M3 Topic 4: Cardiac Output

Question 1

What is cardiac output (CO)?

Answer 1

The amount of blood that is pumped out of the heart each minute

• Variable, can increase according to body demands (e.g. exercise)

Question 2

What is the average cardiac output in a typical healthy adult at rest?

Answer 2

5L/min

• Typical adult blood volume is also 5L, thus, entire blood volume passes through circulation each minute

Question 2

What are the two factors that determine CO?

Answer 2

• Heart rate (HR)
• Stroke volume (SV)

Any changes in these factors will affect CO

Question 3

Heart rate (HR)

Answer 3

Measure of how often the heart contracts in BPM

Question 4

Stroke volume

Answer 4

Volume of blood pumped out of ventricle in single contraction (beat)

Question 5

CO formula

Answer 5

CO = HR x SV

Question 6

What can affect HR?

Answer 6

• Type of activity
• Cardiovascular disease
• ANS (with both parasympathetic and sympathetic nerves)
• Circulating hormones
• Changes in plasma ion levels

Question 7

Activities that affect HR

Answer 7

Exercise = increase in HR
Sleep = decrease in HR

Question 8

Cardiovascular diseases that affect HR

Answer 8

Tachycardia = abnormally fast resting HR
Bradycardia = abnormally slow resting HR

Question 9

Autonomic nervous system impact on HR

Answer 9

Rate of SA node firing AP’s determines HR

• SA node innervated by both parasympathetic and sympathetic nerves

Regulates HR by changing spontaneous depolarisation of pacemaker cells

• Done by altering opening and closing of different ion channels (altering ion permeability)

Question 10

Activation of the sympathetic NS affect on HR

Answer 10

Causes SA node to fire more rapidly, increasing HR, increasing CO

Question 11

Activation of the parasympathetic NS affect on HR

Answer 11

Causes SA node to fire less rapidly, decreasing HR, decreasing CO

Question 12

Circulating hormones affect on HR

Answer 12

Adrenaline

• Released into circulation from adrenal medulla during sympathetic NS activation

Question 13

Changes in plasma ion levels impact on HR

Answer 13

Imbalance in plasma electrolytes can lead to risk of serious damage to heart

• Changes in blood K+ or Ca2+ effects electrical activity, can be life-threatening, risk of arrhythmias

Question 14

What are the two components of SV?

Answer 14

• End diastolic volume (EDV)
• End systolic volume (ESV)

SV = difference between EDV and ESV = volume of blood pumped out of ventricle

Question 15

End diastolic volume (EDV)

Answer 15

Volume of blood in ventricle at the end of diastole (before it begins to contract)

Question 16

End systolic volume (ESV)

Answer 16

Volume of blood left in ventricle at the end of systole (after contraction)

Question 17

What is the average SV in a healthy adult at rest?

Answer 17

70mL/beat

Question 18

What is the key factor that determines ESV?

Answer 18

Contractility of the heart, the ability of the heart to generate force

Question 19

What is the key factor that determines EDV?

Answer 19

Venous return (VR)

• Amount of blood being returned to heart
• Larger VR = larger EDV

Question 20

Frank-Starling Law of the Heart

Answer 20

Increases in EDV lead to an increase in SV

• The more blood returned to heart, the greater the stretch on ventricular wall, increases force of contraction so heart can eject more blood

Question 21

Factors that affect the VR and EDV

Answer 21

• Venous pressure
• Duration of ventricular diastole

Question 22

What are the 3 physiological mechanisms that help maintain venous pressure and VR?

Answer 22

• Skeletal muscle pump
• Respiratory pump
• Sympathetic constriction of veins

Question 23

Skeletal muscle pump

Answer 23

Skeletal muscle surround many deep veins in body, those adjacent to these veins contract and relax, acting as pump that squeezes veins, propelling blood back towards heart

• Action is activity dependent. E.g. standing still for long period time causes blood to pool in feet and legs, reducing VR

Question 24

Respiratory pump

Answer 24

Breathing in increases abdominal pressure, squeezes adjacent veins and propels blood back to heart

Question 25

Sympathetic constriction of veins

Answer 25

Veins are innervated by sympathetic NS - Activation of sympathetic NS causes constriction (narrowing) of veins, propelling blood back to heart

Question 26

How does a decrease in the duration of ventricular diastole decrease VR?

Answer 26

If ventricle relaxation and blood filling time is decreased, less blood will be returned to the heart before next contraction - Extremely high HR can thus lead to decrease of VR

Question 27

Factors that affect the ESV and cardiac contractility

Answer 27

- Activation of contractile proteins - Chemicals (positive and negative inotropes)

Question 28

Contractile proteins

Answer 28

Muscle (including cardiac muscles) contains contractile proteins - Activation of these contractile proteins allow muscle to generate force and shorten

Question 29

How are contractile proteins regulated?

Answer 29

Regulated by Ca2+ - If more of these ions enter cytoplasm of cell, more contractile proteins are activated, more force generated, cardiac contractability increased

Question 30

Positive inotropes

Answer 30

Substances that increase cardiac contractability - Includes hormones such as adrenaline and noradrenaline

Question 31

Negative inotropes

Answer 31

Substances that decrease cardiac contractibility