Regulation of stroke volume Flashcards
What set the HR and how?
Pacemaker cells in the sinoatrial node
SA reaches threshold first and evokes an AP and wave of depolarisation through the atria
The atria contracts and after a short delay so do the ventricles.
What effect does sympathetic and parasymthapetic fibres do to heart rate?
Sympathetic = increases HR
Parasympathetic = decreases HR
How does the sympathetic nervous system act?
Sympathetic fibres release noradrenaline and adrenaline.
These act on (adrenergic receptors) B1 receptors on SA node
What is the effect of sympathetic nervous system on HR (detailed)?
Increases slope of pacemaker potential
- increase ion flow through funny Na and Ca channels causing pacemaker cells to depolarise to threshold sooner and evoke AP
- Decreases cardiac interval and increases HR
What nerve supplies parasympathetic supply to the heart?
Vagus nerves
What neurotransmitter is release by vagus nerve and what does it act on?
Acetylcholine
Acts on muscarinic receptors on SA node
What is the response of the heart to parasympathetic activation?
Hyperpolarises cells and decreases slope of pacemaker potential
- Take longer to reach threshold and evoke AP
- Increase cardiac intervals and decrease HR
What is the effect of sympathetic nervous system on myocytes and how does it do this?
Increases contractility
- More release of calcium
- More cross bridges between actin and myosin
- Shorter &stronger contractions
- Calcium taken up quicker
What is the effect of parasympathetic nervous system on contractility and why?
Little effect on contractility
- Vagus nerve innervates the SA nod but not ventricular myocytes
What is preload?
How full the ventricle is before it starts contracting (i.e. the EVD)
What affects preload?
The state of contraction of the venules and veins
What is contractility?
How strong a contraction is produced for any given preload or afterload
What affects contractility?
Sympathetic system
What is afterload?
How difficult it is for the heart to pump blood out. (i.e. the TPR)
What is afterload affected by?
The state of contraction of the arterioles
How can veins and venules increases EDV?
Veins and venules are squeezed which pushes more blood back to the heart
Describe Starling’s curve?
Energy of contraction is proportional to the initial length of the cardiac muscle
Tension is proportional to preload (length)
Starlings law: what happens at peak tension?
Optimal number cross bridges forming between actin and myosin
Gives really strong muscle contraction
Starlings law: what happens at longest muscle
Muscle at larger length so fewer cross bridges can form
Weaker contraction
Starlings law: At smallest and least tension
Too much overlap and actin and myosin filaments are interfering with each other
Less cross bridges
Insufficient contraction
What happens to SV when there’s a small EDV?
Vessels not being stretched
Weak contraction
Small stroke volume
What happens to SV when there’s a large EDV?
Stronger contraction
Bigger stroke volume
What effect does increasing venous return have on SV?
Increases EDV and stretches cardiac muscle more.
Increasing stroke volume.
What effect does decreasing venous return have on SV?
Decreases EDV
Decreasing stroke volume
What determines afterload and what is it dependent on?
The arterial pressure against which the blood is ejected - this in turn depends on the total peripheral resistance (TPR)
What effect does increasing TPR have on SV?
Stroke volume will decrease
(more energy is “wasted” building up enough pressure to open the aortic valve)
What stops the muscles contracting during systole?
Mitral valve
- Ventricle has to build up pressure to push the mitral valve closed
Aortic valve
- Aortic pressure affected by how much blood your trying to pump out and how easy it is to get it into the arterioles
What happens to TPR if arterioles are constricted?
Increases:
- Aortic pressure will increase
- Ventricle will have to work harder to push open aortic valve
- Has less energy to eject blood
- Stroke volume decreases
What does cardiac output equal?
HR x stroke volume
What does CO determine?
How much blood and thus O2 is getting to tissues
What happens to CO and SV if you increase HR and why?
CO increases
SV decreases
Increased HR = shorter cardiac intervals
- Shorter rapid filling phase
- Decreased EDV and preload
- Cardiac muscle will stretch less
- Decreased strength of contraction
- Stroke volume will fall
What 4 things happen during exercise?
Increased HR
Increased contractility
Increased venous return
Decrease in total peripheral resistance
How does exercise increase HR?
Decreases vagal tone
Increased sympathetic tone
How does exercise increase contractility?
Increased sympathetic tone
Alters inotropic state and shortens systole
How does exercise increase venous return?
Skeletal muscle pump
- Squishes veins/venules which pushes more blood back to the heart increasing EDV
How does exercise increase TPR?
Arteriolar dilation in muscles, skin and heart
- Makes it easier for the heart to pump blood
Reduces afterload
By how much can exercise mechanisms increase CO?
4 fold
