control of cardiac output Flashcards
what is cardiac output and what does it affect ?
- amount of blood ejected from the heart per minute
- affects blood pressure and blood flow
what is the equation for blood flow (CO) ?
CO = Blood Pressure / Total peripheral resistance
what is preload ?
Stretching of heart at rest, increases stroke volume, due to Starling’s law.
describe starlings law of the heart
- energy of cardiac muscle contraction is relative to the muscle fibre length at rest.
- greater stretch of ventricle in diastole then greater energy of contraction and greater stroke volume achieved in systole
- more blood in more blood out
what happens during an unstretched fibre ?
*Overlapping actin/myosin
*Mechanical interference
*Less cross-bridge formation available for contraction
what happens during a stretched fibre ?
- Less overlapping actin/myosin
- Less mechanical interference
- Potential for more cross-bridge formation
- Increased sensitivity to Ca2+ ions
what are the roles and effects of starlings law ?
- balances output of R and L ventricles
- Responsible for fall in cardiac output during a drop in blood volume or vasodilation (eg. hemorrhage, sepsis).
*Restores cardiac output in response to intravenous fluid transfusions.
*Responsible for fall in cardiac output during orthostasis (standing for a long time) leading to postural hypotension & dizziness as blood pools in legs.
*Contributes to increased stroke volume & cardiac output during upright exercise.
what is afterload ?
- the amount of pressure that the heart needs to exert to eject the blood during ventricular contraction, reduces stroke volume, due to Laplace’s law.
- increased by increasing pressure & radius
- Reduced by increasing wall thickness
describe laplace’s law
- 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 contraction energy is needed 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)
why does radius affect wall stress/afterload ?
Small ventricle radius
Greater wall curvature
More wall stress directed towards centre of chamber
Less afterload
Better ejection
Larger ventricle radius
Less wall curvature
More wall stress directed through heart wall
More afterload
Less ejection
what is the importance of laplace’s 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
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.
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 is acute rise in blood pressure offset by ?
Starling’s law - increased stretch give increased contraction and increased stroke volume
Local positive inotropes (substance that changes force of contraction, eg noradrenaline)
Baroreflex - decreased sympathetic tone which decreases blood pressure
what does chronic increase in arterial blood pressure cause ?
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, high blood pressure will reduce cardiac output
what is hypertrophy ?
Increased radius (r) eg. Heart failure where heart does not contract properly (myocardial infarction, cardiomyopathies, mitral valve regurgitation) blood left in ventricle leading to eventual volume overload.
Increased pressure (P) eg. Pressure-overload heart failure due to increased pressure/afterload in chamber (hypertension, aortic stenosis).
The heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres), increasing wall thickness. This decreases wall stress per sarcomere and therefore afterload so maintains SV and CO.
what is energy of contraction ?
Energy of contraction is the amount of work required to generate stroke volume.
Depends on Starling’s Law and contractility
what are the functions of stroke work ?
Contracts until chamber pressure > aortic pressure (isovolumetric contraction).
Ejection from ventricle.
describe increased preload during excercise
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, shown by an increase in the width of the PV loop.
describe increased afterload during hypertension
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 increasing end-systolic volume (ESV).
