Determining Heart Rate and Stroke Volume

Question 1

What makes up stroke volume?

Answer 1

End Diastolic volume - End systolic volume

Question 2

How do you calculate cardiac output?

Answer 2

CO = HR x SV
CO = HR x (EDV - ESV)

Question 3

What is average cardiac output volume?

Answer 3

5L.min-1

Question 4

What is the SAN

Answer 4

The heart pacemaker which regulates heart rate via autonomic innervation.

Question 5

What does sympathetic innervation of the SAN do?

Answer 5

Increases HR
Decreases AVN delay
Increase force of contraction atria and ventricles and thus increase SV

Question 6

What receptors are involved in sympathetic innervation of the heart?

Answer 6

Beta-1 receptors (GPCRs) stimulation increases Ca channel opening via PKA mediated phosphorylation.

Question 7

What does parasympathetic innervation of the SAN and AVN do?

Answer 7

Decreases HR
increase AVN delay

There is tonic vagal activity at the SAN.

Question 8

How is the heart rate increased?

Answer 8

Sympathetic innervation is increased and parasympathetic innervation is reduced which increases the heart rate.

Question 9

How is heart rate controlled?

Answer 9

Changes in the shape of the sino-atrial node (SAN) action potential bring about changes in heart rate (covered in more detail next year).

Question 10

Why is the heart rate lowered at rest compared to the original heart rate set by the SAN?

Answer 10

Hyperpolarise the cell
Threshold is increased
Decreased electrical gradient so it takes longer to reach threshold.

Question 11

How do you calculate stroke volume and what are some average measurements for it?

Answer 11

SV = EDV - ESV

Systole (70ml) = End diastole (120ml) - End systole (50ml)

Question 12

What does calcium ions regulate in the heart?

Answer 12

Regulates the number of actin-myosin cross-bridges formed and therefore the force of contraction.

The more cross-bridges formed, the stronger the force of contraction.

Question 13

How can force of contraction be increased?

Answer 13

Increasing the calcium sensitivity of the contractile apparatus

Increases calcium concentration in the cell.

Question 14

Explain the process of excitation-contraction coupling!

Answer 14

During the action potential there is an influx of calcium through L type calcium channels.
This stimulates the Ryanidine receptors on the sarcoplasmic reticulum.
The Ca channels open and calcium is releasd from the SR.
The calcium interacts with the contractile apparatus to cause contraction.
This process elongates the action potential.

Question 15

What is starlings law of the heart?

Answer 15

Force of ventricular contraction is dependent on the length of ventricular muscle fibres in diastole.

Therefore more blood going into the heart, the more blood comes out of the heart.

Question 16

What affect does stretching the sarcomere have?

Answer 16

Increased sarcomere length increases the force of contraction due an increase in the number of cross-bridges formed.

This is an intrinsic property of cardiac muscle so no external force is required.

Question 17

What does starlings law of the heart ensure?

Answer 17

ensures that the right stroke volume = left stroke volume

any factors which affect right ventricular EDV and SV, after a couple of beats will affect left ventricular filling from the pulmonary circulation so left SV will match right SV

Question 18

What would happen if the RSV > LSV?

Answer 18

congestion of the pulmonary circulation since less blood is being pumped out of the left ventricle per beat.

Question 19

What would happen if the LSV> RSV?

Answer 19

congestion of systemic circulation since less blood is being pumped out of the right ventricle per beat.

Question 20

What factors affect venous return to the right ventricle?

Answer 20

Blood volume
Skeletal muscle pump
Respiratory pump
Venous Tone
Gravity

Question 21

What affect does blood volume have on the venous return to the right ventricle?

Answer 21

Increased blood volume might occur during renal failure, will lead to an increase in venous return. (therefore have a higher EDV and SV)

Decrease in blood volume will lead to a decrease in EDV and a decrease in SV.(this can be caused be a haemorrhage or dehydration)

Question 22

What affect does the skeletal muscle pump have on the venous return to the right ventricle?

Answer 22

Valves ensure unidirectionality of blood through the heart.
Blood is propelled through the valves by the skeletal muscle pump.
When the skeletal muscle contracts it squashes the venous vessels pushing blood forwards towards the heart and prevented backflow via valves.

Question 23

What affect does the Respiratory muscle pump have on the venous return to the right ventricle?

Answer 23

The action of breathing also helps to return blood to the right heart.
When we breathe in, the diaphragm flattens and presses down on the abdomen causing an increase in pressure in the abdomen.
At the same time the chest wall expands and decreases pressure with the thorax.
This pressure gradient essentially helps to “suck” blood from the abdominal vena cava into the thoracic vena cava and into the right ventricle.
This increases the filling of the heart and therefore EDV and SV.

Question 24

What affect does the venous tone have on the venous return to the right ventricle?

Answer 24

Venous circulation holds up to 70% of blood volume at any one time.
Increasing the tone will lead to a smaller volume in the veins for a given pressure and thus more blood is moved. (venoconstriction mediated by sympathetic activity)

This is the only mechanism which we have reflex control over.

Question 25

What affect does the gravity have on the venous return to the right ventricle?

Answer 25

Lying down:uniform distribution of blood across the body so CVP and VR is maintained.

Standing:
Redistribution of blood due to gravity.
venous pooling in lower extremities
reduction in thoracic blood volume.
CVP and VR fail
EDV and SV fall.

Question 26

What is Pre Load

Answer 26

Any factor which influences the stretch of cardiac muscle cells before contraction.

Question 27

Which period of the cardiac cycle shortens when increasing HR?

Answer 27

Diastole

Question 28

How can a greater stroke volume be achieved with the same volume of filling?

Answer 28

By increasing the amount of blood ejected during systole.

This however will leave less blood in the ventricles after systole and therefore lead to a lower ESV.

Question 29

What is the mechanism that increases SV at a given EDV by decreasing ESV?

Answer 29

By increasing contractility / ionotropy:

• Increase sympathetic innervation stimulating B1 receptors
• Leads to increased Ca influx during Ap and thus more Ca released from SR
• Increassed cross bridge formation and therefore greater binding of Ca to Troponin
• Increased force of contraction leading to a greater SV
• Thus ESV is decreased.

Question 30

What is a change in contractility?

Answer 30

a specific change in stroke volume brought about by a change in the number of cross-bridges made between actin and myosin at a give EDV.

Question 31

What is after load?

Answer 31

The load against which the heart must contract to eject the stroke volume

Eg. high aortic pressure makes it more difficult to eject SV

Question 32

How can SV be increased?

Answer 32

By starlings law of the heart:
        Ca2+ influx/release from SR   
 length-dependent increase  in sensitivity of contractile apparatus to Ca2+ 
  increase cross-bridge formation 
 increase  force of contraction 
  increase SV

By increasing contractility:
increase Ca2+ influx/release from SR
length-independent increase in sensitivity of contractile apparatus to Ca2+ ( increased TN-C binding affinity).
increased cross-bridge formation
increased force of contraction
Increased SV

Question 33

What would an increase in EDV cause the SV to do on a pressure-volume loop?

Answer 33

Increases SV by widening the loop to the right

Question 34

What would an decreasein ESV cause the SV to do on a pressure-volume loop?

Answer 34

Increases SV by widening the loop to the left

Question 35

What regulates ESV?

Answer 35

Contractility and inotropy.

Question 36

What regulates cardiac output?

Answer 36

HR – which is determined by the balance between the influence of the parasympathetic nervous system and the sympathetic nervous system on the SAN.
EDV – which is mainly determined by venous return to the heart which determines central venous pressure and ventricular filling (intrinsic influences of the heart).
ESV – which is determined by the contractility/inotropy of the heart which is influenced by extrinsic factors on the heart including sympathetic nerve activity and circulating hormones.

Question 37

What keeps the heart rate down at rest?

Answer 37

At rest the parasympathetic nervous system is keeping the heart rate down.

Question 38

What receptors are involved in parasympathetic innervation of the heart?

Answer 38

M receptor stimulation increases K channel opening

Question 39

How can stroke volume be increase?

Answer 39

Increase End diastolic volume

Decrease End systolic volume

Question 40

How can increased Calcium in the sarcomere lead to increased SV?

Answer 40

Increased calcium would lead to an increased number of cross bridges forming and therefore a greater force of contraction meaning that the end systolic volume is decreased as more blood is pumped oout of the heart.

Question 41

How does an increased End diastolic volume increase SV?

Answer 41

More blood filling the heart during diastole stretches the muscle fibres.
Greater stretch in fibres increases sarcomere length and thus number of cross bridges formed leading to a greater force of contraction and more blood being pumped out (increased SV)