6A - Control of heart rate

Question 1

What controls the regular beating of the heart?

Answer 1

Cardiac muscle.

Question 2

What does it mean that the heart is myogenic?

Answer 2

It can contract and relax without receiving signals from nerves - this pattern of contractions controls the regular heartbeat.

Question 3

How is heart rate controlled?

Answer 3

1) SAN in wall of RA sends out regular waves of electrical activity (excitation) to the atrial walls.
2) This causes the atria to contract at the same time.
3) Waves transferred from SAN to AVN (atrioventricular node).
4) Slight delay at AVN before electrical impulse is passed onto the bundle of His.
5) Bundle of His is a group of muscle fibres responsible for conducting the waves of excitation between the ventricles to the apex (bottom) of the heart. The bundle splits into finer muscle fibres in the right and left ventricle walls called the Purkinye tissue.
6) The Purkinye tissue carries the wave of excitation into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.

Question 4

What does the sinoatrial node do?

Answer 4

Sends out regular waves of electrical activity (excitation) to the atrial walls.

It is like a pacemaker setting the rhythm of the heartbeat.

Question 5

Why can’t the wave of excitation be passed straight from the atria to the ventricles?

Answer 5

Because a band of non-conducting collagen tissue (atrioventricular septum) prevents this.

Question 6

Why does the AVN delay the transmission of the impulse to the bundle of His?

Answer 6

To make sure the atria have emptied before the ventricles contract.

Question 7

How do the ventricles contract?

Answer 7

From the bottom (apex) up.

Question 8

What branch of the nervous system does control of heart rate involve?

Answer 8

Autonomic.

Question 9

What does the SAN do?

Answer 9

Generates electrical impulses that cause the cardiac muscles to contract.

Question 10

The rate at which the SAN fires is controlled by what?

Answer 10

Unconsciously by a part of the brain called the medulla oblongata.

Question 11

Why do animals need to alter their heart rate?

Answer 11

To respond to internal stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen.

Question 12

What are stimuli in the blood detected by?

Answer 12

Pressure receptors and chemical receptors.

Question 13

What are the pressure receptors called?

Answer 13

Baroreceptors.

Question 14

What are baroreceptors?

Answer 14

Pressure receptors.

Question 15

Where are baroreceptors?

Answer 15

Aorta and carotid arteries.

Question 16

Where are the carotid arteries?

Answer 16

(Major arteries) In the neck.

Question 17

What are baroreceptors stimulated by?

Answer 17

High and low blood pressure.

Question 18

What are the chemical receptors called?

Answer 18

Chemoreceptors.

Question 19

What are chemoreceptors?

Answer 19

Chemical receptors.

Question 20

Where are chemoreceptors?

Answer 20

Aorta, carotid arteries and medulla.

Question 21

What do chemoreceptors do?

Answer 21

Monitor the oxygen level in the blood and also carbon dioxide and pH (which are indicators of O2 level).

Question 22

How are electrical impulses from receptors sent to the medulla?

Answer 22

Along sensory neurones.

Question 23

What does the medulla do?

Answer 23

Processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (which are part of the autonomic nervous system).

Question 24

Where are the cardioacceleratory and cardioinhibitory centres?

Answer 24

Medulla oblongata.

Question 25

What does the cardioacceleratory centre do to HR?

Answer 25

Increases it.

Question 26

What does the cardioinhibitory centre do to HR?

Answer 26

Decreases it.

Question 27

How is the cardioacceleratory centre linked to the SAN?

Answer 27

By the sympathetic NS.

Question 28

How the the cardioinhibitory centre linked to the SAN?

Answer 28

By the parasympathetic NS.

Question 29

How is heart rate controlled when there are high blood O2 levels/low CO2/high pH levels?

Answer 29

Chemoreceptors (in carotid arteries) detect chemical changes in blood.

Increases frequency of impulses sent to the medulla/cardioinhibitory centre.

Increases frequency of impulses sent along parasympathetic neurones.

These secrete acetylcholine, which binds to receptors on the SAN.

Effector = cardiac muscles.

HR decreases to return O2, CO2 and pH levels back to normal.

Question 30

How is heart rate controlled when there are low blood O2 levels/high CO2/low pH levels?

Answer 30

Chemoreceptors (in carotid arteries) detect chemical changes in blood.

Increases frequency of impulses sent to the medulla/cardioacceleratory centre.

Increases frequency of impulses sent along sympathetic neurones.

These secrete noradrenaline, which binds to receptors on the SAN.

Effector = cardiac muscles.

HR increases to return O2, CO2 and pH levels back to normal.

Question 31

Explain how heart rate is controlled DURING exercise

Answer 31

Increase in respiration increases CO2 levels and decreases pH levels in blood.

Chemoreceptors (in carotid arteries) detect chemical changes in blood.

Increases frequency of impulses sent to the medulla/cardioacceleratory centre.

Increases frequency of impulses sent along sympathetic neurones.

These secrete noradrenaline, which binds to receptors on the SAN.

Effector = cardiac muscles.

HR increases to return O2, CO2 and pH levels back to normal.

Question 32

Explain how heart rate is controlled AFTER exercise

Answer 32

Decrease in respiration decreases CO2 levels and increases pH levels in blood.

Chemoreceptors (in carotid arteries) detect chemical changes in blood.

Increases frequency of impulses sent to the medulla/cardioinhibitory centre.

Increases frequency of impulses sent along parasympathetic neurones.

These secrete acetylcholine, which binds to receptors on the SAN.

Effector = cardiac muscles.

HR decreases to return O2, CO2 and pH levels back to normal.

Question 33

What is the equation for blood pressure?

Answer 33

BP = CO x Total peripheral resistance

Total peripheral resistance = diameter of blood vessels (Increase in diameter = decrease in resistance).

Question 34

What is the equation for cardiac output?

Answer 34

CO = SV x HR

Question 35

What happens to BP when CO decreases?

Answer 35

BP decreases.

Question 36

What happens to BP when HR decreases?

Answer 36

BP decreases.

Question 37

How is heart rate controlled when there is high blood pressure?

Answer 37

Baroreceptors (in walls of carotid arteries and aorta) detect high BP.

Increase in impulses sent to the medulla/cardioinhibitory centre.

Increase in impulses sent along parasympathetic neurones.

These secrete acetylcholine, which binds to receptors on the SAN.

Effector = cardiac muscles.

Heart rate slows down to reduce blood pressure back to normal.

Question 38

How is heart rate controlled when there is low blood pressure?

Answer 38

Baroreceptors (in walls of carotid arteries and aorta) detect low BP.

Increase in impulses sent to the medulla/cardioacceleratory centre.

Increase in impulses sent along sympathetic neurones.

These secrete noradrenaline, which binds to receptors on the SAN.

Effector = cardiac muscles.

Heart rate speeds up to increase blood pressure back to normal.