Control of Cardiac Output Flashcards
Define cardiac output
What is heart rate and stroke volume?
Cardiac output (CO) – the amount of blood ejected from the heart per minute.
How often the heart beats per minute (heart rate, HR) and how much blood (ml) is ejected per beat (Stroke volume, SV)
What is the equation that links cardiac output, heart rate and stroke volume?
And does cardiac output affect blood pressure and blood flow? If so what is the equation for that links these 3 things?
CO = HR x SV
Cardiac output affects blood pressure and blood flow.
BP = CO (blood flow) x TPR
What are preload and afterload?
What are heart rate and contractility?
• Preload - Stretching of heart at rest, increases stroke volume, due to Starling’s law. If more blood enters the heart it contacts harder.
• Afterload - Opposes ejection, reduces stroke volume, due to Laplace’s law. Applying stress to the wall of the heart prevents it from contracting.
1. Afterload - resistance to ejection.
2. Preload – stretching of left ventricle on filling.
• Heart rate & contractility – SA node pacemaker but we also have the sympathetic and parasympathetic nerves which control heart rate.
• Strength of contraction due to sympathetic nerves & circulating adrenaline increasing intracellular calcium.
3. Heartrate - bpm, autonomic nervous system
4. Contractility - strength of contraction.
What is the energy of contraction and what does it depend on?
Energy of contraction is the amount of work required to generate stroke volume.
Depends on Starling’s Law and contractility
What are the two functions of stroke volume?
- Contracts until chamber pressure > aortic pressure (isovolumetric contraction).
- Ejection from ventricle.
What does Starlings Law state (Preload)
‘Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest’
Greater stretch of ventricle in diastole (blood entering).
…then greater energy of contraction.
…and greater stroke volume achieved in systole.
• more blood in = more blood out
• Intrinsic property of cardiac muscle
• (nerves, hormones etc. not involved)
Describe Starlings experiments
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Describe the molecular basis of Starlings Law
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What are the roles and effects of Starlings Law? (5)
- Balances outputs of the right ventricle and left ventricle – important.
- Responsible for fall in CO during a drop in blood volume or vasodilation (eg. haemorrhage, sepsis).
- Restores CO in response to intravenous fluid transfusions.
- Responsible for fall in CO during orthostasis (standing for a long time) leading to postural hypotension & dizziness as blood pools in legs.
- Contributes to increased SV & CO during upright exercise.
What is afterload?
Afterload opposes the contraction that ejects blood from the heart and is determined by wall stress directed through the heart wall. Stress through the wall of the heart prevents muscle contraction
More energy of contraction is need to overcome this wall stress to produce cell shortening and ejection. Laplace’s law describes parameters that determine afterload:
Wall tension (T), pressure (P), and radius (r) in a chamber (ventricle)
T α P r
Wall stress (S) is made up of tension (T) over wall thickness (w)
S = T/W so S = Pr/ 2W
What is the importance of Laplaces Law?
- Opposes Starling’s law at rest
- Increased preload gives increased stretch of chamber (Starling’s law)
- This increases chamber radius (decreases curvature) – increase afterload
- In a healthy heart, Starling’s Law overcomes Laplace’s – so ejection is OK.
- Facilitates ejection during contraction
- Contraction reduces chamber radius so less afterload in ‘emptying’ chamber.
- This aids expulsion of last portion of blood and increases stroke volume.
- Contributes to a failing heart at rest and during contraction
- In a failing heart the chambers are often dilated and radius is large - so increased afterload opposing ejection.
- Laplace’s Law is good ejection with small radius, bad with large radius.
What does Laplaces Law State?
Laplace’s law states that increased blood pressure (P) will increase wall stress This will increase afterload & reduce ejection.
Acute rises in blood pressure offset by…
Starling’s law - increased stretch give increased contraction and increased SV
Local positive inotropes (noradrenaline)
Baroreflex - decreased sympathetic tone which decreases blood pressure.
Chronic increase in arterial blood pressure
Increased energy expenditure attempts to maintain stroke volume but ultimately stroke volume will gradually decrease.
Decrease in blood pressure would increase efficiency of the heart.
This is why blood pressure needs to be kept fairly constant during exercise, a high BP will reduce CO.
Describe Laplace’s law & hypertrophy in heart failure
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Describe Starling’s law & ventricular pressure-volume loop
During exercise increased venous return leads to increased preload and more stretch.
This causes a shorter isovolumetric contraction phase, and increase in SV due to Starlings law.
More blood back to the heart, more blood ejected from the heart.
Describe Laplace’s law & ventricular pressure-volume loop
With high blood pressure (in red) increased afterload.
Longer time spent in isovolumetric contraction to increase pressure in the chamber above that in aorta to open the valve. This uses more energy and lowers the force of contraction reducing stroke volume.
