Flashcards in The Heart as a Pump Deck (122)
Where does the heart pump blood to?
Exchange vessels in the lungs, and then through resistance vessels to the exchange vessels in the tissue
Via what does blood return to the heart?
What do resistance vessels do?
Restrict blood flow, to drive supply to the hard to perfuse areas of the body
What do the capacitance vessels enable?
The system to vary the amount of blood pumped around the body
What do the capacitance vessels act as?
A storage facility
Why is the storage facility of capacitance vessels important?
Means that different amounts of blood can be in the circulation, so supply can change to meet demand
What would happen if the circulatory system was just a simple system with pumps and pipes?
It wouldn’t work very well as the blood would go to the easiest places to perfuse
How do the resistance vessels prevent blood just going to the places that are easiest to perfuse?
They stop blood going to some areas, so the blood has to go elsewhere
Which of the circulations is high pressure?
Which of the circulations is low pressure?
What must be true of the outputs of the left and right sides?
They must be equal over time
What do the atria act as?
‘Priming pumps’ for the ventricles
What is meant by the atria acting as priming pumps?
They fill with blood so they can fill the ventricles
What does the right side of the heart do?
Pumps deoxygenated blood from the right ventricle through the pulmonary artery to the lungs for oxygenation
What is between the right ventricle and the pulmonary artery?
The pulmonary valve
How does blood in the lungs return to the heart?
Through the pulmonary vein into the left atrium
What is between the left atrium and left ventricle?
The mitral valve (also known as the bicuspid valve)
What does the left ventricle do?
Pumps oxygenated blood to the body through the aorta
What is between the left ventricle and the aorta?
The aortic valve
How does blood from tissues return to the heart?
Through the superior and inferior vena cava into the right atrium
What is between the right atrium and right ventricle?
The tricuspid valve
What is the typical pressure in the left atrium?
What is the typical pressure in the left ventricle?
120mmHg systole / 10mmHg diastole
What is the typical pressure in the aorta?
120mmHg systole / 80mmHg diastole
What is the typical pressure in the right atrium?
What is the typical pressure in the right ventricle?
25mmHg systole / 4mmHg diastole
What is the typical pressure in the pulmonary artery?
25mmHg systole / 10mmHg diastole
What is systole?
Contraction and ejection of the blood from the ventricles
What is diastole?
Relaxation and filling of the ventricles
How much does each ventricle pump per beat?
What is how much each ventricle pumps per beat known as?
The stroke volume
What type of muscle is heart muscle?
A specialised form of cardiac muscle
Where does ventricular contraction start from?
The apex of the heart
Describe the connections of cells in the cardiac muscle?
Cells are discrete, but interconnected electrically
How does the connections of cardiac muscle differ from skeletal muscle?
Skeletal muscle has fused cells
How doe the connections of cardiac muscle differ from smooth muscle?
Smooth muscle has individual cells
Why are the cardiac muscles connected?
So electrical signals can pass freely through the cells
Why is the ability for electrical signals to pass through cells important?
Because it allows the heart to beat in sync
What do the cardiac myocytes contract in response to?
An action potential in the membrane
What does an action potential in cardiac myocytes cause?
A rise in intracellular calcium, the key driver of muscle contraction
How long is the cardiac action potential?
Lasts for the duration of a single contraction of the heart, which is about 280ms
How does the duration of the cardiac output compare to usual?
Relatively long in terms of electrophysiology
What is the purpose of the long duration of the cardiac action potential?
So the contraction of the heart lasts long enough
What are action potential triggered by?
A spread of excitation from cell to cell
What determines the pathway of blood through the heart?
The 4 valves
What are the 4 valves in the heart?
What determines the opening and closing of heart valves?
The differential pressure on each side
What do valves have?
What is the purpose of cusps?
They are pushed open to allow blood flow, and close together to seal and prevent backflow
What do cusps of the mitral and tricuspid valves attach to?
Papillary muscles via chordae tendinae
What is the purpose of the papillary muscle connections?
It prevents the inversion of valves on systole
Where are the pacemaker cells?
In the sinoatrial nodes
What do the pacemaker cells do?
Generate an action potential
What happens once the pacemaker cells have generated an action potential?
The activity spreads over the atria in atrial systole
What happens when the signal reaches the atrioventricular node?
It is delayed for about 120ms
What is the purpose of the delay facilitated by the atrioventricular node?
So the ventricles don’t contract at the same time as the atria
What happens in terms of excitation form the AV node?
Excitation spreads down the Purkinje fibres in the septum between the ventricles. Then, it spreads through the ventricular myocardium
In what manner does excitation spread through the ventricular myocardium?
From inner (endocardial) to outer (epicardial) surface
In what manner does the ventricle contract?
From the apex up
What is the result of the ventricle contracting from the apex out?
It forces blood through the outflow valves
What are the phases of the cardiac cycle?
1. Atrial contraction
2. Isovolumetric contraction
3. Rapid ejection
4. Reduced ejection
5. Isovolumetric relaxation
6. Rapid filling
7. Reduced filling
What do Wiggers diagrams show?
The changes that occur in pressure, and the simultaneous changes in volume
What do Wiggers diagrams typically start at?
How are Wiggers diagrams usually plotted?
For the left side of the heart
How would a Wiggers diagram of the right side of the heart appear?
Very similar, but at lower pressures
What is meant by the cardiac cycle?
One pump of the heart
What does the cardiac cycle consist of?
One cycle of systole and diastole
What is phase 1 of the cardiac cycle?
What happens to atrial pressure in phase 1?
It rises due to atrial systole
What is the rise in atrial pressure called?
The A wave
How full are the ventricles from the previous contraction in phase 1?
To 80% capacity
What does atrial contraction provide?
The final bit of filling of the ventricles
What does the amount the atria fill the ventricles vary with?
Age and exercise
What happens as the atria contract?
They push blood from the atria to the ventricles, giving a small rise in pressure in the ventricles
Why is the pressure rise in the ventricles during phase 1 only small?
Because the atria are only providing the final 10% of ventricular filling
What does the P wave in the ECG signify?
The onset of atrial depolarisation
What state are the valves in during phase 1?
The mitral and tricuspid valves are open, and the aortic and pulmonary valves are closed
What has happened to ventricular volumes at the end of phase 1?
They are maximal
What is maximal ventricular volume termed?
End diastolic volume (EDV)
Typically, what is EDV?
What is phase 2 of the cardiac cycle?
What happens in isovolumetric contraction?
There is a rapid rise in ventricular pressure as the ventricles contract. This means that intraventricular pressure exceeds atrial pressure, so the mitral valve closes
What does the mitral valve closure cause?
A C wave in the atrial pressure curve
Why does the mitral valve closure cause a C wave in the pressure curve?
As the valve is being pushed back into the atria, there is a transient increase in pressure at the C wave
Why is this stage termed isovolumetric contraction?
Because there is no change in ventricular volume, as all the valves are closed so there is nowhere for blood to go
What does the QRS complex in the ECG signify?
The onset of ventricular depolarisation, and therefore contraction
What does closure of the mitral and tricuspid valve result in?
The first heart sound
What is the first heart sound termed?
How can valve closures be heard?
Using a stethoscope
What sound does S1 make?
What is phase 3 of the cardiac cycle?
When does ejection begin?
When intraventricular pressure exceeds the pressure within the aorta
Why does ejection begin when intraventricular pressure exceeds the pressure within the aorta?
Because it causes the aortic value to open
What does ejection cause?
A rapid decrease in the volume of blood in the ventricles, as it rapidly moves into the aorta
What happens to atrial pressure during phase 3?
It initially decreases as the atrial base is pulled downwards as the ventricle contracts
What is the decrease in atrial pressure during phase 3 called?
The X descent
What happens to volume of blood in the atria during phase 3?
At this point, blood is continuing to flow into the atria from their respective venous inputs
Why does blood flow into the atria during phase 3?
To fill them with blood for the next cycle
What is phase 4 of the cardiac cycle?
What happens in reduced ejection?
Repolarisation of the ventricle leads to a decline in tension, the rate of ejection begins to fall
What is ventricular repolarisation depicted by?
The T wave of the ECG
What happens to atrial pressure in phase 4?
It gradually rises due to the venous return from the lungs
What is the rise in atrial pressure in phase 4 called?
The V wave
What is phase 5 of the cardiac cycle?
What causes isovolumetric relaxation?
When the intraventricular pressure falls below aortic pressure due to a decline in tension, there is a brief backflow of blood which causes the aortic valve to close.
What does the closure of the aortic valve cause in phase 5?
A dicrotic notch in the aortic pressure curve
Why does the closure of the aortic valve cause a dicrotic notch in the aortic pressure curve?
Due to elastic reverberation
What does closure of the aortic and pulmonary valve result in?
The second heart sound
What is the second heart sound?
Why does volume remain constant in isovolumetric relaxation?
All the valves are closed
What is the volume in the left ventricles after phase 5 termed?
End systolic volume
What does stroke volume equal?
What is phase 6 of the cardiac cycle?
When does rapid ventricular filling begin?
When the intraventricular pressure falls below the atrial pressure, causing the mitral valve to open
What does the opening of the mitral valve cause?
Passive blood flow into the ventricles, and hence a fall in atrial pressure
What is the fall in atrial pressure in rapid filling called?
What sound does the rapid filling make?
It is normally silent, however a third heart sound is normally present
What does the presence of the third heart sound indicate?
It is normal in children, but can be a sign of pathology in adults
What is phase 7 of the cardiac cycle?
What happens as the ventricle reaches its inherent relaxed volume?
The rate of filling slows down (diastasis)
What is reduced filling driven by?