Control of Cardiac Output - Lecture 12

Question 1

What is cardiac output?

Answer 1

Stroke volume x Heart rate
○ Stroke volume = volume of blood ejected by each ventricle per beat
○ Heart rate = number of heart beats per minute

Question 2

What is the cardiac output, heart rate and stroke volume at rest?

Answer 2

○ CO = 5L
○ HR = 70 bpm
○ SV = 70ml

Question 3

What is the cardiac output, heart rate and stroke volume during exercise?

Answer 3

○ CO = 20L
○ HR = 190-200 bpm
○ SV = 105ml

Question 4

What is the cardiac output, heart rate and stroke volume at rest for an athlete?

Answer 4

○ CO = 5.5L
○ HR = 40 bpm (Lower)
○ SV = 140ml (Higher)

Question 5

What is the cardiac output, heart rate and stroke volume during exercise for an athlete?

Answer 5

○ CO = 40L (Higher)
○ HR = 190 bpm
○ SV = 210ml (Higher)

Question 6

Why do athletes have a higher stroke volume?

Answer 6

Physiological left ventricular hypertrophy = chamber sizes increase

Question 7

What are factors affecting heart rate called?

Answer 7

Chronotrophic effects

Question 8

What factors affect heart rate?

Answer 8

○ Autonomic innveration
○ Hormones
○ Venous return

Question 9

How does the parasymphatetic system affect heart rate?

Answer 9

○ Cardioinhibitory centre is activated
○ Impulses increase in the vagus nerve as well as acetylcholine
○ Pacemaker cells slow down
○ Heart rate decreases
○ Negative chronotrophic effect

Question 10

How does the sympathetic system affect heart rate?

Answer 10

○ Cardioacceleratory centre is activated
○ Noradrenaline is secreted from the sympathetic ganglia or from adrenal medulla
○ Pacemaker cells speed up
○ Increases heart rate
○ Positive chronotrophic effect

Question 11

Where is the cardioregulatory system found?

Answer 11

Medulla obolongata

Question 12

What does depolarisation at the SA node involve?

Answer 12

○ HCN channel opens to allow entry of Na+: Increases membrane potential
○ At threshold potential: calcium channel opens

Question 13

What does repolarisation at the SA node involve?

Answer 13

○ K+ leaves the cell
○ Membrane potential decreases

Question 14

How does parasympathetic system affect repolarisation?

Answer 14

○ K+ channel is sensitive to acetylcholine
○ More K+ leaves the cell
○ More -ve membrane potential
○ Takes longer to reach threshold potential so HR slows down

Question 15

How does sympathetic system affect depolarisation?

Answer 15

○ Increases depolarisation
○ Reduces repolarisation
○ Threshold potential is reached quicker
○ HR increases

Question 16

What is vagal tone?

Answer 16

○ Parasympathetic activity is higher than sympathetic activity at rest which reduced the rate of SA node
○ Athletes have higher parasympathetic activity so lower HR at rest

Question 17

How does venous return affect heart rate?

Answer 17

If venous return increases:
○ Atria and pacemaker cells are stretched more
○ Stretch receptors activate sympathetic nervous system
○ Heart rates increases

Question 18

What is the equation for stroke volume?

Answer 18

End diastolic volume - end systolic volume

Question 19

What factors affect end diastolic volume?

Answer 19

Preload

Question 20

What factors affect end systolic volume?

Answer 20

○ Preload
○ Contractility
○ Afterload

Question 21

What is preload?

Answer 21

The degree to which ventricular muscle cells are stretched at the end of diastole

Question 22

What is contractility?

Answer 22

The force produced by ventricular muscle cells during systole at a given period

Question 23

What is afterload?

Answer 23

The force the ventricle needs to overcome to open the semilunar valve and eject blood

Question 24

What factors affect preload?

Answer 24

○ Rate of venous return
○ Available ventricular filling time
○ Preload is directly proportional to EDV

Question 25

What is Frank-Starling law?

Answer 25

The force developed in a muscle fibre is dependent on the extent it is stretched

Question 26

What factors affect venous return?

Answer 26

○ Posture: blood pools in leg venins when standing due to gravity so venous return decreases
○ Skeletal muscle pump: movement of skeletal muscles constricts veins aiding venous return so venous return increases’
○ Respiratory pump: Inspiration reduces intrathoracic pressure whilst increasing intraabdominal pressure so increases venous return
○ Venous capacitance: SNS activity reduces compliance and increases central venous pressure so increases venous return

Question 27

What factors affect contractility?

Answer 27

○ Autonomic innervation
○ Hormones

Question 28

What are factors affecting contractility called?

Answer 28

Inotropic effects

Question 29

How does sympathetic and parasympathetic system affect contractility?

Answer 29

○ Sympathetic: positive inotropic effects
- Increases force of contraction and velocity of conduction
○ Parasympathetic: negative inotropic effects

Question 30

What factor affects afterload?

Answer 30

Vascular tone

Question 31

What is vascular tone?

Answer 31

○ Vasodilation reduces afterload
○ Vasoconstriction increaseas afterload

Question 32

How does increased afterload affect stroke volume?

Answer 32

○ Decreases stroke volume
○ Shorter ejection
○ Longer isovolumetric contraction
○ Increased ESV
○ Prolonged increases in afterload damage the myocardium and lead to heart failure