Module 5: Section 5

Question 1

Cardiac output (CO)

Answer 1

• The volume of blood that is pumped by each ventricle over a minute of time
• Same for both sides of the heart

Question 2

Equation for CO

Answer 2

Heart rate x Stroke volume = Cardiac Output

Question 3

Heart Rate (HR)

Answer 3

• At rest heart rate is 70 bpm
• This increases with an increase of activity

Question 4

Stroke volume (SV)

Answer 4

• Each time a ventricle contracts, it ejects a certain amount of blood
• At rest, SV is approximately 70ml

Question 5

Result of CO equation

Answer 5

• Amounts to almost 5L of blood being pumped out of each ventricle every minute at rest
• During exercise can increase to around 20L/min

Question 6

How to change your heart rate?

Answer 6

• Influence the rate at which the SA node depolarizes
• Heart is innervated by the sympathetic and parasympathetic nervous systems

Question 7

Parasympathetic system - changes heart rate

Answer 7

• Via the vagus nerve has rich innervation to the atria
• Mainly innervates the SA and AV nodes in the atria, with only minimal innervation to the ventricles

Question 8

Sympathetic system - changes heart rate

Answer 8

• Innervates the atria, including both nodes
• More richly innervates the ventricles

Question 9

Parasympathetic stimulation - 4 effects on the heart

Answer 9

1. Slowing heart rate
2. Reducing the AV nodes excitability
3. Shortening atrial action potentials
4. very little stimulation of the ventricles

Question 10

Slowing the heart rate

Answer 10

• The parasympathetic system releases acetylcholine at the SA node, which increases K⁺ permeability and hyperpolarizes the membrane
• This makes it harder to reach threshold and decreases the slope of the pacing current (If), slowing the firing rate of the SA node and reducing heart rate

Question 11

Reducing the AV nodes excitability

Answer 11

• Effect on AV node are same as SA node
• Increased K+ permeability hyperpolarizes the AV node membrane and makes the node less excitable

Question 12

Shortening atrial action potentials

Answer 12

• Parasympathetic stimulation increases K⁺ permeability in atrial cells, causing faster repolarization
• This shortens the plateau phase, so less Ca²⁺ enters the cell, resulting in a weaker atrial contraction

Question 13

Very little stimulation of the ventricles

Answer 13

• Very little parasympathetic innervation to the ventricular muscle cells so it has very little effect on the ventricles
• Majority of its innervation is on the SA node, which initiates atrial contraction and the AV node

Question 14

Sympathetic stimulation - 4 effects on the heart

Answer 14

1. Increasing heart rate
2. Increasing AV nodes excitability
3. Enhancing conduction speeds
4. Increasing contractility of cardiac muscles

Question 15

Increasing heart rate

Answer 15

• The sympathetic system releases norepinephrine at the SA node, which enhances If and T-type Ca²⁺ currents
• This increases the pacing current slope, allowing the membrane potential to reach threshold faster and increasing heart rate

Question 16

Increasing AV nodes excitability

Answer 16

• Release of norepinephrine at the AV node also increases its excitability by decreasing the AV node delay
• Allows the wave of excitation to reach the ventricles faster

Question 17

Enhancing conduction speeds

Answer 17

The sympathetic system increases the conduction speed of the AV node, bundle of His, and Purkinje fibres, allowing the wave of excitation to travel through the heart faster, reducing delay and improving coordination of contractions

Question 18

Increasing contractility of cardiac muscles

Answer 18

• The sympathetic system increases Ca²⁺ permeability during the plateau phase, allowing more Ca²⁺ to enter and enhance CICR (calcium-induced calcium release)
• This strengthens atrial and ventricular contractions, increasing stroke volume and reducing end systolic volume

Question 19

Injury to the vagus nerves

Answer 19

• If vagus nerves were cut, influence of parasympathetic system would be removed and resting heart rate would increase
• Pacing rate of SA node is around 100bpm, parasympathetic system reduces it to 70bpm

Question 20

Activity

Answer 20

• HR has to increase to deliver O2 to working muscles
• Parasympathetic input decreases and an increase in sympathetic input
• Sympathetic system dominates and increases HR

Question 21

Fever

Answer 21

HR will increase to increase the amount of blood flow to the tissues and dissipate heat through the skin

Question 22

2 types of controls for stroke volume

Answer 22

1. Extrinsic control
2. Intrinsic control

Question 23

Extrinsic control - stroke volume

Answer 23

• Factors outside the heart that influence contractility
• Primary extrinsic control is the sympathetic nervous system

Question 24

Intrinsic control - stroke volume

Answer 24

• Refers to factors within the heart itself that control contractility
• Primary intrinsic control is the volume of blood in the ventricle at the end of diastole

Question 25

Relationship Between end-diastolic volume (EDV) and stroke volume (SV)

Answer 25

- as more blood fills the ventricles, more blood is pumped out - The ventricles never completely empty during systole

Question 26

Length-Tension Relationship in Cardiac Muscle

Answer 26

- Stretching of cardiac muscle fibres (from increased blood volume) enhances tension and contraction strength - At rest, the fibres are below optimal length, so increasing stretch increases force of contraction up to an optimal point

Question 27

Frank-Starling Law of the heart

Answer 27

- Relationship between EDV and SV - Increasing EDV increases SV, the greater the diastolic filling, the greater the systolic emptying - Allows for beat-to-beat regulation of stroke volume

Question 28

Two forces that affect EDV

Answer 28

1.Preload 2. Afterload

Question 29

Preload

Answer 29

- Amount of blood returning to the ventricle - Also called venous return - Preload is increased by increasing venous return and therefore EDV and stroke volume - Preload stretches the right or left ventricles to its greatest dimensions

Question 30

Afterload

Answer 30

- The force that the ventricle is pushing against, mainly due to aortic pressure - Increased afterload causes the aortic valve to close earlier, decreasing stroke volume - However, if blood volume returns to normal, the next contraction force increases due to a higher EDV

Question 31

Ejection Fraction

Answer 31

It's the percentage of blood ejected from the ventricles during systole relative to the amount present before contraction

Question 32

Ejection fraction equation

Answer 32

Stroke volume/end diastolic volume = ejection fraction

Question 33

Normal Ejection Fraction

Answer 33

- At rest, normal ejection fraction is ~60%, meaning 40% of blood remains in the ventricles as a reserve - Remaining 40% can be used when the body needs to increase cardiac output