S2) Control of Cardiac Output

Question 1

What are the 7 phases of the cardiac cycle?

Answer 1

• Atrial contraction
• Isovolumetric contraction
• Rapid ejection
• Reduced ejection
• Isovolumetric relaxation
• Rapid filling
• Reduced filling

Question 2

How can the cardiac cycle be split into 2 phases?

Answer 2

• Systole: isovolumetric contraction, rapid ejection, reduced ejection
• Diastole: isovolumetric relaxation, rapid filling, reduced filling, atrial contraction

Question 3

What happens to systole and diastole when the HR increases?

Answer 3

When the heart rate increases, systole stays the same but diastole gets shorter

Question 4

In terms of left atrial pressure, left ventricular volume, and the ECG, explain the changes occuring in Phase 1: Atrial Contraction

Answer 4

At the end of Phase 1, ventricular volumes are maximal: termed the End-Diastolic Volume (EDV) typically ~120 ml

Question 5

What is End Diastolic volume?

Answer 5

End-Diastolic Volume is the maximal ventricular volume and occurs at the end of atrial contraction (~120 ml)

Question 6

In terms of left ventricular pressure, left atrial pressure, left ventricular volume, the ECG and the phonocardiogram, explain the changes occuring in Phase 2: Isovolumetric Contraction

Answer 6

Question 7

In terms of aortic pressure, left atrial pressure and left ventricular volume, explain the changes occuring in Phase 3: Rapid Ejection

Answer 7

Question 8

In terms of left ventricular pressure, left atrial pressure and the ECG, explain the changes occuring in Phase 4: Reduced Ejection

Answer 8

Question 9

In terms of aortic pressure, left ventricular pressure, left ventricular volume and the phonocardiogram, explain the changes occuring in Phase 5: Isovolumetric Relaxation

Answer 9

Question 10

In terms of left atrial pressure and left ventricular pressure, explain the changes occuring in Phase 6: Rapid Filling

Answer 10

Question 11

In terms of left ventricular volume, explain the changes occuring in Phase 7: Reduced Filling

Answer 11

Question 12

What is cardiac output?

Answer 12

Cardiac output is the volume of blood pumped per minute by the left side of the heart

CO = HR x SV

Question 13

Explain how end diastolic volume is determined by the filling of the heart

Answer 13

• During diastole, the ventricles fill as the venous pressure drives blood into them
• The passive stretch of the ventricular wall causes intra ventricular pressure to rise, until it matches venous pressure, when no more filling will occur

Question 14

How is stroke volume determined?

Answer 14

• Stroke volume is determined by how much the ventricle contracts during systole
• All myocardial cells normally contract, so active tension is changed by factors which act directly upon individual myocardial cells

Question 15

Define the terms preload and afterload

Answer 15

• Afterload is the load the heart must eject blood against (~equivalent to aortic pressure)
• Preload is the amount the ventricles are stretched in diastole (related to the EDV or cVP)

Question 16

Define the term total peripheral resistance

Answer 16

Total peripheral resistance (aka systemic vascular resistance) is the resistance to blood flow offered by all the systemic vasculature

Question 17

What are the effects of changing total peripheral resistance?

Answer 17

- If TPR decreases and CO is unchanged:

I. Arterial pressure will decreases

II. Venous pressure will increase

- If TPR increases and CO is unchanged:

I. Arterial pressure will increase

II. Venous pressure will decrease

Question 18

What are the effects of changing cardiac output?

Answer 18

- If CO increases and TPR is unchanged:

I. Arterial pressure will increase

II. Venous pressure will decrease

- If CO decreases and TPR is unchanged:

I. Arterial pressure will decrease

II. Venous pressure will increase

Question 19

Explain how the heart responds to an increased demand for blood

Answer 19

• Arterioles and precapillary sphincters dilate
• Total peripheral resistance falls
• Heart pumps more blood so aBP does not fall and cVP doesn’t rise

Question 20

Referring to the profile of pressure changes in the internal jugular vein, identify the following component areas:

Answer 20

Question 21

What is the Frank-Starling Law of the Heart?

Answer 21

Frank – Starling law of the heart: the more the heart fills, the harder it contracts (up to a limit)

Question 22

How does the Frank Starling Law of the heart explain the relationship between the stroke volume and central venous pressure?

Answer 22

• Increased venous pressure causes the heart to fill more
• Increased force of contractions increases the stroke volume

Question 23

Explain how the Starling Law of the heart ensures that both sides are balanced

Answer 23

The increased stroke volume with increased filling of the heart ensures that both sides of the heart maintain the same output as the pulmonary and systemic circulations operate in series

Question 24

The relationship between left ventricular pressure and volume is illustrated in the Ventricular Compliance Curve.

Outline this

Answer 24

• In diastole, ventricle fills until the walls stretch enough to produce an intraventricular pressure equal to the venous pressure.
• The higher the venous pressure, the more the heart fills

Question 25

What is contractility?

Answer 25

Contractility (inotropy) is the force of contraction for a given fibre length and is stimulated by the sympathetic nervous system and circulating adrenaline

Question 26

Explain how the CVS responds to eating a meal

Answer 26

Local vasodilation in the gut

Question 27

Explain how the CVS responds to standing up

Answer 27

• Standing up causes ‘pooling’ of blood in legs due to gravity as both arterial and venous pressures have changed in the same direction
• Baroreceptor reflex and autonomic nervous system increase HR and TPR

Question 28

Explain how the CVS responds to exercise

Answer 28

• Muscle pumping and venoconstriction returns more blood to the heart
• Later, decreased TPR also increases venous return