuspid valve
- Mitral valve
- Semilunar valves
The Cardiac Cycle
The four chambers of the heart are continually contracting and relaxing in a sequence known as the cardiac cycle. Contraction of a chamber is SYSTOLE (pronounced sistolee) and relaxation DIASTOLE (pronounced diastole). The left and right sides of the heart actually contract simultaneously but in order to understand how blood moves through the circulatory system we will consider each half separately.
- Right atrium receives blood from
- Superior vena cava (SVC) - carries blood from upper body (head, arms)
- Inferior vena cava (IVC) - carries blood from lower body (chest, abdomen, legs)
- Blood flows from right atrium, across tricuspid valve, into right ventricle
- Blood leaves right ventricle and enters pulmonary artery
- Backflow into RV prevented by semilunar pulmonic valve
- Deoxygenated blood arrives at lungs via pulmonary artery
- Oxygenated blood leaves lungs via pulmonary vein
- Blood from pulmonary vein enters left atrium
- Blood flows from left atrium, across mitral valve, into left ventricle
- Left ventricle has a thick muscular wall / generates high pressures during contraction
- Blood from LV is ejected, across aortic valve, into aorta
TASK: using a simple diagram (boxes will do), draw arrows showing how the blood moves through the chambers and blood vessels.
A common exam question at both GCSE and A Level is why is the muscle of left ventricle is thicker than right ventricle? If you’ve done a heart dissection at school, this will certainly be something which the teachers pointed out and there is in fact a considerable difference between the two chambers. The reasons for this are outlined below:
- The pressure of the blood in the aorta is higher than pulmonary artery
- The left ventricle must therefore generate more pressure to overcome pressure of aorta
- Therefore, thicker muscle required in left ventricle
The problem of backflow:
- Between each chamber of the heart are valves which prevent the blood being forced back into the chamber from which it was just pushed out. Between the atria and the ventricles are the tricuspid and mitral valves (mitral is on the left and tricuspid on the right). These are known as the atrioventricular valves. If you’ve dissected a heart you will have seen fibrous strands leading from ‘flaps’ at the top of the ventricles. These strands (cordae tendinae) are attached to papillary muscles which contract during ventricular systole which generates tension pulling the AV valves shut.
- The pulmonary artery and the aorta also contain valves to prevent the blood from these vessels falling back into the ventricles. These are known as the Semilunar valves (pulmonic and aortic). They do not work in the same way as the AV valves. Instead, the pressure of blood within the vessel actually causes the closure of the semilunar valves.
At several points so far, pressure has been mentioned. It is an important aspect of the cardiac cycle and a factor which can be used to identify which stage of the cardiac cycle a heart is in. In fact, examiners love to provide you with pressure graphs and ask you to analyse the cardiac cycle. It is therefore worth us spending a little time going over the principles of ‘Isovolumetric contraction’ - it sounds worse than it is!
As a chamber fills with blood, the pressure is going to rise. When a chamber contracts, the pressure is going to rise. Changes in pressure affect whether a valve is open or closed. Fluids always move from areas of high pressure to areas of low pressure. Let us think through the cardiac cycle in terms of pressure:
- As the blood passes into the atria, the valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles.
- The intraventricular pressure rises as the ventricles fill with blood. This closes the AV valves.
- Contraction of the ventricles means that the intraventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at).
- The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle.
All good text books should have a pressure graph for you to look at and try to understand how the pressure changes relate to the cardiac cycle.
Electrical Activity of the Heart - Controlling the Cardiac Cycle
The heart has a unique ability to beat (contract) on its own. The cardiac muscle cells are therefore myogenic. Regulation of this contraction though is required to ensure that the muscle cells contract in a specific way and that your heart can respond to meet the energy demands of your body. Nervous and hormonal stimulation both have an effect on the way that the heart contracts.
On the right atrium is a structure called the Sinoatrial Node, or the SAN. This bundle of cells acts as a pacemaker controlling the rate of contraction - the heart rate. Stimulation of this node initiates a wave of electrical impulses which spread aross the atria causing atrial systole. If cardiac cells are stained with a voltage sensitive dye then a wave of contraction can be seen rippling across the atria (all muscular contraction relied on electrical changes).
The electrical signal in the atria is picked up by a second node, the AtrioVentricular Node (or the AVN) which passes the signal down to the apex of the heart (bottom of the ventricles). This is passed through specialised conducting cardiac muscle fibres called the Bundle of His. From the apex, the electrical activity is spread throughout the ventricles along Purkinje fibres. This means that the ventricles contract from the bottom up once they have filled with blood.
You may have heard the terms fibrillation or VF (ventricular fibrillation) on TV shows or films. This refers to changes in the electrical activity of the heart muscle cells. Fibrillation occurs when the cells are not contracting in a regular fashion which, if it’s happening in the ventricles will mean that blood is not being forced into the blood vessels.
QUESTION: How and why would using a defibrillator help in this situation?
Pulling it all together
You should now understand the following things:
- The structure of the heart - atria, ventricles, valves, blood vessels leading to and from the heart.
- The route blood takes through the heart - the cardiac cycle
- The terms systole and diastole
- The function and role of valves within the heart
- The changes in pressure within the chambers and blood vessels
- The electrical control of the cardiac cycle.
TASK: Test your knowledge by rearranging the following statements into a logical order!
- Atria receive blood from veins and store it prior to each heart beat
- Ventricular diastole
End of cardiac cycle, all chambers relax. Aortic and pulmonary valves close → 2nd heart sound. This prevents backflow into ventricles.
- Pressure of RA > RV - forces tricuspid valve to open
Pressure of LA > LV - forces mitral valve to open
- Atrial systole
SAN stimulated and wave of electrical activity spreads across atria.
Both atria contract and move blood across AV valves into ventricles. This reduces volume of atria but increases pressure
- Atria fill up again to start next cycle. The volume increases while pressure decreases.
- Ventricular systole
Contraction of ventricles increases pressure. The AV valves close as blood is forced against them → 1st heart sound. This prevents backflow into atria. Instead, blood is ejected into arteries through aortic and pulmonary valves.
- Electrical signal picked up by AVN. Bundle of His transfers electrical activity down to apex of heart and along purkinje fibres to intiate contraction of the ventricles.