3. The heart as a pump Flashcards
Describe the two circulations of the heart
Systemic - left heart, contraction of the left ventricle pumps blood to the aorta
Pulmonary - Right heart, contraction of the right ventricle pumps blood into the pulmonary arteries
NB. these both have the same cardiac output to ensure that blood does not accumulate in one circulation and can be removed from the other
Describe the left heart and it’s actions
Pumps blood into the aorta and the systemic circulation
The blood pressure in the systemic circulation must be kept high to allow efficient distribution of blood to the organs of the body
Blood flow to the organs is determined by the state of constriction of muscles within the small arteries to that organ
Describe the right heart and it’s actions
Pumps blood into the pulmonary artery and pulmonary circulation
No need for high pressure as there is no distribution of blood to different organs - only lungs
Lungs are efficiently perfused at a lower pressure because the pulmonary resistance is much lower than the systemic vascular resistance - lower resistance equates to lower pulmonary arterial pressures:
-If the resistance is lower, then a lower pressure is required to push the cardiac output through the vessels
-Flow = pressure/resistance
Describe the effect of an increased pulmonary resistance e.g. due to hypoxia
An increased resistance in the pulmonary circulation would cause there to be an increased pressure and so this would lead to pulmonary arterial hypertension - right heart damage and right heart failure
Describe the pulmonary arteries compared to the systemic arteries and explain their difference
Pulmonary arteries (from right heart) have larger diameter and thinner walls because they operate at a lower pressure
What is meant by Starling’s law of the heart?
Concept that the heart when working normally will pump out from the ventricles whatever volume of blood is delivered from the aorta
SO if more blood is delivered, the ventricle expands to a greater diameter and increases the strength of contraction
What is Starling’s law of the heart controlled by?
Pre-load
Define ‘pre-load’ and explain this
This is the degree of stretching experienced by the ventricle during diastole
Proportional to the end diastolic volume
How is Starling’s law of the heart controlled by pre-load?
According to the law, an increased pre-load results in an increased stroke volume and hence, increased cardiac output e.g. during exercise
What is the limit to the concept of Starling’s law?
If the ventricle expands beyond a certain volume, the mechanism fails
In heart failure, the ventricles are overstretched and this weakens the heart, meaning that Starling’s law does not hold
Describe the mechanism of Starling’s law
One theory:
Actin and myosin filaments have excess overlap at rest
Stretching increases the level of overlap and so increases the force of contraction (?)
Define ‘afterload’
This is the resistance that the heart sees - of aorta and large arteries
Dependent on diameter and elasticity of the tissue
What are the effects of after-load?
Increased afterload can decrease the cardiac output - outside a certain parameter, the heart cannot compensate for changes in after-load
What is afterload affected by?
Afterload is increased by lower compliance and damage to the aorta
What is compliance?
How much the aorta will distend under a given pressure load
This is the reciprocal of afterload
The less the compliance of the aorta, the greater the afterload
Give an example of when the compliance of the arteries may decrease and the effects of this
If the elasticity of the aorta declines and is e.g. replaced by collagen as in the elderly then compliance of arteries will decrease
SO afterload will increase
The heart will be working harder to maintain the same cardiac output
Describe the impact of increased afterload
The greater the afterload, the more resistance the ventricle has to contraction
SO there is a longer period of ‘isovolumetric’ contraction of the ventricle before the aortic valves open and a shorter period of ejection
SO there is a smaller stroke volume
Compare the ejection of blood volume from each ventricl
Both ventricles eject the same volume of blood
This balances the output of the two sides of the heart
This prevents blood from accumulating in only one of the pulmonary or systemic circulations
Describe how heart failure can arise from increased afterload
If there is increased systemic afterload (vascular resistance) over a prolonged period of time (months, years) then gradually the heart dilates (enlarges) and is unable to pump all of the blood delivered to it from the pulmonary vein
The dilation of the heart passes the limit of Starling’s law
The heart cannot pump as much blood as a normal heart
This results in left heart failure
Give the different heart valves
Atrio-ventricular valves -Right = tricuspid -Left = mitral/bicuspid Semi-lunar valves -Right (pulmonary) -Left (aortic)
Why are there no valves between the vena cava and pulmonary vein and the left and right atria?
How does this differ in heart failure?
When the atria contract, the ventricles ‘suck’ and the blood is forced into the ventricle rather than the vein
In heart failure, the blood can go into the vein and this causes raised venous pressure
Describe the structure of the heart valves and how they work
Heart valves are kept in position by the ‘chordae tendineae’
These are fibrous tendons which attach to the valves
These are also attached to ‘papillary muscles’
Papillary muscles are the first part of the ventricles to contract during systole
They pull on the chordae tendineae and pull the valves closed
Give the sequence of the cardaic cycle
One cardiac cycle is equivalent to one heartbeat
- Atrial systole begins - atrial contraction forces a small amount of additional blood into the relaxed ventricles
- Atrial systole ends and atrial diastole begins
- Ventricular systole - 1) Ventricular contraction pushes the AV valves closed but does not create enough pressure to open the SL valves 2) As the ventricular pressure rises and exceeds the pressure in the arteries, the SL valves open and blood is ejected
- Ventricular diastole - 1) Ventricles relax and pressure in the ventricles drops and blood flows back against the cusps of the SL valves forcing them closed - blood flows into the relaxed atria 2) All the chambers are relaxed and the ventricle fills passively
Describe atrial systole and it’s role
Atrial systole is not essential for normal cardiac output at rest
The elastic recoil of the ventricular walls enlarging during diastole is enough to suck blood into the ventricles without atrial contraction
BUT atrial contraction is necessary to fill the ventricles during exercise as diastole is shortened with increased heart rate - there is less time to fill the expanding ventricles and the atrial contraction gives the blood an extra ‘push’ which helps fill the ventricles
Give the sequence of valve movements in the left and right heart
Left heart:
At the start of systole, left ventricle starts to contract and mitral valve closes
As pressure rises above diastolic, the aortic valve opens
At the end of systole, pressure in the ventricle decreases and aortic valve closes
When the pressure is near zero, mitral valve opens
Similar sequence in the right heart
Explain the heart sounds
S1: The closure of the AV valves followed by the opening of the aortic and pulmonary artery valves is the first heart sound (lub)
S2: The closure of the pulmonary and aortic valves is the second heart sound (dupp)
S3: - faint, low-pitched, due to turbulent flow during the filling NB. may also be heard in adults with heart disease - is s
What is the S3 sound of the heart?
Sometimes a third heart sound can be heard in children and young adults
This is a faint, low-pitched sound that coincides with the period of rapid ventricular filling - due to turbulent filling
This may be heard in adults with heart disease - is usually a sign of serious heart damage; possible damaged valves
What is meant by Jugular Venous Pulse (JVC)?
There are no valves between the vena cava and pulmonary vein and the atria - this is because the flow here is at such a low pressure that a valve would significantly increase resistance to flow SO when the right atrium contracts, a backpressure occurs in the jugular vein
This is felt as a faint pulse and forms the JVP
There are three peaks in the venous pulse:
A: Atrial contraction just before the tricuspid valve closes
C: Rising pressure in the atrium once tricuspid closes
V: As the valve bulges again the ventricle reaches peak contraction
Describe venous return to the heart and how it is mediated
Three main factors
1) One way valves in the veins - prevents backwards flow but lack of muscle activity in e.g. legs can lead to pooling and stasis of blood - can lead to clot or thrombosis formation
2) Muscular pumps - Contraction of muscles in the limbs squeezes the veins
3) Thoraco-abdominal pump - During inspiration, pressure in the thoracic cavity is reduced and this pulls blood into the inferior vena cava - On exhalation thoracic pressure increases and blood is forced into the right atrium
Draw a diagram of the pressure profiles in the left atrium, left ventricle and aorta for a single cardiac cycle
Indicate the points at which the cardiac valves open and close and the periods of iso-volumetric contraction and relaxation
Need to find this
