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? What is it related to?

Answer 2

• Amount the ventricles are stretched (filled) in diastole.

- Related to the end diastolic volume or central venous pressure.

Question 3

What is total peripheral resistance?

Answer 3

Resistance to blood flow offered by all the systemic vasculature (systemic vascular resistance).

Question 4

Which type of blood vessel offers greatest resistance?

Answer 4

Arterioles

Question 5

What is the effect of arteriolar contraction on blood pressure in the CVS?

Answer 5

• Pressure on arterial side increases.

- Pressure in capillaries and on venous side decreases.

Question 6

What are the effects of changing total peripheral resistance (if CO is unchanged)?

Answer 6

• Increased TPR… increased arterial pressure and decreased venous pressure.
• Decreased TPR… decreased arterial pressure and increased venous pressure.

Question 7

What are the effects of changing cardiac output (if TPR is unchanged)?

Answer 7

• Increased CO… increased arterial pressure and decreased venous pressure.
• Decreased CO… decreased arterial pressure and increased venous pressure.

Question 8

What happens in the CVS when a tissue required more blood?

Answer 8

1. Arterioles and precapillary sphincters dilate.
2. Fall in peripheral resistance.
3. Heart needs to increase CO so that arterial blood pressure (aBP) doesn’t fall and central venous pressure (CVP) doesn’t rise.

Question 9

What are cardiac output and stroke volume? What are their typical values in average man at rest?

Answer 9

• Cardiac output = stroke volume x heart rate
• Stroke volume = end diastolic volume - end systolic volume
• Typical SV = 70ml, typical heart rate = 70 beats/min, so typical CO = 4.9 l/min

Question 10

How can stroke volume be increased?

Answer 10

By increasing EDV or decreasing ESV.

Question 11

Why does increasing venous pressure increase stroke volume?

Answer 11

• The ventricle fills until the walls stretch enough to produce an intraventricular pressure equal to the venous pressure (depending on ventricular compliance).
• According to Frank-Starling Law of the Heart, the more the heart fills (increased venous return), the harder it contracts - increased LV end diastolic pressure and increased preload (volume).
• The harder the heart contracts, the bigger the stroke volume (as decreased ESV).

Question 12

Why does increasing myocyte stretching (i.e. increased venous return) cause increased contraction (i.e. increased LV end diastolic pressure)?

Answer 12

1. If sarcomere length is too short, filament overlap interferes with contraction.
2. Muscle fiber stretching increases calcium sensitivity.

Question 13

Give an example of an intrinsic control mechanism of CO.

Answer 13

Pulmonary and systemic circulations operate in series - same volume of blood pumped to body must also be pumped to lungs.
Starling’s Law causes increased heart filling to increase stroke volume. So both sides of heart pump maintain same output.
= intrinsic control mechanism.

Question 14

Give an example of an extrinsic control mechanism of CO.

Answer 14

• Contractility = force of contraction for a given fiber length. Increase in contractility increases contraction force for a given left EDP.
• Extrinsic factors such as sympathetic stimulation and circulating adrenaline can increase contractility,

Question 15

What is the effect of the autonomic nervous system on CO?

Answer 15

• ANS controls contractility and heart rate.
• Decrease in BP reduces parasympathetic NS activity and stimulates sympathetic NS - increases heart rate and contractility,

Question 16

How does the CVS respond to eating a meal?

Answer 16

1. Local vasodilation in gut… decreased TPR,
2. Decreased arterial pressure and increased venous pressure.
3. Increased heart rate and stroke volume… increased CO.
4. Increased arterial pressure and decreased venous pressure,

Question 17

How does the CVS respond to standing up?

Answer 17

1. Standing causes ‘pooling’ of blood in legs due to gravity.
2. Decreased venous pressure, decreased CO and decreased arterial pressure - cannot adjust by intrinsic mechanisms.
3. Baroreceptor reflex and ANS increase heart rate and TPR.

Question 18

Which condition occurs with deficient baroreceptor and ANS reflexes on standing?

Answer 18

Postural hypertension

Question 19

How does exercise increase CO?

Answer 19

Increased venous pressure, heart rate and contractility act to increase CO

1. Initially, muscle pumping and venoconstriction increase venous return to heart.
2. Later decreased TPR also increases venous return.
3. Very early response of increased heart rate - increased sympathetic NS.
4. Increased contractility - increased sympathetic NS.