Preload

Question 1

When does heart failure develop?

Answer 1

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

Question 2

What two things are needed in order for the heart to act effectively as a pump?

Answer 2

Co-ordinated transmission of excitation

Co-ordination of excitation- contraction coupling in the different regions

Question 3

Define cardiac output

Answer 3

Cardiac output is the volume of blood pumped into the aorta

Question 4

What is the name of the blood returning into the right atrium?

Answer 4

Venous return.

Under normal conditions this should be equal to cardiac output

Question 5

What are the two variables that can be changed to alter cardiac output?

Answer 5

Stroke volume and heart rate

Cardiac output is a measure of cardiac work

Question 6

Define preload

Answer 6

Preload is the degree of stretch of the ventricular myocardium at the end of diastole

Question 7

Name three determinants of preload

Answer 7

Circulating fluid volume (venous return)
Venous tone
Myocardial compliance

Question 8

What does excessive preload lead to?

Answer 8

Increased atrial pressure
Increased venous pressure
Signs of congestion

Question 9

What are the three possible mechanisms for regulating stroke volume?

Answer 9

Mechanical

Neuronal: Increased activity of sympathetic nerves to the heart

Hormonal: increased plasma adrenaline

Question 10

What did the Frank- Starling law of the heart demonstrate?

Answer 10

Intrinsic autoregulation of the heart - allows stroke volume to increase without an increase in heart rate

Question 11

How are right ventricular and left ventricular stroke volumes kept equal?

Answer 11

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

Question 12

What is responsible for central venous pressure?

Answer 12

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

Question 13

How does the arterial baroreceptor reflex respond to an increase in blood volume?

Answer 13

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.

Question 14

What is meant by the pressor reflex and when is this seen?

Answer 14

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 )

Question 15

Where is the integrating centre for baroreceptor reflexes?

Answer 15

Medulla Oblongata:

Meddulary Cardiovascular centre

Question 16

What are atrial receptors?

Answer 16

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

Question 17

What are the consequences of increased blood volume on atrial volume receptors?

Answer 17

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

Question 18

Haemorrhage results in reduced preload. How do the atrial volume receptors respond to this?

Answer 18

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

Question 19

What is the role of atrial natriuretic peptide?

Answer 19

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