Control of Cardiac Output Flashcards
What is cardiac output (CO)?
CO is the amount of blood ejected from the heart per minute
What factors affect Cardiac Output (CO)?
- Heart rate (HR)
- Stroke Volume (SV)
cardiac output changes according to demand
What is Heart rate?
How often the heart beats per minute (bpm)
What is stroke volume?
How much blood (ml) is ejected per heartbeat
Outline the equation used to calculate Cardiac Output?
CO = HR x SV
At rest what is the average CO value?
At rest: 70 bpm x 70 ml = 5 litres/min
During exercise what is the average CO value?
during exercise: 180 bpm x 120 ml = 20 litres/min
What does CO affect?
Cardiac output affects blood pressure and blood flow
Give the equation used to calculate blood pressure
BP = CO x TPR
BP - blood pressure
CO - cardiac output
TPR - total peripheral resistance
What is heart rate controlled by?
Heart rate and contractility is controlled by the SA node pacemaker which is controlled by the autonomic nervous system (para/sympathetic)
What affects the strength of heart contraction?
The strength of contraction is due to parasympathetic nerves and circulating adrenaline, increasing intracellular [Ca2+]
What is preload?
The stretching of the left ventricle on filling (heart at rest) which increases SV due to Starling’s Law
What is Afterload?
Opposes ejection, reducing SV due to Laplace’s Law
What is contractility?
The strength of heart contraction
What is heart rate?
Beats per minute controlled by the ANS
What is the Energy of Contraction?
The amount of work required to generate the Stroke volume
What is the energy of contraction dependent on?
It’s dependent on Starling’s Law and contractility
What are the 2 functions carried out by stroke work?
- contracts until chamber pressure > aortic pressure
2. Ejection from ventricle
What are the respective effects of preload and afterload on the stroke volume?
preload increases SV, afterload opposes SV
What is Starling’s Law?
The energy of contraction of cardiac muscle is relative to the muscle fibre length at rest.
Greater the stretch of the ventricles in diastole, the greater the energy of contraction and stroke volume in systole
more blood in = more blood out
How can we calculate Stroke volume?
SV = EDV - ESV
SV - stroke volume
EDV - end diastolic pressure
ESV - end systolic pressure
How does excess filling of ventricles occur?
Due to overstretched muscles, decreasing SV
consideration with fluid replacement
Describe Starling’s Law on a molecular basis when fibres are unstretched
Overlapping of actin / myosin
mechanical inference - less cross-bridge formation available for contraction
Describe Starling’s Law on a molecular basis when fibres are stretched
Less overlapping actin and myosin
less mechanical inference so potential for more crossbridge formation and increased Ca2+ sensitivity
What are the effects of Starling’s Law?
- Balances outputs of right & left ventricles
- Decreases CO during vasodilation or blood volume
drops (e.g. haemorrhages / sepsis etc.) - Restores CO after intravenous fluid transfusions
- Falls in CO during orthostasis leading to postural
hypertension & dizziness due to blood pooling in legs - Contributes to increased SV and CO during upright
exercise
What is Afterload Laplace’s Law?
Laplace’s Law describes parameters that determine afterload
- Afterload opposes ejection of blood from the heart and
is determined by wall stress directed through the heart
wall. Stress through the heart wall reduces muscle
contraction
How does Laplace’s Law affect Energy of contraction?
More energy of contraction is required to overcome wall stress to produce cell shortening and ejection
Outline the equation for Laplace’s Law
T = Pr/2
(divided by 2 as chambers have 2 directions of curvature)
T - wall tension
P - pressure
r - radius in chamber
wall tension T = wall stress S x wall thickness w
T = SW so SW = Pr/2 or S = Pr/2w
How is afterload increased?
Afterload increased by increasing pressure and radius
How do we decrease afterload?
Afterload is reduced by increasing wall thickness
What is afterload wall stress determined by?
Mainly, wall stress is affected by radius
Describe the effects of a small ventricle radius
- greater wall curvature
- more wall stress directed to centre of chamber
- less afterload
- better ejection
Describe the effects of a large ventricle radius
- less wall curvature
- more wall stress directed through heart wall
- more afterload
- less ejection
