Regulation of the Stroke Volume and Heart Rate Flashcards
What is the function of the sympathetic nervous system in the regulation of heart rate?
Sympathetic nervous system releases noradrenaline plus circulating adrenaline from adrenal medulla. Both act on Beta 1 receptors on sinoatrial nodes. That acts to increase the slope of the pacemaker potential. It does this because it increases the ion flow through funny sodium channels and through the calcium channel. This causes the pacemaker cells to depolarise to threshold sooner and evoke their action potential.
This therefore increases heart rate.
How does the parasympathetic nervous system regulate the heart rate ?
Vagus nerve releases acetylcholine. This acts as a cholinergic muscarinic receptor on the sinaotrial node. This hypopolarises the pacemaker cells and decreases the pacemaker potential. This means that the pacemaker cells take longer to reach threshold and fire their action potentials. That will increase the cardiac interval and therefore decrease the heart rate (bradychardia)
What is preload ?
Preload is how filled the ventricle is before it starts contracting
What does starlings law state?
Starlings law states that the energy of contraction is proportional to initial length of the cardiac muscle fibre
When is the muscle contraction the strongest ?
Muscle contraction is the strongest at the peak tension. This is because there is an optimal number of cross bridges forming between the filament, the actin, the thick filament and myosin.
What streches the cardiac muscles?
The end diastolic volume (the volume of blood filled into the ventricles before they contract) streches the cardiac muscles.
How is the stroke volume controlled?
When venous return is increased, the end diastolic volume will increase and therefore the stroke volume will increase. When venous return is decreased the end diastolic volume will decrease and the stroke volume will decrease.
What is afterload ?
Afterload is the load against which the heart muscles tries to contract
What happens during the isometric contraction phase ?
During the isometric contraction phase, enough pressure needs to be built up in the left ventricle so the pressure in the left ventricle exceeds the pressure in the aorta and that pushes the aortic valve open.
Why does the mitral valve need to be closed during systole?
During systole, the muscle is trying to shorten and first thing stopping that is the mitral valve. Therefore the ventricle needs to bulid up pressure to push the mitral valve closed. The aortic valve being closed is stoping the left ventricle from contracting.
What does afterload depend on?
Afterload is determined by the arterial pressure against which the blood is ejected - this in turn depends on the total peripheral resistance (how constricted and dilated the arterioles are)
If total peripheral resistance increases, the stroke volume will decrease
What is the ionotropic effect in the sympathetic nervous system?
Beta 1 receptors on the myocytes are activated and this increase contractility. This is termed an ionotropic effect. During this, there is more release of calcium within the cariomyocytes that causes more crossbridges to form between the actin and myosin filaments and therefore result in a stronger and shorter contraction. As well increased release of calcium more calcium is being taken up as well
What is the effect of the parasympathetic nervous system?
Parasympathetic nervous system has very little effect. Vagus nerve does not innervate the ventricular muscle. Acetylcholine will not be being released and this has a little affect on the contractility.
What is cardiac output ?
Cardiac output = Stroke volume X heart rate
Cardiac output determines how much blood and oxygen is getting to your tissues.
What increases cardiac output?
increasing heart rate will cause a small increase in the cardiac output but stroke volume starts to decrease.
What does shortening of the cardiac interval result in?
Shortening the cardiac interval cuts into the rapid filling phase, that reduces the end diastolic volume and therefore reduces preload according to starlings law that will reduce the force of contraction and therefore reduce stroke volume.
How is the cardiac output effected during exercise ?
- HR increases
- Contractility of the heart increases
- Venous return increases
- Total peripheral resistance decreases
By working together these mechanisms can increase your cardiac effort by 4-6 folds.
How does the heart rate increase during exercise ?
This happens due to decreased vagal tone (tonic activity). At rest some acetylcholine is being released and that slows down the intrinsic heart rate of pacemaker cells, therefore decreased vagal tone will increase the heart rate.
Increased HR during exercise also happens due to increased sympathetic tone and that also causes heart rate to increase
How does contractility of the heart increase during exercise ?
This is controlled by the sympathetic nervous system, by increased sympathetic tone. It alters the iontropic state. There is stronger force of contraction and more crossbridges forming between actin and myosin filaments and you also get a shorter contraction which shortens the systole, allowing more time for diastole therefore the heart has more time to fill with blood.
How does venous return increase during exercise ?
This is due to venoconstriction and skeletal/respiratory pumps pushing more blood back to the heart. This increases the diastolic volume and therefore maintain preload.
How does total peripheral resistance decrease during exercise ?
This is due to the arterioles suppling the regions of the body that are most important during exercise such as skeletal muscle, cardiac muscle and skin, dialate. The arterioles dilating makes it easier for the heart to pump blood. This reduces afterload
