11. Control of Cardiac Output Flashcards
What is Cardiac Output?
The volume of blood ejected by each ventricle per minute.
CO = HR x SV
What is heart rate?
Number of contractions per minute.
What is Stroke Volume?
Volume of blood ejected by each ventricle per beat.
Define chronotropic effect.
An influence upon the HR.
What increases the HR?
- Noradrenaline and adrenaline have positive chronotropic effects i.e. increase HR
- Mechanism of Action? Stimulation of hyperpolarisation-activated ion channels
Define Tachycardia
Under pathological conditions a fast HR is termed tachycardia
What decreases the HR?
Parasympathetic nerve
Acetylcholine has a negative chronotropic effect i.e. slows HR
Mechanism of Action? (+) of muscarinic (ACh)-receptor gated K-channels; A component of ‘vagal tone’
Define Bradycardia.
Under pathological conditions a slow HR is termed bradycardia
What is End Diastolic Volume?
volume of blood in a ventricle at the end of loading or filling
What is End Systolic Volume?
volume of blood in a ventricle at the end of contraction (systole).
How does the autonomic nervous system innervate the heart?
Parasympathetic Nerve (Vagus Nerve) to SA node- Acetylcholine is released
Sympathetic Nerve to SA node and bottom of the heart releases noradrenaline/ adrenaline from the adrenal medulla.
What regulates Blood Volume reflexes?
Sensory nerve fibres receive signals from;
- Baroreceptors in the wall of internal carotid artery
- Carotid Body Chemoreceptors
- Baroreceptors in the aorta.
3 Key factors influencing Venous Return/ Central Venous Pressure
- Posture (gravity) - Standing = ↓ venous return
- Muscle Pump: Venomotor tone involving constriction of veins by skeletal muscles. Venous return is facilitated by valves which prevent backflow of blood.
- Respiratory Pump: inspiration creates an internal pressure difference by lowering intra-thoracic pressure and increasing intra-abdominal pressure.
What is the Frank-Starling Law of the Heart?
the force or tension developed in a muscle fibre depends on the extent to which the fibre is stretched
more in = more out
How does preload volume determine EDV and ESV?
Ventricular filling relies on atrial filling which is determined by venous return of blood
↑ Rate of ventricular filling = ↑ SV (proportional)
↑ EDV increase the initial length (stretch) of cardiac muscle fibres ↑ SV and CO
Define Contractility.
contractile ability of cardiac muscle at a given preload
How does contractility determine EDV and ESV?
- ↑ force of ventricular contraction = ↑ stroke volume
- SNS & adrenaline exert positive inotropic effects
- SNS increases velocity of conduction of cardiac impulse
- PNS (ACh) has a negative inotropic influence; mainly exerted by on the atria as vagal nerve doesn’t innervate ventricles
What are inotropic effects?
Substances that alter the contractility of the heart have inotropic effect.
Define Afterload.
- the amount of tension that the contracting ventricle must generate to force open the aortic valve to eject blood.
- Ease of which the blood exits the ventricles and circulatory system.
What influences Afterload?
Influenced by blood vessel tone (i.e. vasoconstriction, vasodilation)
How does Afterload determine EDV and ESV?
- A high opposing pressure = greater isovolumetric contraction + a shortened ejection period + larger residual ESV.
- Increased afterload = decreased SV and decreased CO
- Sustained increases in afterload will weaken the myocardium = heart failure.
- Afterload is increased by factors that elevate blood pressure
