Preload Flashcards
What are the Major Determinants of Cardiac Output?
Preload, After Load, Heart Rate and Myocardial Contractility
What is the Equation of Cardiac Output?
Cardiac Output = Stroke Volume x Heart Rate
What is Heart Failure?
Inability of the heart to maintain normal cardiac output
What does a Reduced Cardiac Output result in?
Under filling of the arterial circulation
What is Preload?
The volume of blood in the ventricle at the end of diastole (end diastolic pressure)
What is Venous Return?
Volume of blood that comes back to the RA per unit time
What is the Frank-Starling Mechanism?
An intrinsic cardiac autoregulatory mechanism (reflex)
What happens in the Frank-Starling Mechanism?
It ensures that stroke volume changes in proportion to the change in end diastolic volume
When does Cardiac Output Vary?
In response to physiological and pathological factors
What does the parasympathetic innervation effect in relation to the Major Determinants of Variation?
Decreases Heart Rate which increases Cardiac Output
What does the sympathetic innervation effect in relation to the Major Determinants of Variation?
Increases contractility which leads to a increased stroke volume
How Is Stroke Volume Regulated?
Mechanical Means: Increased Preload
Neuronal: Increases sympathetic innervation
Hormonal: Increased plasma epinephrine
All = Increased Stroke Volume
How do we work out Mean Arterial Pressure?
Mean Arterial Pressure = Cardiac Output x Total Peripheral Resistance
What is Starlings Law of the Heart?
The energy of contraction of a cardiac muscle fibre is proportional to the initial fibre length at rest. Ensures that the right ventricular output equals left ventricular output.
What Governs Central Venous Pressure? (and therefore preload?)
Volume of blood, distribution of blood, sympathetic nerve activity, gravity and movement
What are the Determinants of Preload?
Circulating fluid volume, venous return, venous tone, heart rate, myocardial compliance
What does excessive preload cause?
Increased atrial pressure, increased venous pressure , signs of congestion
What do baroreceptors detect?
High pressure. Increased/Decreased blood volume causes an increase or decrease in arterial pressure which is sensed by the baroreceptors which either fire more or less depending on whether the arterial pressure has increased or decreased.
What do baroreceptors cause?
Higher/Lower sympathetic outflow to arterioles, veins and heart or the depressor reflex (reduce volume and pressure) or the opposite pressor reflex by increasing or decreasing the parasympathetic outflow to the heart
What receptors also contribute to CVRS homeostasis?
Atrial Receptors
What is the role of Atrial Receptors?
They are low pressure stretch receptors in the walls of the atria that act as volume receptors. Increased blood volume = stimulation of atrial volume receptors
What do atrial receptors do when increased blood volume is detected?
Inhibit ADH secretion and sympathetic vasoconstrictor pathways to kidneys leading to diuresis
What receptors contribute to restring blood volume following haemorrhage?
Both baroreceptors and atrial volume receptors
What are Natriuretic Peptides?
Peptides released into the blood when blood volume in the atria is increased
What is the role of Natriuretic Peptides?
Atrial Natriuretic Peptide and Brain NP reduce blood volume by stimulating excretion of salt and water by the kidney. Relax vascular smooth muscle/vasodilation so reduces TPR and inhibits RAAS.
