Lecture 21 : Control of the Heart

Question 1

What are intrinsic or local controls?

Answer 1

Controls that originate entirely from within a system

Question 2

What are extrinsic controls?

Answer 2

Controls that involve input from outside - in particular hormonal and nervous effects

Question 3

How do intrinsic and extrinsic controls work together?

Answer 3

Both forms of control operate continuously to adjust the heart and vessels, so that both are always functioning appropriately

Question 4

What is a chronotropic factor?

Answer 4

Anything affecting heart rate

Question 5

What is an inotropic factor?

Answer 5

Anything affecting stroke volume

Question 6

How is stroke volume calculated?

Answer 6

The difference between the end diastolic volume (EDV) and the end systolic volume (ESV)
* SV= EDV-ESV
* With every beat the heart pumps about 60% of the blood in its chambers (65-70 mL)

Question 7

What are the 3 intrinsic and extrinsic controls of stroke volume?

Answer 7

1. Preload
2. Afterload
3. Contractility

Question 8

How is preload an intrinsic control?

Answer 8

• Determines the intial (pre-contraction) muscle fibre length
• Left ventricle - when EDV and EDP increase = preload increase
• LVEDV and LVEDP will be used as the index of preload of the left ventricle

Question 9

How is pre load related to ‘Starling’s law of the heart’ or ‘Frank-Starling mechanism’?

Answer 9

If LVEDV increases, the left ventricle responds by doing more work
- Extra work done by the left ventricle produces a larger stroke volume
- More fill = more empty

Question 10

What accounts for the Frank-Starling mechanism?

Answer 10

• Length-tension relationship
• Increase in Ca2+ sensitivity
• Increase in number and strength of cross-bridges

Question 11

What is afterload?

Answer 11

• Intrinsic control
• The tension the fibres must generate before they can shorten
• Afterload is the pressure that the ventricles must overcome to force open the aortic and pulmonary valves
• Increase in arterial pressure = increase afterload (hypertension)

Question 12

How is afterload determined?

Answer 12

Mean aortic or mean arterial pressure will be used as the index of afterload of the left ventricle

Question 13

Describe the extrinsic control of contractility:

Answer 13

• Catecholamines from the adrenal medulla, and sympathetic nerves in the heart, alter ventricular function without a change in LVEDV
• Improve contractiliy
• More “effective” pump
• Causes contraction and relaxation to occur more quickly

Question 14

If contractility increases what is the affect on stroke volume?

Answer 14

Stroke volume increases

Question 15

What is the ejection fraction?

Answer 15

Blood pumped out of the heart per beat
- The ratio of stroke volume (SV) to end-diastolic volume (EDV)

Question 16

What is the ejection fraction (EF) equation?

Answer 16

EF = SV/EDV
EF = 70/130
* Expressed as %, EF normally averages between 50-75% under resting conditions

Question 16

Describe the overall inotropic effect:

Answer 16

• Increase sympathetic nerve activity to heart
• Increase EDV
• Increase plasma epinephrine
  -> Increase stroke volume

Question 16

If contractility increases what happens to EF?

Answer 16

Ejection fraction increases

Question 17

Which mechanism has the major control on heart rate?

Answer 17

Extrinsic mechanisms (nervous and endocrine)

Question 18

What role do intrinsic mechanisms play in controlling heart rate?

Answer 18

• Very limited role
• Right atrial pressure has small effect of AP generation by SA node

Question 19

How do extrinsic mechanisms of heart rate operate?

Answer 19

• Tonic
• Mostly active and control CV system by altering level of their control, rather than switching on/off
• Without nervous control HR would be ~110bpm at rest

Question 20

What is a result of increasing parasympathetic nervous activity to the SA node?

Answer 20

Reduces HR: 110bpm-70bpm

Question 21

What is the result of increasing sympathetic nervous activity?

Answer 21

• Increases frequency of AP generation in the SA node
  -> Increase HR: 110bpm-170bpm

Question 22

What effect do agents that alter heart rate have?

Answer 22

They have a chronotropic effect

Question 23

How are chronotropic effect of the nervous system brought about?

Answer 23

Alterations to the slope of the slow diastolic pacemaker potential through changes in the permeability of the SA node cell membrane to K+, Na+ and Ca2+

Question 24

What are 6 responses of the heart to increased sympathetic activity?

Answer 24

1. Increase heart rate 2. Increase conduction velocity 3. Reduction in duration of diastole 4. Small reduction in duration of systole 5. Synchronisation of contractions 6. Elevated contractility - SV maintained despite HR increase

Question 25

What is cardiac work?

Answer 25

The work the heart has to do - Metabolic cost (O2 demand) is determined by tension development in the cardiac muscle (Law of laplace)

Question 26

What is the law of laplace?

Answer 26

* Muscle tension (T) to develop ventricular pressure (P) depends on radius of the ventricle (r) and the thickness of the ventricle wall (u) * Tension = transmural pressure x radius/ 2 x wall thickness * 𝑻 =𝑷 𝑿 𝒓/ 𝟐 𝑿 u

Question 27

What is the relationship between tension and pressure, radius and thickness?

Answer 27

T α P T α r T α 1/u

Question 28

What affects metaboloic cost more, tension development or muscle shortening?

Answer 28

Tension development

Question 29

What is needed in order for a constant afterload?

Answer 29

Tension that has to be generated by the ventricular muscle increases as the radius (LVEDV) increases

Question 30

What is needed for a constant radius?

Answer 30

Tension that has to be generated by the ventricular muscle increases as the afterload increases

Question 31

What is acute heart failure (preload)?

Answer 31

* Left ventricle unable to contract effectively at a normal LVEDV * It fills more - larger LVEDV - in order to generate a normal stroke volume - Continuously larger volume (radius) - Ventricle generates more tension - Dilated ventricle has thinner wall (u) = problem

Question 32

What is arterial hypertension (afterload)?

Answer 32

* Ventricle has to generate higher pressure than normal - Requires greater tension development * Hypertrophy of the ventricular wall increases wall thickness, bringing wall tension development back towards normal * Larger mass of heart muscle will require more oxygen (and coronary blood flow reduced due to thick muscle)