Preload Flashcards
Define the determinants of cardiac output Define the term preload Explain what is meant by the Frank-Starling mechanism Describe the factors affecting venous return Describe the reflex responses to increased blood volume Describe the reflex responses to decreased blood volume Explain for ANP defends against volume overload (19 cards)
When does heart failure develop?
Heart failure develops when the heart is unable to maintain a normal cardiac output at normal filling pressures. Reduced cardiac output leads to under filling of the arterial circulation
What two things are needed in order for the heart to act effectively as a pump?
Co-ordinated transmission of excitation
Co-ordination of excitation- contraction coupling in the different regions
Define cardiac output
Cardiac output is the volume of blood pumped into the aorta
What is the name of the blood returning into the right atrium?
Venous return.
Under normal conditions this should be equal to cardiac output
What are the two variables that can be changed to alter cardiac output?
Stroke volume and heart rate
Cardiac output is a measure of cardiac work
Define preload
Preload is the degree of stretch of the ventricular myocardium at the end of diastole
Name three determinants of preload
Circulating fluid volume (venous return)
Venous tone
Myocardial compliance
What does excessive preload lead to?
Increased atrial pressure
Increased venous pressure
Signs of congestion
What are the three possible mechanisms for regulating stroke volume?
Mechanical
Neuronal: Increased activity of sympathetic nerves to the heart
Hormonal: increased plasma adrenaline
What did the Frank- Starling law of the heart demonstrate?
Intrinsic autoregulation of the heart - allows stroke volume to increase without an increase in heart rate
How are right ventricular and left ventricular stroke volumes kept equal?
Under stable conditions input volume and output volumes are equal because the more blood there is in the ventricles at the end of diastole, the harder the heart will contract to eject the blood.
The Frank Starling mechanism provides intrinsic autoregulation of stroke volume in response to variation in venous return and to changes in the peripheral resistance of the systemic arterial circulation
What is responsible for central venous pressure?
Volume of blood in the circulation.
Distribution of blood between central and peripheral veins.
Sympathetic nerve activity which regulates peripheral nervous tone
Gravity and movement
Thoracic pump
How does the arterial baroreceptor reflex respond to an increase in blood volume?
An increase in blood volume leads to an increase in arterial pressure. This results in a increase in firing of arterial baroreceptors and consequently a decrease in sympathetic outflow to the heart, arterioles and veins; along with a increase in parasympathetic outflow to the heart.
What is meant by the pressor reflex and when is this seen?
Pressor reflex is seen in response to a decrease in arterial pressure (as a result of decreased blood volume). As a result there is a decrease in firing of arterial barorecptors and a decrease in parasympathetic outflow to the heart.
There is also a increase in sympathetic outflow to the heart (causing for a increase in heart rate) and arterioles and veins (leading to vasoconstriction )
Where is the integrating centre for baroreceptor reflexes?
Medulla Oblongata:
Meddulary Cardiovascular centre
What are atrial receptors?
Unlike baroreceptors which are high pressure sensors located the aorta; atrial receptors are low pressure stretch receptors that are located in the walls of the atria and act as volume receptors
What are the consequences of increased blood volume on atrial volume receptors?
Increase in blood volume leads to a increase in stimulation of atrial volume receptors.
This leads to the reflex inhibition of ADH secretion from the posterior pituitary.
Simultaneously there is inhibition of sympathetic vasoconstriction pathways to the kidneys, leading to diuresis.
All of this allows for a decrease in blood volume, a decrease in central venous pressure and so a decrease in stroke volume, cardiac output and mean arterial pressure
Haemorrhage results in reduced preload. How do the atrial volume receptors respond to this?
Blood loss = a decrease in venous return.
As a result this sees a decrease in the discharge rate of atrial volume receptors.
At this time a number of things happen
There is a increase in ADH release, resulting in a increase in blood volume.
There is also a activation of sympathetic pathways to the kidneys; where a number of responses occur.
The renal vasculature constricts causing urine volume to fall and so blood volume to increase.
Release of renin from the juxtaglomerular apparatus allows the production of angiotensin II. This constricts renal vascularture and causes the secretion of aldosterone from the adrenal cortex.
Urine volume falls as there is retention of sodium and water…leading to to a increase in blood volume
What is the role of atrial natriuretic peptide?
ANP is released into circulation when the atrial walls are stretched by an increase in blood volume. It acts to:
Reduce blood volume by stimulating excretion of salt and water by the kidneys.
Relax vascular smooth muscle (vasodilator)
Inhibits the renin-angiotensin-aldosterone system
