S2L2 - Control Of Cardiac Output Flashcards
What formula is used to work out the cardiac output?
CO = SV x HR
What is stroke volume?
The difference between the end diastolic volume and the end systolic volume
How is venous pressure related to the filling of the ventricles?
There is passive filling of the ventricles during diastole until intraventricular pressure is equal to venous pressure. Therefore the higher the venous pressure, the more the ventricles will fill in diastole.
What determines stroke volume?
The amount that the ventricle contracts during systole
What changes the amount that myocardial cells contract?
May be changed by two different factors: mechanical (intrinsic) or chemical (extrinsic)
What is preload?
preload is the amount of sarcomere stretch experienced by cardiac muscle cells, called cardiomyocytes, at the end of ventricular filling during diastole
Related to EDV and central venous pressure.
What is afterload?
Afterload is the pressure that the heart must work against to eject blood during systole (ventricular contraction). Afterload is proportional to the average arterial pressure.
What are the 2 mechanical factors that effect the stroke volume?
Preload
Afterload
How does preload affect the stroke volume?
The greater the preload (determined by the venous pressure) the more the myocardium is stretched before contraction. The more the myocardium is stretched, the harder it will contract. The greater the contraction, the larger the stroke volume.
How does afterload affect stroke volume?
Afterload determines the effect of a given force of contraction during systole. If total peripheral resistance is low then it will be easy to eject blood from the ventricle. Therefore if the afterload is low, the stroke volume will increase.
What increases stroke volume?
Rises in venous pressure by increasing pre-load
Decreases in total peripheral resistance by decreasing after load.
What substances have a positive inotropic affect?
Noradrenaline and adrenaline
What is a positive inotropic effect?
Something that increases the contraction of the myocardial cells.
Do noradrenaline and adrenaline act on the sympathetic or parasympathetic nervous system?
Act on the sympathetic nervous system via GPCR receptors.
How does an increase in sy,pathetic activity affect stroke volume?
Increase in stroke volume
What happens if total peripheral resistance falls?
Arterial pressure falls and venous pressure rises. As venous pressure rises, preload increases. Increased preload results in increased stroke volume. Increased stroke volume increases arterial pressure and reduces venous pressure.
What is total peripheral resistance?
Systemic vascular resistance. The resistance to blood flow offered by all the systemic vasculature
What happens to pressure of fluid in a tube as it encounters a resistance?
Pressure decreases
What happens if the total peripheral resistance falls in a system but the cardiac output remains the same?
Less resistance means the arterial pressure will fall and the venous pressure will rise as the blood gets to veins more readily
What happens if the TPR increase and the CO is unchanged?
The arterial pressure increases and the venous pressure decreases.
What happens to the blood pressure if the TPR is unchanged and the CO increases?
The arterial pressure will increase and the venous pressure will decrease.
What happens to the blood pressure if the CO decreases and the TPR is unchanged?
Decrease in arterial blood pressure and increase in venous pressure
What physiological changes happen if tissues demand for blood increases?
Arterioles and pre-capillary sphincters dilate, decreasing TPR. This makes the arterial blood pressure fall and venous blood pressure increase. The cardiac output increases aswell to stop arteriole blood pressure dropping.
How does the heart detect changes in blood pressure?
Detects changes in blood pressure in arterial blood pressure and central venous pressure
How does the heart respond to changes in arterial blood pressure and central venous pressure
By intrinsic and extrinsic mechanisms
What is the calculation for cardiac output?
Stroke volume x heart rate = cardiac output
What is the calculation for stroke volume?
End diastolic volume - end systolic volume = stroke volume
When does ventricular filling stop?
When intraventricular pressure is equal to venous pressure
What is the ventricular compliance curve?
A graph showing the relationship between LV pressure (y) and LV volume (x).
What affects the compliance of LV volume to LV pressure?
Diseased states
What is the frank-starling law of the heart?
The more the heart fills, the more the fibres stretch, the harder the heart can contract (up to a limit). The harder the heart contracts, the bigger the stroke volume.
Larger EDV = larger SV
What is the starling curve?
A graph of stroke volume (y) against LV end-diastolic pressure. An increase in LV end-diastolic pressure increases the stroke volume as the fibres are more stretched and are therefore able to contract harder.
Describe the length tension curve for cardiac muscle
The greater the resting length of the fibres, the larger the contractile force produced.
What happens if sarcomere length is too short?
Filament overlap interferes with contraction
Why do stretched muscle fibres produce a greater contractile force
As filament overlap doesn’t happen and therefore doesn’t interfere with contraction
Cardiac muscle gets an increase in calcium sensitivity as the muscle fibres are stretched.
What is an intrinsic control mechanism for blood pressure?
Increased stroke volume and increased filling of the heart. Both sides of the heart maintain the same output.
How does a change in contractility affect the slope of the starling curve?
Increased contractility results in an increase in the force of contraction for a given EDP. This makes the curve more steep.
Decreased contractility results in a decrease in force of contraction at a given EDP. The curve becomes flatter and shorter.
What extrinsic factors affect the heart to influence blood pressure?
Sympathetic stimulation and circulating adrenaline can increase contractility
What is afterload?
The pressure the heart has to pump against (in the arteries)
When is the arterial pressure increased?
When peripheral resistance is increased. This means the heart has to work harder, resulting in hypertension
What factors determine the cardiac output and therefore also the end systolic volume?
The force of contraction - determined by the EDV (preload) and the contractility (sympathetic/adrenaline)
The difficulty of ejecting the blood - determined by aortic impedance.
How are contractility and heart rate controlled?
By the autonomic nervous system
How does standing up affect blood pressure?
Blood pools in the legs. This reduces venous pressure and therefore cardiac output. A decrease in cardiac output decreases the arterial pressure.
If both arterial and venous pressure drop, what mechanism is used to increase the blood pressure?
Baroreceptor reflex and autonomic nervous system (increases heart rate and TPR)
How is cardiac output increased in exercise?
Increased venous return due to a muscle pumping and venoconstriction. TPR later falls in response to increased venous return.
Increased heart rate and contractility due to increased sympathetic drive.
Increased HR, contractility and venous pressure increases CO
How is the right atrial pressure/ central venous pressure estimated?
By measuring the blood pressure of the right internal jugular vein pulse as it is a column of blood directly connected to the right atrium.
How is the pulse of the right internal jugular vein measured?
The right internal jugular vein lies behind the sternocleidomastoid muscle.
Can also be measured with a central line inserted into the internal jugular vein or SVC
What conditions increase the JVP?
If the right side of the heart doesn’t pump out properly (stenosed pulmonary valve)
Volume overload with IV infusion
Impaired filling of the heart (stenosed tricuspid valve)
Low arterial BP (stab wound)
Where is the biggest resistance to blood in the CVS?
Arterioles
Describe how constriction of arterioles affects blood pressure?
Pressure in the capillaries and on the venous side falls
Pressure on the arterial side will rise.
What is the typical stroke volume?
70ml
How can SV be increased?
Increased EDV
Decreasing ESV
When does the ventricular compliance curve show decreased compliance?
Hypertrophy
Narrow lumen
Higher venous pressure
When does the ventricular compliance curve show increased compliance?
Decreased venous pressure
Wider lumen
What is demand led pumping?
Occurs when there is an increase in metabolism and there is a higher demand for circulating blood.
TRP falls to supply more blood, fall in arterial pressure, increase in venous pressure. Heart pumps more due to increase central venous pressure
What is postural hypertension?
Hypotension on standing up due to blood pooling in the legs
Why is the pulse seen with a central line described as biphasic?
As it produces 2 waves for each beat of the heart.
1st wave = atrial contraction
2nd wave = venous filling of the right atrium against a closed tricuspid valve.
What is the typical duration of a cardiac AP?
0.28 s
What type of anaemia can result from aortic valve stenosis?
Microangiopathic haemolytic anaemia
