3.4.8 Control of Heart Rate

Question 1

Describe how the cardiac cycle is controlled by electrical activity (7x)

Answer 1

1. SAN generates waves of electrical activity across atrial walls causing atria to contract
2. Non-conducting tissue prevents waves of electrical activity from being passed directly from atria to ventricles
3. ∴ Waves of electrical activity go to ventricles via AVN
4. Delay at AVN allows atria to completely empty before ventricles contract
5. AVN sends waves of electrical activity down the bundles of His (on apex)
6. Purkyne tissue carries waves of electrical activity into muscular walls of ventricles
7. Ventricles contract from the bottom upwards

Question 2

What part of the brain controls the rate at which SAN fires (i.e. heart rate)

Answer 2

Medulla oblongata

Question 3

Why do animals need to alter their heart rate?

Answer 3

To respond to internal stimuli

(e.g. to prevent fainting due to low blood pressure or to make heart rate is high enough to supply body weight enough oxygen)

Question 4

Name the receptors that detect a change from the set point (those involved in heart rate)

Answer 4

• Baroreceptors (pressure receptors)
• Chemoreceptors (chemical receptors)

Question 5

Where are baroreceptors found?

Answer 5

• Aorta
• Carotid arteries (major arteries in neck)

Question 6

Where are chemoreceptors found?

Answer 6

• Aorta
• Carotid arteries
• Medulla

Question 7

What do baroreceptors monitor?

Answer 7

Blood pressure

Question 8

What do chemoreceptors monitor?

Answer 8

They monitor oxygen level in blood and also CO₂ and pH (which are indicators of O₂ level)

Question 9

Describe how the body restores blood pressure when there’s high blood pressure

Answer 9

• Baroreceptors detect high blood pressure
• Impulses sent to medulla which sent impulses along parasympathetic neurones
• Increased frequency of impulses (to/from medulla)
• Secrete acetylcholine (neurotransmitter) which binds to receptors in SAN
• Cardiac muscles (effector)
  
  • Heart rate slows down to reduce blood pressure back to normal

Question 10

Describe how the body restores blood pressure when there’s low blood pressure

Answer 10

• Baroreceptors detect low blood pressure
• Impulses sent to medulla which sent impulses along sympathetic neurones
• Increased frequency of impulses (to/from medulla)
• Secrete noradrenaline which binds to receptors in SAN
• Cardiac muscles (effector)
  
  • Heart rate speeds up to increase blood pressure back to normal

Question 11

Describe how the body restores O₂/CO₂/pH levels when there’s high blood O₂, low CO₂ or high pH levels

Answer 11

• Chemoreceptors detect chemical changes in blood
• Impulses sent to medulla which sends impulses along parasympathetic neurones
• Increased frequency of impulses (to/from medulla)
• Secrete acetylcholine which bind to receptors on SAN
• Cardiac muscles (effector)
  
  • Heart rate decreases to return O₂, CO₂ and pH levels back to normal

Question 12

Describe how the body restores O₂/CO₂/pH levels when there’s low blood O₂, high CO₂ or low pH levels

Answer 12

• Chemoreceptors detect chemical changes in blood
• Impulses sent to medulla which sends impulses along sympathetic neurones
• Increased frequency of impulses (to/from medulla)
• Secrete noradrenaline which bind to receptors on SAN
• Cardiac muscles (effector)
  
  • Heart rates increases to return O₂, CO₂ and pH levels back to normal