Lecture 5 - Control of Cardiac Output Flashcards
Define Afterload
the load the heart must eject blood against
roughly equivalent to aortic pressure - if the aortic pressure rises ( due to a rise in TPR) the Left ventricle must push harder to overcome this pressure rise
rise in TPR will reduce the venous pressure, so reduce the filling of the heart
Define Preload
amount the ventricles are stretched (filled) in diastole - related to the end diastilolic volume or central venous pressure
Define Total Peripheral Resistance
or systemic vascular resistance -
resistance to blood flow offered by all systemic vasculature - if TPR increases then the resistance to heart pumping out blood increases
what occurs to pressure of blood as it encounters resistance ?
arterioles are the main resistance vessel - the narrow lumen gives a high resistance - so the provide a significant pressure drop
arterioles can constrict to increase resistance - so pressure in capillaries and venous side will fall
consequently the pressure in the arteriole side will rise
What happens in the TPR Falls ?
if TPR falls and CO is unchanged
blood flows through easier - less pressure lost to the resistance (now lower)
so venous pressure is higher
and arterial pressure is lower
What happens in the TPR rises ?
if TPR rises and CO is unchanged
blood flows through is harder - more pressure lost to the resistance (now higher)
Venous pressure is lower
arterial pressure is higher
What happens if Cardiac output rises ?
if CO rises and TPR is unchanged
blood flows through is harder - as higher pressure from heart - higher pressure means a greater resistance
Venous pressure is lower
arterial pressure is higher
What happens if Cardiac output Falls ?
if CO falls and TPR is unchanged
blood flows through is easier - due to a lower pressure produced in heart - so less resistance
Venous pressure is higher
arterial pressure is lower
how does the body respond to changes in demand for blood ?
- The heart must meet changes in demand for blood
- If the tissues need more blood the arterioles and precapillary sphincters will dilate
- Therefore peripheral resistance falls
- The heart needs to pump more so that arterial pressure does not fall and venous pressure doesn’t rise
- The heart ‘sees’ changes in this demand as changes in arterial blood pressure (aBP) and central venous pressure (CVP)
- The heart responds to changes in CVP and aBP by INTRINSIC - starling law and EXTRINSIC - autonomic nervous system, mechanisms
cardiac output and stroke volume
Cardiac Output = Stroke Volume x Heart Rate
• Stroke Volume = end diastolic volume – end
systolic volume (the volume of blood left in the heart after systole)
• SV = EDV - ESV
• Typical stroke volume in ‘average text-book man at
rest’ is about 70ml
• Can increase SV by increasing EDV or decreasing
ESV
what is ventricular filling ?
filling of the ventricles , which occurs during diastole
the mitral and tricuspid valves are open, while the pulmonary and aortic valves are closed
ventricles fills until walls stretch enough to produce an intraventricular pressure equal to the venous pressure
the higher the venous pressure, the more the heart fills
the more the heart fills the higher the left ventricular pressure - this relationship is the ventricular compliance curve
compliance can increase or decrease in disease states
what is the frank sterling law of the heart
and hence the starling curve - sketch and explain
check against lecture
• Like skeletal muscle – if you stretch the fibres of the heart before contracting, it will contract harder
• The more the heart fills, the harder it contracts (up to a limit)
• This is the Frank – Starling Law of The Heart
• The harder the heart contracts, the bigger the stroke volume
• An increase in venous pressure will fill the heart more
– How much the ventricles fill depends on the compliance
starlings law of the hear ensures both sides are balanced
The increased stroke volume with increased filling of the heart is and INTRINSIC control mechanism • It ensures that both sides of the heart pump maintain the same output • The pulmonary and systemic circulations operate in series – The same volume of blood pumped to the body must also be pumped to the lungs
Increasing venous return leads to increased
left ventricular end-diastolic pressure
(LVEDP) and volume (‘increased preload’).
This causes an increase in stroke volume, so
that the extra blood is pumped out of the
ventricle. - not vital
sketch the length tension curve for cardiac muscle
If sarcomere length is too short filament
overlap interferes with contraction
• In cardiac muscle also get an increase in
calcium sensitivity as the muscle fibres
are stretched
check against lecture
what is contractility ?
contractility is the force of contraction for a given fiber length
extrinsic factors such as sympathetic stimulation and circulating adrenaline can increase contractility
what factors determine CO
• How much the ventricle empties (end systolic volume) depends on
1. How hard it contracts
determined by the end diastolic volume (how much the heart fills)
and contractility (increased by sympathetic drive)
2. How hard it is to eject blood
determined by aortic impedance (roughly arterial pressure)
- Cardiac Output = Stroke Volume x Heart Rate
- Contractility and heart rate are controlled by the autonomic nervous system
• A decrease in arterial BP will reduce parasympathetic NS activity and stimulate sympathetic NS increase heart rate and increase contractility
