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 (will still be lower than arterial)

- 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) the bigger the stroke volume

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 need a good blood supply to increase absorption reduce in TPR, reduce in arterial pressure, increase in venous pressure, increase HR, increase SV, increase cardiac output

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 - increased venous pressure, increased heart rate and increased contractility - Later, decreased TPR also increases venous return

Question 29

afterload

Answer 29

load the heart must eject blood against

Question 30

preload

Answer 30

amount the ventricles are stretched/filled in diastole

Question 31

central venous pressure

Answer 31

pressure in the large veins draining into the heart

Question 32

arterial pressure

Answer 32

pressure in the large arteries

Question 33

total prtipheral resistance

Answer 33

resistance to the blood flow offered by all the systemic vasculture

Question 34

relationship between pressure and resistance

Answer 34

pressure increases as resistance increases to maintain flow

Question 35

which vessel has the greatest resistance

Answer 35

arterioles, they have a very narrow lumen and can constrict so increases the resistance

Question 36

order of pressure in vessels

Answer 36

highest to lowest arteries, arterioles, capillaries, venule, veins

Question 37

explain this

Answer 37

in B, the middle vessel is much smaller so **increased resistance** so **pressure** on **arteriole** end **increases**, the **pressure** on the **venule** end **decreases** to maintain flow

Question 38

describe what happens if there is a fall in peripheral resistance

Question 39

descrive ventricular filling

Answer 39

- 70% of the blood flowing into the ventricle from the atria is passivly due to pressure gradient - ventricle fills until the intraventricular pressure = venous pressure - higher the venous pressure the more the heart fills

Question 40

describe decreased compliance

Answer 40

as you fill the heart the pressure increases - occurs in a heart that is hypertrophied

Question 41

factors that determine cardiac output

Answer 41

1. how **hard ventricle contracts** which depends on the end **diastolic volume** (how much the heart fills) and **contractility** **2**. how hard it is to eject blood (arterial pressure) 3. CO = SV x HR

Question 42

effects of TPR (occurs when lumen is smaller)

Answer 42

- make it harder for the blood to pump blood out - reduces the venous pressure so reduces the filling of the heart

Question 43

roles of the TPR (occurs when the vessel lumen is smaller)

Answer 43

- harder for blood to pump out - reduces the venous pressure so less blood can fill the heart

Question 44

Which pressure is important for the filling of the heart

Answer 44

venous pressure (blood comws from the vena cave)

Question 45

relationship between venous and arterial pressure

Answer 45

as one increases the other decreases to maintain the flow of the blood

Question 46

demand led pumping

Answer 46

if metabolism of body drops TPR reduces to increase blood supply arterial pressure drops venous pressure increases so heart pumps more

Question 48

conditions that increase jugular venous pressure

Answer 48

- right side of heart doesnt pump out blood properly - volume overload - impaired filling of the heart