Control of cardiac output Flashcards
Discuss cardiac hyper trophy
This is the thickening or enlargement of the heart as a response to increased workload or stress.
What is the definition of cardiac output
Cardiac output- the amount of blood ejected from the heart per minute
What is the definition of heart rate and stroke volume
heart rate- how often the heart beats per minute
Stroke volume- how much blood (ml) is ejected per beat
What is the equation for cardiac output
cardiac output= heart rate x stroke volume
what is the formula for blood flow
Blood flow (CO)= Blood pressure (BP) / Total peripheral resistance (TPR)
Explain what preload and afterload is
Preload- Stretching of the heart at rest, increases stroke volume due to Starling’s law
After load- Opposes ejection, reduces stroke volume due to Laplace’s law.
Explain how preload and afterload are important in stroke volume
Heart rate and contractility- SA node (bundle of cells in right atria) pacemaker as well as sympathetic and parasympathetic nerves control the heart rate
Strength of contraction due to sympathetic nerves and circulating adrenaline increases intracellular calcium which leads to depolarisation
Explain preload- Starling’s law of the heart
- energy of contraction of cardiac muscle is relative to the muscle fibre length at rest
-The greater the stretch of ventricle during relaxation (diastole) then the greater energy of contraction
-therefore greater stroke volume is achieved in systole (contraction)
Explain preload as the molecular basis of starling’s law
There are two fibres:
Unstretched fibre- Overlapping of actin and myosin. There is mechanical interference ad less cross-bridge formation available for contraction
Stretched fibre- Less overlapping of actin and myosin, less mechanical interference and more potential for cross bridge formation due to increased sensitivity to Ca2+ ions
Explain the roles and effects of Starling’s law (preload)
1) Balances outputs of the right ventricle and left ventricle
2) Responsible for fall in cardiac output during a drop in blood volume or vasodilation
3) Restores cardiac output in response to intravenous fluid transfusions
4) Responsible for fall in cardiac output during orthostasis (standing for long period of time) leading to postural hypotension and dizziness as blood pools in legs.
5) Contributes to increased stroke volume and cardiac output during upright exercise
Explain afterload-Leplace’s law
-Afterload opposes the contraction that ejects blood from the heart and is determ by wall stress directed through the heart wall.
-more energy of contraction is needed to overcome this wall stress to produce cell shortening ejection.
Leplace’s law describes parameters that determine afterload
Describe the parameters in Leplace’s law that determines afterload
Wall tension (T)
Pressure (P)
Radius in a camber
Explain wall tension, pressure and radius in a chamber’s relationship
Wall tension is proportional to pressure and radius in the chamber
What is the equation for wall stress
Wall stress (S) = Tension (T)/ wall thickness (w)
Why does the radius affect wall stress/afterload?
Small ventricle radius:
-greater wal curvature
-more wall stress directed towards centre of chamber
-less afterload
-better ejection
Larger ventricle radius:
-less wall curvature
-more wall stress directed through the heart wall
-more afterload
-less ejection
Huge theoretical radius:
-negligible wall curvature
-virtually all stress directed through wall
Explain the importance of Laplace’s law
Laplace law is important because:
1) Opposes Starling’s law at rest:
-increased preload gives an increased stretch of chamber (describes Starling’s law). This increases chamber radius (decreases curvature) and increases afterload in a healthy heart.
2) Facilitates ejection during contraction:
-Contraction reduces chamber radius so less afterload as chamber empties, this aids expulsion of the last portion of blood and increases stroke volume
3) Contributes to a failing heart at rest + during contraction:
-in a failing heart the chambers are often dilated and the radius is large- so increased afterload opposing ejection
Describe Laplace’s law in simple terms
Laplace’s law means good ejection with a small radius, bad with a large radius
Describe Laplace’s law
Laplace’s law states that increased BP will increase wall stress- this will increase afterload and reduce ejection
What are acute (small) rises in blood pressure offset by?
Acute rises in blood pressure offset by:
Starling’s law- increased stretch gives increased contraction and increased stroke volume. Local positive inotropes (substance that changes force of contraction e.g. noradrenaline)
Baroreflex- decreased sympathetic tone which decreases blood pressure
What happens when theres a chronic increase in arterial 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 the efficiency of the heart
What happens when theres is an increased radius in the heart according to Laplace’s law
Increased radius can lead to heart failure as the blood is left in the ventricle and leading to eventual volume overload
What happens when there is an increased pressure according to Laplace’s law
Increased pressure can lead to pressure overload, heart failure due to hypertension in the chambers
How does the heart compensate for these changes
The heart compensates with ventricular hypertrophy, this increases wall thickness. In turn, wall stress decreases and therefore afterload
However this requires more 02- the amount of energy required will increase so contractility will decrease so can lead to a cycle of heart failure
Explain the energy of contraction
Energy of contraction is the amount of work required to generate stroke volume .
This depends on Starling’s law and contractility
Stroke work carries out two functions:
1) contracts until chamber pressure > aortic pressure (isovolumetric contraction
2) Ejection from ventricle
Explain preload and ventricular pressure volume loop during exercise
Exercise increases preload
During exercise, increased venous return leads to increased preload and higher end diastolic volume (EDV)
The ventricle will eject blood to the same end systolic volume (ESV) so there is an increase in stroke volume
Explain afterload ad ventricular pressure-volume loop
Hypertension leads to increased afterload which reduces ejection of blood
-there is a longer time spent in isovolumetric contraction to increase pressure in the chamber above than in the aorta to open the valve.
-this uses more energy and lowers the force of contraction by reducing stroke volume and increasing end -systolic volume (ESV)
