5 - 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 in diastole (related to end diastolic volume and central venous pressure)

Question 3

What is total peripheral resistance?

Answer 3

Resistance to blood flow offered by all the systemic vasculature

Question 4

Constriction of arterioles increases resistance. What is the effect on blood pressure on the arterial and venous sides of the arteriole?

Answer 4

Arterial side - increased pressure

Venous side - decreased pressure

Question 5

If total peripheral resistance falls and cardiac output is unchanged what is the effect on arterial and venous blood pressure?

Answer 5

Arterial pressure - decreases

Venous pressure - increases

Question 6

If total peripheral resistance increases and cardiac output is unchanged what is the effect on arterial and venous pressure?

Answer 6

Arterial pressure - increases

Venous pressure - decreases

Question 7

If TPR is unchanged what is the effect on arterial and venous pressure of an:

• Increase in cardiac output
• Decrease in cardiac output

Answer 7

• Increase in cardiac output
  
  • Arterial pressure - increases
  • Venous pressure - decreases
• Decrease in cardiac output
  
  • Arterial pressure - decreases
  • Venous pressure - small increase (not emptying the heart as well)

Question 8

Why is there a small drop in central venous pressure when cardiac output is reduced? CVP falls even more during heart failure, what can this lead to?

Answer 8

• The heart isn’t emptying as well = increased preload
• In heart failure, this can lead to peripheral or pulmonary oedema

Question 9

If tissues need more blood, the ……….. will dilate and peripheral resistance falls. Why does the heart begin to pump more?

Answer 9

• arterioles
• So the arterial pressure doesn’t fall and central venous pressure doesn’t rise

Question 10

Stroke volume = …………. - …………..

Answer 10

Stroke volume = end diastolic volume - end systolic volume

Question 11

What is ventricular compliance?

Answer 11

The relationship between LV volume and LV pressure. The more the heart fills, the higher the pressure. Compliance is increased or decreased in disease states.

Question 12

Describe the Frank-Starling law of the heart.

Answer 12

• If cardiac muscle is stretched more, it will contract harder (up to a limit)
• The harder the heart contracts, the bigger the stroke volume

Increasing venous return (& LVEDP/V) leads to an increase in stroke volume

Question 13

Why does the Frank-Starling mechanism occur?

Answer 13

• Due to the optimum sarcomere length, if it is too short the filaments overlap and interfere with each other
• In cardiac muscle, there is also an increase in calcium sensitivity as the fibres are streched

Question 14

What is contractility? What extrinsic factors can affect contractility?

Answer 14

Force of contraction for a given fibre length. Affected by sympathetic stimulation and circulating adrenaline.

Question 15

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

Answer 15

• Increased afterload - makes it harder for the heart to pump
• Increased TPR also reduces venous pressure
• Filling of the heart and stroke volume are reduced

Question 16

What two factors determine how much the ventricle can empty?

Answer 16

1. How hard the heart contracts (EDV and contractility)
2. How hard it is to eject blood (aortic impedance)

Question 17

Describe how the heart responds to changes in blood pressure during exercise.

Answer 17

• As the metabolism of the body increases, the TPR will fall to supply more blood
• Arterial pressure will drop and venous pressure will increase
• The heart responds to this by pumping more
• The heart also pumps more (increased rate and contractility) due to decreased parasympathetic drive and increased sympathetic drive

Question 18

What blood pressure changes occur when standing up and how does the body protect against postural hypotension?

Answer 18

• Standing up causes blood to pool in the legs, both venous and arterial pressure are reduced
• The baroreceptor reflex and ANS increase heart rate and TPR (no intrinsic mechanisms)

Question 19

How is the jugular venous pressure measured and what does it represent?

Answer 19

• Measured in right internal jugular vein ~4cm above the sternal angle
• Normal range is 5 to 8 cm H2O
• Can also be measured with a central line
• Represents the blood pressure in the right atrium (preload)

Question 20

What conditions may increase JVP?

Answer 20

• Where the right side of the heart isn’t pumping blood properly
• Volume overload with IV infusion
• Something that impairs filling of the heart