Session 6 Flashcards
What is blood draining into the veins determined by?
The balance between:
Rate at which blood enters the veins
Rate at which the heart pumps it out
This all determines how distended the veins become in particular the central veins.
What does total peripheral resistance mean?
Combined resistance of resistance vessels which determine distribution and flow;
measurement of the difficulty of blood leaving vessels.
What happens when total peripheral resistance changes and cardiac output remains the same?
When TPR decreases (easier for blood to leave the arteries), arterial pressure will fall (does not have to rise as high to push the same amount of blood out), venous pressure will rise (alters in opposite direction).
When TPR increased, arterial pressure will rise and venous pressure will fall.
What happens if cardiac output changes and TPR stays the same?
If cardiac output rises and TPR does not change (more blood is pumped through a given resistance), arterial pressure will rise and venous pressure will fall (as more blood is removed from the heart to be pumped out).
If cardiac output falls, arterial pressure will fall and venous pressure will rise.
Describe changes in demand for blood
TPR is inversely proportional to the body’s need for blood.
Within individual tissues, the actions of vasodilator metabolites and other mechanisms will modify flow resistance through arterioles to suit metabolic demand.
Across the whole body the effect of these mechanisms is to make up the total peripheral resistance.
If metabolism changes, TPR changes and generates signals in the form of changes in arterial and venous pressure.
What happens after eating a meal?
The gut needs more blood.
Gut tissues produce local vasodilators which dilate arterioles.
Total peripheral resistance falls.
If cardiac output does not change, arterial pressure will fall due to decreased total peripheral resistance and venous pressure will rise.
Blood pressure may drop so low fainting could occur.
What is demand-led pumping?
If the body needs more blood, the heart needs to pump more to meet the demand.
Demand is expressed as changes in arterial and venous pressure signals.
Heart is sensitive to these changes therefore it is able to meet demand and restore pressures back to normal.
If the heart responds to falls in arterial pressure and rises in venous pressure by pumping more blood, then it will meet the demand and bring arterial and venous pressures back to normal.
What is Stroke Volume?
The difference between: End diastolic volume (after filling during diastole) and End systolic volume (after contraction during systole).
Note: ventricles rare fill to the maximum volume - max diastolic volume.
What happens during ventricular filling?
In diastole the ventricles are isolated from the arteries (valves are closed) and connected to the veins.
Venous pressure drives blood into the ventricles.
The ventricle fills as the walls stretch passively until the intra-ventricular pressure is equal to venous pressure (at this point there is no pressure gradient driving the filling of blood therefore ventricular filling stops).
Describe the relationship between venous filling and ventricular volume
The higher the venous pressure, the more the heart fills in diastole (within limits).
Venous pressure alters stretch of muscle fibres.
As increased blood causes ventricles to rub against rigid pericardium, curve becomes more steep.
Relationship between venous pressure and ventricular volume is known as the ventricular compliance curve.
Stroke volume increases if venous pressure increases.
Stroke volume is determined by how much the ventricles contract during systole. All myocardial cells normally contract so active tension is changed by factors which act directly upon individual myocardial cells. Discuss the mechanical factors acting on the myocardium.
Because of the valves, the mechanical forces are different in diastole and systole.
In diastole, the ventricle is connected to the veins so venous pressure determines the end diastolic stretch or ‘pre-load’ on the myocardium.
Once systole begins, the ventricles are isolated from the veins but connected to the arteries and the force necessary to expel blood into the arteries or the ‘after load’ determined what happens during systole.
Preload and afterload may vary independently.
What is Starlings Law?
If ventricular muscle is stretched (I.e. The more it fills) before contracting (the preload is increased), it contracts harder (up to a limit) during systole.
Therefore if all other things are equal, increases in venous pressure and therefore in end diastolic volume will lead to increases in stroke volume (the harder it contracts the bigger the stroke volume).
Rise in venous pressure automatically lead to rises in stroke volume
The heart responds to changes in venous pressure by pumping more or less blood (altering stroke volume). “More in more out!”
What is the Starling Curve?
Relates stroke volume to venous pressure
Slope is known as the contractility of the ventricle.
Increase in contractility –> increase in stroke volume.
The extent to which the increase in force of contraction is related to the increase in venous pressure.
What does afterload determine?
The effect of a given force of contraction during systole.
If it is easy to eject blood then the volume in the ventricle will fall a lot in systole but pressure will only rise a little.
Therefore falls in total peripheral resistance increase stroke volume by reducing after load.
If it is difficult to eject blood because blood will not readily leave the arteries (TPR high), then the stroke volume will be less but a much higher arterial pressure will be generated.
What is the End Systolic Volume dependent on?
How much the ventricle empties depends on:
How hard the ventricle contracts (the harder it contracts, the more it ejects).
How hard it is to eject blood (for a given force of contraction)
What is the force of contraction determined by?
End diastolic volume (according to Starling’s law: the more heart fills the harder it contracts)
Contractility (slope of curve relating stroke volume to venous pressure).
Contractility is increased by sympathetic activity (so more blood is ejected). Heart failure reduces contractility.
What does aortic impedance depend on?
Total peripheral resistance
The harder it is to eject blood the higher the pressure rises in the arteries
Aortic impedance also depends on stretchiness of artery.
What happens if arterial pressure falls?
The easier it is to eject blood, the more comes out in systole so if arterial pressure falls, end systolic volume will fall and stroke volume will rise.
What are the chemical factors that can affect contractility (dependent on intracellular [Ca2+})?
The force of contraction of the ventricle always arises with pre-load but the slope of this relationship - the contractility - can be affected by neurotransmitters, hormones or drugs acting on the myocardium.
Noradrenaline and adrenaline increase contractility (how much calcium is available intracellularly) (positive inotropy) so increases in sympathetic activity will increase stroke volume at a given pre-load and after load.
What are the direct effects of arterial and venous pressures on stroke volume?
If venous pressure rises, stroke volume will rise
If arterial pressure falls, stroke volume will rise
Describe the control of Heart Rate
Sympathetic activity increases HR
Parasympathetic activity decreases HR
At rest parasympathetic activity predominates (~60bpm)
Increases in heart rate can be produced by turning off the parasympathetic system and increasing sympathetic stimulation.
The ANS can therefore influence cardiac output by changing heart rate and contractility (sympathetic branch only)
What do baroreceptors do?
Monitor arterial blood pressure.
Located in the walls of the aorta and the carotid sinus at the bifurcation of the common carotid artery, they detect changes in arterial pressure.
This information is released to the medulla where collections of neurones - the cardiovascular centre- modify the behaviour of the heart and circulation via the ANS.
What happens when Baroreceptors detect falls in arterial pressure?
Increase heart rate by reducing parasympathetic activity and increasing sympathetic activity
Increase contractility by increasing sympathetic activity.
So heart rate and stroke volume (dependent on two independent factors: contractility and preload) rise.
So via the ANS, falls in arterial pressure increase cardiac output.