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
→ Increases stroke volume due to Starling’s Law
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 it 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 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
What is an acute rise in blood pressure offset by?
→ Starling’s law
→ Local positive inotropes
→ Baroreflex
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
