Control of cardiac output Flashcards
Define stroke volume
Volume of blood ejected from the heart with each cycle (~ 70ml)
Define end-diastolic volume
The volume of blood in the ventricle immediately before contracting
Define end-systolic volume
The volume of blood in the ventricle immediately before relaxing
Define preload
The level of stretch (tension) in the relaxed muscle immediately before it contracts
Determined by venous return. Increase in return stretches the muscle fibres more before contraction, this increases the contractile force, which increases cardiac output
Define afterload
The force that the muscle has to generate during contraction before it can eject blood.
Mostly affected by changes in arterial pressure. Hypertension increases afterload as it opposes ventricular ejection, stroke volume is limited
What factors affect venous filling pressure?
Rate blood is being pumped by the heart
Rate blood returns from the body
Compliance of veins
Venomotor tone (contraction of smooth muscle in veins)
What is mean circulatory filling pressure?
a measure of the average (arterial and venous) pressure necessary to cause filling of the circulation with blood. Directly proportional to the rate of venous return and thus to cardiac output
The pressure throughout the circulation comes to a level determined by blood volume and compliance of the circulation
What is Starling’s Law of the heart?
An increase in the length (stretch) on cardiac muscle at rest increases the force of contraction.
Therefore increased ventricular filling in diastole leads to an increased stroke volume - thus increases cardiac output.
What is the relationship between venous pressure and ventricular filling?
As the ventricle fills with blood, there is an increase in pressure. Filling stops once pressure in the ventricle equals venous pressure. The higher the venous pressure, the more the ventricle will fill.
Ventricular compliance may be reduced in disease e.g. Ventricular hypertrophy and restrictive cardiomyopathy - muscle become rigid so the heart cannot relax adequately between contractions, harder for the ventricles to fill.
Higher venous pressure required to fill the heart.
What is stroke work?
Stroke volume x pressure required to eject volume
Explain why output from both sides of the heart is equal
Starling’s law of the heart: cardiac output equals venous return. Therefore the volume of blood into the right heart is equal to the volume pumped out into the pulmonary circulation, this enters the left heart and is pumped out.
The output from one ventricle is responsible for the venous return to the other side of the heart in an intact system.
How does the heart respond to an increase in SVR?
If the resistance is increased, it becomes harder to eject blood. This initially reduces stroke volume because the ventricle empties less (blood accumulates in the ventricle).
When the heart refills the end-diastolic volume will be increased. This increases the stroke work (force of contraction) of the heart so stroke volume increases.
Note: End-diastolic volume has increased, but ejection fraction has fallen.
End-diastolic volume continues to increase until a new steady state is reached where stroke volume has returned to normal.
Describe baroreceptor reflexes of the heart
Baroreceptors stretch receptors (visceral afferent nerve fibres) located in the walls of the carotid sinus and the aortic arch.
Impulses are conducted at a lower frequency at low pressures and raised at high pressures.
Increase in pressure stimulates sensory output from the baroreceptors. This inhibits sympathetic outflow to reduce cardiac output at SVR and stimulate parasympathetic nerves to reduce cardiac output.
Baroreceptor responses are rapid, and used to compensate for sudden changes in blood pressure. If the arterial pressure remains high for a prolonged period, the baroreceptor sensitivity adapts to the higher pressure range. Therefore they provide very little long-term control of mean blood pressure.
Describe the changes that occur when end-diastolic volume is high
The diastolic pressure becomes high. Venous pressures are high which increases the chance of oedema.
The heart responds to increase in end-diastolic volume by increasing stroke work. Atrial stretch acts on vasomotor centres of the brain which increase heart rate. Cardiac output is increased.
Describe the bainbridge reflex
Veno-atrial mechanoreceptors monitor venous volume and pressure. Respond to stretch.
Increase in central venous pressure activation of fibres. These act on vasomotor centres of the brainstem resulting in sympathetic stimulation of the cardiac pacemaker cells which increases heart rate (reflex tachycardia)
