Control of Cardiac Output Flashcards
What is the cardiac output equation
Heart Rate x Stroke Volume
What is the cardiac output limited by and why
Limited by venous return as the blood is held in a closed system
Name some factors that influence the venous return
- Skeletal Muscle Pump
- Abdomino-Thoracic Pump
What is the skeletal muscle pump
- Veins have one way valves that prevent back flow
- Surrounding muscle activity acts to propel the blood back towards the heart.
What is the abdomen-thoracic pump
- The great veins (vena cavae) and atria are exposed to intrathoracic pressure
- This pressure is normally negative and becomes more so during inspiration
Why does cardiac output start to decrease if the heart rate does much above 180bpm
Not enough time for ventricular refilling and stroke volume is reduced
Up to 180bpm how does an increase in heart rate affect the cardiac output
It will increase proportionately
At what exercise intensity does stroke volume usually reach its maximum levels
fairly moderate exercise levels
How is the heart rate innervated
Dually innervated
Rate of the SAN is modulated by both the sympathetic and parasympathetic nervous systems
The natural intrinsic rate of the SAN is about 110-120bpm so why is the resting rate about 70
Parasympathetic inhibition dominates at rest
Small increases in heart rate occur by what nervous system
By inhibition of the parasympathetic nervous system.
What neurotransmitters from the SNS act on what receptors on the SAN
Noradrenalin released at nerve endings acts on the Beta-receptors in the SAN
How does sympathetic stimulation affect the action potential graph of pacemaker tissues
Increases the rate of depolarisation after repolarisation has occurred so the threshold potential is reached quicker and the rate of firing increases
Sympathetic fibres on the Right and Left sides of the body innervate what in the heart
RIGHT = SAN, regulates heart rate LEFT = Innervate the ventricles, regulate cardiac contractility
The left and right branches of what nerve innervate the SAN and AVN
Vagus nerve
Where can the parasympathetic ganglia innervating the heart be found
On the cardiac surface or in the heart itself
The postganglionic fibres release what neurotransmitter and act on what receptors in the SAN
Acetylcholine on the muscarinic receptors in the SAN
If an individual has a lower resting heart rate how big does their stroke volume tend to be
Lower heart rate = bigger stroke volume
What is the end-diastolic volume (EDV)
This is the filled volume of the ventricle prior to contraction
What is the end-systolic volume (ESV)
This is the residual volume of blood remaining in the ventricle after ejection
What is the definition of ventricular stroke volume
The difference between the ventricular end-diastolic volume and end-systolic volume
What are the three primary mechanisms that regulate EDV, ESV and therefore SV
Preload
Contractility
Afterload
What is preload
Factors that determine ventricular EDV and end-diastolic fibre length of the ventricular muscle
What is Afterload
Factors which determine ventricular wall stress (chamber radius, wall thickness and resistance to outflow)
What are the 3 things that EDV depend on
Filling pressure
Filling time
Ventricular compliance
What is filling pressure and how can it affect EDV
When venous return increases atrial filling pressure increases and so ventricle filling pressure increases and so EDV rises
What is filling time and how can it affect EDV
Longer filling time = greater EDV
Heart rate increase = diastole shortens more than systole does so EDV decreases
What is ventricular compliance and how can it affect EDV
As an ventricular compliance increases, a given filling pressure will produce an increased ventricular volume and result in a greater EDV.
What are the 4 things that ESV depend on
Preload
Afterload
Heart Rate
Contractility
What is pre load and how does it affect stroke volume
Starlings law of the heart - increased EDV increases stretch on the cardiac muscle and the force of the contraction and thus the stroke volume
What is Starlings law of the heart
the stroke volume of the left ventricle will increase as the left ventricular volume increases due to the myocyte stretch causing a more forceful systolic contraction.
What is after-load and how does it affect ESV
Afterload of the left ventricle is the mean systemic arterial pressure. increased after-load impedes the hearts ability to empty and so increases the ESV.
How does heart rate affect ESV
Increased heart rate = greater Ca2+ entry into myocardial cells and increasing contractility and reducing ESV
Why is the Frank-Starling mechanism physiological important
The heart is able to eject additional blood that was returned to it and keeps the two sides in balance. Otherwise if the RV output exceeds the LV output then blood would be trapped in the lungs, but instead the extra preload in the left means the LV pumps more strongly to keep the sides in balance
What is contractility
Physiological alterations in contractility are associated with change in intracellular calcium conc in the myocyte.
What can increase the force of ventricular contraction without increasing the EDV and stretching the muscle fibres
Contractility
What is an inotrope
a neurotransmitter, hormone or drug that alters the force of contraction of the heart muscle
What are positive and negative inotropes
Positive = increases contractility Negative = decreases contractility
