Session 2: Control Of Cardiac Output

Question 1

What is afterload?

Answer 1

The load the heart must eject blood against (roughly equivalent to aortic pressure)

Question 2

What is preload?

Answer 2

Amount the ventricles are stretched (filled) in diastole - related to EDV or CVP

Question 3

What is total peripheral resistance?

Answer 3

Resistance to blood flow offered by all the systemic vasculature (also referred to as systemic vascular resistance)

Question 4

What happens to pressure of fluid in a tube as it encounters resistance?

Answer 4

• ‘Resistance’ is associated with vasoconstriction
• pressure on arterial side increases
• Pressure of fluid in the capillaries and on venous side fall

Question 5

What is the effect of decreasing TPR (CO unchanged)?

Answer 5

Arterial pressure falls

Venous pressure rises

Question 6

What is the effect of increasing TPR (CO unchanged)?

Answer 6

Arterial pressure rises

Venous pressure falls

Question 7

What is the effect of increasing CO (TPR unchanged)?

Answer 7

Arterial pressure rises

Venous pressure falls

Question 8

What is effect of decreasing CO (TPR unchanged)?

Answer 8

Arterial pressure falls

Venous pressure rises

Question 9

What happens if the demand for blood in tissues increases?

Answer 9

Arterioles and precapillary sphincters dilate

Therefore TPR falls

Question 10

How did cardiac output calculated?

Answer 10

CO = Stroke volume x heart rate

Question 11

How is stroke volume calculated?

Answer 11

SV = EDV - ESV

Question 12

What is the typical stroke volume for a 70kg man?

Answer 12

70ml

Question 13

What is the relationship between venous pressure and filling of the heart?

Answer 13

The higher the venous pressure the more the heart fills

Question 14

What does the Frank-Starling law of the heart state?

Answer 14

If you stretch the fibres of the heart before contracting, force of contraction increases

Question 15

What does compliance mean?

Answer 15

How easily a chamber of the heart or the lumen of a blood vessel expands when it is filled with a volume of blood

Question 16

What does the Starling curve show?

Answer 16

LV end-diastolic pressure and stroke volume have a direct relationship

Question 17

What is the intrinsic control mechanism of the heart?

Answer 17

Increased stroke volume with increased filling of the heart

Question 18

What is contractility?

Answer 18

Force of contraction for a given fibre length

Question 19

What is the effect of increasing arterial pressure on stroke volume?

Answer 19

• arterial pressure is increased when TPR is increased
• therefore venous pressure is reduced
• reduced filing of the heart
• Stroke volume decreases

Question 20

What factors determine cardiac output?

Answer 20

• Stroke volume
• Heart rate
• End-systolic volume
• end-diastolic volume
• arterial blood pressure
• aortic impedance
• contractility
• autonomic nervous system

Question 21

What does increased metabolism of the body result in?

Answer 21

TPR falls
Arterial pressure falls
Venous pressure rises
Heart pumps more blood

Question 22

How does the CVS respond to eating a meal?

Answer 22

Local vasodilation in the gut

• TPR falls
• arterial pressure falls
• Venous pressure rises
• stroke volume increases
• heart rate increases
• cardiac output increases
• arterial pressure rises
• venous pressure falls

Question 23

What effect does standing up have on the CVS?

Answer 23

• ‘pooling’ of blood in legs
• venous pressure falls
• arterial pressure falls
• baroreceptor reflex and ANS increase heart rate and TPR

Question 24

What happens if baroreceptor and ANS reflexes don’t work when standing up?

Answer 24

Postural/orthostatic hypotension

Question 25

What changes occur in the CVS when exercising?

Answer 25

• initially: venoconstriction returns more blood to the heart (because venous pressure rises)
• later: decreased TPR also increases venous return
• very early response of increased heart rate
• increased contractility
• INCREASED CARDIAC OUTPUT