Cardiac Output And Control

Question 1

Define cardiac output

Answer 1

Cardiac output is the volume of blood pumped by each ventricle per minute
The volume of blood in systemic circulation = volume in pulmonary
Therefore each ventricle has roughly the same CO

Question 2

Give the equation for cardiac output

Answer 2

1. Heart rate /bpm - number of beats per minute
2. Stroke volume /ml - volume of blood pumped per beat
3. Cardiac output = HR x SV

Question 3

Explain autonomic control of the heart

Answer 3

1. Autonomic control is regulated by sympathetic and parasympathetic pathways
2. Sympathetic nerves increase heart rate by releasing norepinephrine to cardiac receptors
3. Parasympathetic nerves lower heart rate by the vagus nerve releasing acetylcholine to cardiac receptors

Question 4

Explain how the SA node and AV node are effected by parasympathetic stimulation

Answer 4

1. Upon sympathetic stimulation, the SA node is more easily depolarised, therefore higher rate of depolarisation and heart rate
2. Upon parasympathetic stimulation the rate of depolarisation falls and heart rate is lowered
3. Upon sympathetic = lowers AV nodal delay
4. Upon parasympathetic = increases AV nodal delay

Question 5

Explain what intrinsic and extrinsic control means in relation to stroke volume

Answer 5

1. Intrinsic control refers to the extent of venous return (higher return = more blood ready to be pumped)
2. Extrinsic control refers to the strength/frequency of sympathetic stimulation
3. Intrinsic refers to volume of blood available after diastole, and extrinsic refers to strength of heart contraction
4. Both increase stroke volume

Question 6

Explain Frank-Starling’s Law of the Heart

Answer 6

The systole of the heart pumps the same volume as the volume of blood returned during diastole. So to increase the systole, and thus the stroke volume, there must be increased venous return

Question 7

Explain how optimising tension in cardiac muscle can lead to increased stroke volume

Answer 7

1. The cardiac muscle fibre usually has a length less than the optimal length (I0)
2. I0 length leads to maximum tension at during contraction, leading to a higher stroke volume
3. It was found increasing end-diastolic volume via increased venous return lengthens cardiac muscle fibres
4. Enhancing contractile tension, leading to a stronger contraction and a higher CO

Question 8

Explain the frank-starling curve and then explain how it gets shifted to the left

Answer 8

1. Frank starling curve shows the relationship between stroke volume and end-diastolic volume
2. Upon sympathetic stimulation the contractility (strength of contraction) is increased
3. Leads to leftward/upward shift of the hyperbola

Question 9

Explain the characteristics of heart failure

Answer 9

1. There is a decrease in cardiac contractility
2. Triggering increase of sympathetic activity leading to increase in noradrenaline
3. In the short term it increases HR and contractility but prolonged sympathetic activation leads to adverse remodelling of heart structure
4. Leads to myocardial hypertrophy, myocardial fibrosis and arrhythmias

Question 10

How is arterial blood pressure used as an indicator to the volume of blood in blood vessels

Answer 10

1. If ABP drops it is detected by baroreceptors which triggers the Baroreceptor reflex
2. Baroreceptors sense artery wall stretch, not pressure directly
3. Regulatory negative feedback responses to lowered pressure maintains ABP by increasing blood flow