Control of Cardiac Output Flashcards
what is the definition of cardiac output?
→amount of blood ejected from the heart per minute
what is the equation for cardiac output?
→CO = HR x SV
what is the equation for blood pressure?
→BP = CO x TPR (total peripheral resistance)
what does cardiac output determine?
→blood pressure
→blood flow
what is cardiac output proportional to?
how often the heart beats PM
→ how much blood is ejected per beat
what controls stroke volume? (list)
→ preload
→ heart rate
→ contractility
→ afterload
what is the equation for blood flow (CO)?
CO = BP/TPR
what is preload?
→ The stretching of the heart at rest. Heart is stretched by the ventricles
→ Increases stroke volume due to Starling’s Law.
Increases energy of contraction
what is afterload?
→ Opposes ejection
→Reduces stroke volume due to Laplace’s law
what controls heart rate?
→ sympathetic and parasympathetic nerves control heart rate
What is energy of contraction and what does it depend on?
→ amount of work done required to generate stroke volume
→ depends on Starling’s Law and contractility
What two functions does stroke work carry out?
→ Increases chamber pressure to make it greater than aortic pressure (isovolumetric contraction)
→ Ejection from the ventricle
what do afterload and preload do to the stroke volume?
→ Preload increases the stroke volume and afterload opposes the stroke volume
State Starling’s law
Energy of contraction in cardiac muscle is relative to the muscle fibre length at rest
→the greater the stretch of the ventricle in diastole (blood entering)
→the greater the energy of contraction
→ a greater stroke volume is achieved in systole
what is the equation for stroke volume?
→ SV = end diastolic volume - end systolic volume
Describe an unstretched fibre
→ overlapping actin/myosin
→ mechanical interference
→ Less cross-bridge formation available for contraction
Describe a stretched fibre
Less overlapping actin/myosin
→ Less mechanical interference
→ potential for more cross-bridge formation
→ Increases sensitivity to Ca2+ ions
What are the roles of Starling’s law during preload?
Balances the output of the right and left ventricle which is very important
→ responsible for the fall in cardiac output during a drop in blood volume
→ Restores cardiac output in response to IV fluid transfusions
→ Responsible for fall in cardiac output during orthostasis leading to postural hypotension + dizziness
→ contributes to increased stroke volume and cardiac output during upright exercise
What is afterload determined by?
it is determined by wall stress directed through the heart wall
→stress through the wall of the heart prevents muscle contraction
What is the equation for Laplace’s law?
→ P = 2t/r
T = wall tension
P = pressure
R = radius
what is the equation for wall tension?
→ T = S (wall stress) x ( wall thickness) W
How is afterload increased and how is it reduced ?
→ by increasing pressure + radius and reduced by increasing wall thickness
what happens if there is a small radius (in terms of afterload)?
→ greater wall curvature
→ more wall stress directed towards the center of the chamber
→ less afterload
→ better ejection
what happens if there is a big radius (in terms of afterload)?
→ less wall curvature → more wall stress directed through heart wall more tension in the wall →more afterload → less ejection
How does Laplace’s law oppose Starling’s law at rest?
Increased preload gives increased stretch of chamber
→increases chamber radius and decreases curvature
→ increases afterload
→ opposes ejection of blood from a full chamber
What law takes precedence in a healthy heart?
→Starling’s law overcomes Laplace’s
How does Laplace’s law facilitate ejection during contraction?
→ Ventricular contraction reduces the chamber radius and increases curvature
→Laplace’s law states there will be less afterload in the emptying chamber
→ This aids expulsion and increases stroke volume
what do the chambers look like in a failing heart?
→ In a failing heart the chambers are often dilates - increased radius
How does Laplace’s law contribute to a failing heart at rest and during contraction?
→ In a failing heart the chambers are often dilated
→increased afterload opposing ejection
What does Laplace’s law state about blood pressure and wall stress?
→ Increased blood pressure will increase wall stress which increases afterload and reduce ejection
What is an acute rise in blood pressure offset by?
→ Starling’s law-increased stretch give increased contraction and increased stroke volume
→ Local positive inotropes- noradrenaline
→ Baroreflex– decreased sympathetic tone which decreases blood pressure.
What is the baroreflex?
→ decreased sympathetic tone
→ decreasing blood pressure
What is chronic increase in arterial blood pressure caused by?
→ increased energy expenditure to maintain stroke volume
→ ultimately decrease in stroke volume
what does decreasing blood pressure do to the heart?
→ increases the efficiency of the heart
What happens if there is an increased radius in the heart?
→Heart failure where the heart does not contract properly (MI, cardiomyopathies, mitral valve re-gurgitation)
→ blood is left in the ventricle leading to eventual volume overload
What happens if there is increased pressure in the heart?
→ pressure overload heart failure due to increased pressure
What happens with increase in radius and pressure?
→ wall stress increases which opposes ejection
how does the heart compensate for an increase in radius + pressure?
→ the heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres)
→ Increasing wall thickness
→ decreases wall stress per sarcomere and therefore afterload so maintains SV and CO
why does ventricular hypertrophy eventually cause heart failure?
→ the more sarcomeres used the more O2 is used
→ amount of energy required continues to increase
→ contractility decreases and produces more heart failure
describe Laplace’s law and the ventricular pressure-volume loop with high blood pressure
→ increased afterload
→ a longer time is spent in isovolumetric contraction to increase pressure in the chamber to be > aorta to open the valve
→ this uses more energy and lowers the force of contraction and SV
→end systolic volume increases
describe Starling’s law and the ventricular pressure-volume loop during exercise
→ increased venous return leads to an increase in EDV →increased preload →more stretch →shorter isovolumetric contraction phase → increase in SV due to Starlings law. →More blood back to the heart → more blood ejected from the heart
Describe the importance of Laplace law
Opposes Starling’s law at rest
Increased preload gives increased stretch of chamber (Starling’s law)
This increases chamber radius (decreases curvature) – increasing afterload
In a healthy heart, Starling’s Law overcomes Laplace’s – so ejection is OK.
2.Facilitates ejection during contraction
Contraction reduces chamber radius so less afterload as the chamber empties.
This aids expulsion of last portion of blood and increases stroke volume.
3.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.
What does a high blood pressure mean for cardiac output?
a high blood pressure will reduce cardiac output.
What does higher energy used in isovolumetric contraction mean for ejection?
Higher energy used in isovolumetric contraction reduces energy available for ejection
What happens to afterload in hypertension?
– increased afterload
There is a longer time spent in isovolumetric contraction to increase pressure in the chamber above that in the aorta to open the valve.
This uses more energy and lowers the force of contraction reducing stroke volume and an increasing end-systolic volume (ESV).
More energy required
What are the functions of the stroke volume?
Contracts until chamber pressure > aortic pressure (isovolumetric contraction).
Ejection from ventricle.
What does energy of contraction depend on?
Depends on Starling’s Law and contractility
