Cardiac Control

Question 1

What is the cardiac output

Answer 1

Cardiac output is the amount of blood ejected from the heart per minute.

Question 2

How is cardiac output calculated

Answer 2

It can be calculated by

multiplying heart rate by stroke volume.

Question 3

What is blood pressure equal to

Answer 3

Blood pressure equals cardiac output multiplied by total 
peripheral resistance (or TPR and it is mainly provided by arterioles). Heart rate covered by when it 
contracts and relaxes is generated by the SAN and affected by autonomic nervous control.

Question 4

What effects the strength of contraction

Answer 4

The strength of contraction can be affected by neurotransmitters and sympathetic innervation.

Question 5

What increases and decreases stroke volume

Preload

Answer 5

Starling’s law) and afterload (Laplace’s law) increase stoke volume and reduce stroke volume
respectively.

Question 6

When is the heart contracting

Answer 6

The heart is working during

isovolumetric contraction and then the final bit of contraction during ejection.

Question 7

What happens if isovolumetric contraction is longer than usual

Answer 7

If the period of isovolumetric contraction is longer than usual, there will be less
energy available for ejection and the heart becomes inefficient.

Question 8

What does starling law also suggest - what is the law

Answer 8

Starling’s law of the heart also governs how hard the heart has to contract.
This law states that the greater the stretch of the ventricle in diastole, then the
greater the energy of contraction and therefore the greater the stroke volume.
‘The energy of contraction of cardiac muscle is relative to the muscle fibre
length at rest’.

Question 9

What happens to the stroke volume when the volume of blood entering the heart increases

Answer 9

This means that the more blood enters the heart, the greater the stroke volume.

Question 10

How can stroke volume be shown on a graph

Answer 10

This

can be expressed on a graph of central venous pressure vs stroke volume (see right).

Question 11

How did starling discover the volume of blood stroke volume property of the heart

Answer 11

Starling
discovered this property of the heart was discovered by injecting a bolus of fluid to a heart that
resulted in an increase in end diastolic volume. The result was an increase in the strength of
contraction and therefore an increase of stroke volume. When fluid was removed, the end diastolic
volume was decreased and the result was a decrease in the strength of contraction of the heart and
therefore a decrease in stroke volume.

Question 12

What is the main cause of heart conditions - why is this property important

Answer 12

Many heart problems are associated with it having too little
or too much fluid. This property of the heart is very useful during exercise as muscles will contract
veins that act as reservoirs for blood.

Question 13

During exercise what happens

Answer 13

This will increase the amount of blood entering the heart and
therefore increase the central venous pressure.

Question 14

What is the molecular reason behind starlings law

Answer 14

According to Starling’s law, the result is an increase in
stroke volume. The molecular reason behind Starling’s law can be explained using actin and myosin.
When the fibre is unstretched, there is actin/myosin overlapping and less cross-bridge formation
potential. When the fibre is stretched, there is less actin/myosin interference meaning there is more
potential for cross-bridge formation and therefore contraction.

Question 15

What is starlings law responsible for

Answer 15

There are a number of important effects of Starling’s law. It is responsible for balancing out the 
outputs of the right and left ventricles that can be distorted by an abnormal increase or decrease in 
blood pressure (e.g. when standing up for too long, blood pools in one’s leg meaning less blood 
returns to the right hand side of the heart and therefore less blood is pumped to other parts of the 
body like the brain and lungs which causes one to faint).

Question 16

How is cardiac output decreased

Answer 16

Cardiac output decrease caused by a drop in
blood volume is also due to Starling’s law as is the restoration of cardiac output due to intravenous
fluid injection. It also contributes to increased SV and CO during exercise.

Question 17

What factor effects cardiac output + law

Answer 17

The other important factor affecting cardiac output is
afterload that is governed by Laplace’s law. This property of
the heart is antagonistic to that of preload as it reduces
contraction of the heart.

Question 18

What properties give the wall on the heart tension

Answer 18

There are two major properties in
the wall of the heart that give it tension to oppose
contraction. The first thing is the pressure inside the
ventricle. The higher the pressure inside the ventricle, the more energy is required to overcome this
pressure during ejection. The second parameter opposing contraction is the radius of the heart. The
bigger the radius, the more the tension is directed outwards along the heart walls.

Question 19

What happens if the radius of the heart is smaller

Answer 19

If the radius is
smaller, this tension is directed more inwards towards the centre of the heart.
Force directed
towards the centre will result in less afterload and better ejection whilst the opposite is true for if the
radius was bigger (see above right the mechanics).

Question 20

What is the Laplace law

Answer 20

Laplace’s law can be formulised; tension in the

wall equals the pressure multiplied by the radius over 2.

Question 21

How does thickness effect tension

Answer 21

One other factor affecting tension of the
wall is the thickness of the wall. The thicker the wall, the more easily the opposition to contraction is
overcome.

Question 22

How can tension and thickness be formularised

Answer 22

This can be formulised; tension equals to wall stress multiplied by wall thickness. These
two equations both describe the tension in the wall so can be combined; wall stress equals to
pressure multiplied by radius all over wall thickness times 2.

Question 23

What is key between the two laws

Answer 23

Starling’s law and Laplace’s law both oppose each other. At rest, Starling’s law overcomes Laplace’s
law so there is good ejection. This means during contraction, chamber radius is reduced in emptying
and this aids expulsion and therefore increases stroke volume. Laplace’s law even aids with the final
emptying of the heart.

Question 24

What law dominates the other and in what situation

Answer 24

However, in a failing heart, Laplace’s law begins to dominate Starling’s law. An
acute rise in blood pressure can be overcome by Starling’s law.

Question 25

When is more energy required to maintain stroke volume and how does this show contrast in laws

Answer 25

However, a chronic increase in blood
pressure means more energy is used to maintain stroke volume and stroke volume will ultimately
decrease (volume of blood remaining in the heart at the end of systole is high). This eventually leads
to volume overload due to the heart not emptying and this increases the radius of the heart. This
means Laplace’s law now opposes Starling’s law further. If blood pressure is high, then the heart will
experience pressure overload due to it trying to overcome the high blood pressure.

Question 26

What increases wall stress and reduces cardiac output

Answer 26

Both pressure or

volume overload increase wall stress and reduce cardiac output.

Question 27

How can the heart overcome and maintain cardiac output - what is the name of this

Answer 27

The heart responds by increasing in

wall thickness in order to try and maintain cardiac output. This is known as ventricular hypertrophy.

Question 28

What is the negative effect of hypertrophy

Answer 28

The negative affect of this however is that more energy to contract the heart and therefore the heart
needs to pump harder to supply cardiac muscles with oxygen. This spirals out of control and usually
ends in heart failure.